Transcript - 09 ELECTRONICS Mehran UET
Slide 1
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 2
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 3
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 4
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 5
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 6
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 7
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 8
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 9
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 10
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 11
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 12
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 13
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 14
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 15
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 16
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 17
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 18
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 19
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 20
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 21
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 22
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 23
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 24
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 25
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 26
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 27
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 28
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 29
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 30
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 31
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 32
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 33
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 34
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 35
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 36
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 37
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 38
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 39
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 40
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 41
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 42
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 43
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 44
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 45
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 46
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 47
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 48
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 49
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 50
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 51
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 52
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 53
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 54
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 55
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 56
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 57
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 58
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 59
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 60
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 61
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 62
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 63
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 64
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 65
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 66
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 67
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 68
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 69
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 70
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 71
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 72
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 73
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 74
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 2
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 3
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 4
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 5
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 6
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 7
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 8
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 9
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 10
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 11
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 12
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 13
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 14
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 15
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 16
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 17
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 18
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 19
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 20
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 21
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 22
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 23
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 24
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 25
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 26
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 27
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 28
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 29
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 30
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 31
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 32
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 33
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 34
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 35
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 36
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 37
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 38
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 39
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 40
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 41
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 42
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 43
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 44
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 45
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 46
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 47
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 48
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 49
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 50
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 51
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 52
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 53
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 54
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 55
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 56
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 57
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 58
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 59
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 60
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 61
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 62
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 63
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 64
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 65
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 66
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 67
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 68
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 69
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 70
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 71
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 72
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 73
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5
Slide 74
SIGNAL ENCODING TECHNIQUES
Engr. Mehran Mamonai
Department of Telecommunication
Encoding Techniques
• Digital data, digital signal
– Equipment less complex and expensive than digital-to-analog
modulation equipment
• Analog data, digital signal
– Permits use of modern digital transmission and switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals E.g.,
unguided media
• Analog data, analog signal
– Analog data in electrical form can be transmitted easily and cheaply
i.e. Done with voice transmission over voice-grade lines
Digital Data, Digital Signal
• Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
Terms (1)
• Unipolar
– All signal elements have same sign
• Polar
– One logic state represented by positive voltage the
other by negative voltage
• Data rate
– Rate of data transmission in bits per second
• Duration or length of a bit
– Time taken for transmitter to emit the bit
Terms (2)
• Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
• Mark and Space
– Binary 1 and Binary 0 respectively
Interpreting Signals
• Need to know
– Timing of bits - when they start and end
– Signal levels
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Data rate
• An increase in data rate increases bit error rate
– Signal to noise ratio
• An increase in SNR decreases bit error rate
– Bandwidth
• An increase in bandwidth allows an increase in
• data rate
Comparison of Encoding Schemes (1)
• Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Spectral efficiency (also called bandwidth efficiency)
– With no dc component, ac coupling via transformer Possible
– Concentrate power in the middle of the bandwidth, Transfer
function of a channel is worse near band edges.
• Clocking
– Synchronizing transmitter and receiver
– Ease of determining beginning and end of each bit position
– External clock
– Sync mechanism based on signal
Comparison of Encoding Schemes (2)
• Error detection
– Can be built in to signal encoding
• Signal interference and noise immunity
– Some codes are better than others
• Cost and complexity
– The higher the signal rate to achieve a given data
rate, the greater the cost
Encoding Schemes
•
•
•
•
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
NonReturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
NonReturn to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
NRZ (L & I)
Differential Encoding
• Data represented by changes rather than
levels
• More reliable detection of transition rather
than level
• In complex transmission layouts it is easy to
lose sense of polarity
NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
Multilevel Binary
• Use more than two levels
• Bipolar-AMI
–
–
–
–
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolarAMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
• Not as efficient as NRZ
– Each signal element only represents one bit
– In a 3 level system could represent log23 = 1.58
bits
– Receiver must distinguish between three levels
(+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase
• Manchester
–
–
–
–
–
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
• Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
Manchester Encoding
Differential Manchester Encoding
Biphase Pros and Cons
• Con
– At least one transition per bit time and possibly two
– Maximum modulation rate is twice NRZ
– Requires more bandwidth
• Pros
– Synchronization on mid bit transition (self clocking)
– No dc component
– Error detection
• Absence of expected transition
Modulation Rate
Scrambling
• Use scrambling to replace sequences that would produce
constant voltage
• Filling sequence
– Must produce enough transitions to sync
– Must be recognized by receiver and replace with original
– Same length as original
•
•
•
•
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
– Amplitude difference of carrier frequency
• Frequency shift keying (FSK)
– Frequency difference near carrier frequency
• Phase shift keying (PK)
– Phase of carrier signal shifted
Modulation Techniques
Amplitude Shift Keying
• One binary digit represented by presence of
carrier, at constant amplitude
• Other binary digit represented by absence of
carrier
where the carrier signal is Acos(2πfct)
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, Amplitude is one
– i.e. presence and absence of carrier is used
• Inefficient modulation technique since it is much more
susceptible to noise
– Atmospheric and impulse noises tend to cause rapid
fluctuations in amplitude
• Linear modulation technique
– Good spectral efficiency
– Low power efficiency
• Up to 1200bps on voice grade lines
• Used for carrying digital data over optical fiber
Amplitude Shift Keying
Frequency Shift Keying
• Two binary digits represented by two
different frequencies near the carrier
frequency
• where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Frequency Shift Keying
•
•
•
•
Most common form is binary FSK (BFSK)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Used for high-frequency (3 to 30 MHz) radio
transmission
• Can be used at higher frequency on LANs using
co-axial
• Amplitude of the carrier wave is constant
– Power-efficient
Binary Frequency Shift Keying
FSK on Voice Grade Line
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than
one bit
Multiple Frequency-Shift Keying (MFSK)
Binary Phase-Shift Keying (BPSK)
• Linear modulation technique
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK
– Two phases represent two binary digits
• Differential PSK
– Phase shifted relative to previous transmission rather
than some reference signal
• Binary 0 – signal burst of same phase as previous signal burst
• Binary 1 – signal burst of opposite phase to previous signal burst
Binary Phase-Shift Keying (BPSK)
Differential PSK
Four-level PSK (QPSK)
Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than one
amplitude
– 9600bps modem use 12 angles , four of which have
two amplitudes
• Offset QPSK (orthogonal QPSK)
– Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to Analog
Modulation Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies, but
to offset of modulated frequency from carrier at high
frequencies
– Bandwidth of modulated signal (BT)
– ASK, PSK BT =(1+r)R
– FSK BT= 2DF+(1+r)R
– R = bit rate
– 0 < r < 1; related to how signal is filtered
– DF = f2-fc=fc-f1
– In the presence of noise, bit error rate of PSK and QPSK
are about 3dB superior to ASK and FSK
Performance of Digital to Analog
Modulation Schemes
• Bandwidth of modulated signal (BT)
• MPSK
• MFSK
– L = number of bits encoded per signal element
– M = number of different signal elements
Multilevel PSK
• Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
•
•
•
•
D = modulation rate, baud
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
Quadrature Amplitude
Modulation (QAM)
• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on
the same carrier frequency
Quadrature Amplitude Modulation
(QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless
• Logical extension of QPSK
• Send two different signals simultaneously on
same carrier frequency
–
–
–
–
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
QAM Modulator
QAM Levels
• Two level ASK
– Each of two streams in one of two states
– Four state system
– Essentially QPSK
• Four level ASK
– Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
– Increased potential error rate
Analog Data, Digital Signal
• Digitization
– Conversion of analog data into digital signal
• Once analog data have been converted to
digital signals, the digital data:
– Digital data can then be transmitted using NRZ-L
– Digital data can then be transmitted using code
other than NRZ-L
– Analog to digital conversion done using a codec
Digitizing Analog Data
Digital Coding Schemes
• Pulse code modulation (PCM)
• Delta modulation (DM)
Pulse Code Modulation(PCM) (1)
• Based on the sampling theorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation
(PAM) samples
• The digital signal consists of block of n bits, where each nbit number is the amplitude of a PCM pulse
• E.g.: 4 bit system gives 16 levels
• Quantized
– Quantizing error or noise
– Approximations mean it is impossible to recover original
exactly
– Leads to quantizing noise
Pulse Code Modulation(PCM) (2)
• 8 bit sample gives 256 levels
• If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency,
the samples contain all the information of the
original signal
– (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• 8000 samples per second of 8 bits each gives
64kbps
• Quality comparable with analog transmission
PCM Example
PCM Block Diagram
Delta Modulation
• Analog input is approximated by a staircase
function
– Move up or down one level () at each sample
interval
• The bit stream approximates derivative of
analog signal (rather than amplitude)
– 1 is generated if function goes up
– 0 otherwise
Delta Modulation - example
Delta Modulation
• Two important parameters
– Size of step assigned to each binary digit (δ)
– Sampling rate
• Accuracy improved by increasing sampling
rate
– However, this increases the data rate
• Advantage of DM over PCM is the simplicity
of its implementation
Delta Modulation - Operation
Delta Modulation - Performance
• Good voice reproduction
– PCM - 128 levels (7 bit)
– Voice bandwidth 4khz
– Should be 8000 x 7 = 56kbps for PCM
• Data compression can improve on this
– e.g. Interframe coding techniques for video
Analog Data, Analog Signals
• Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing ( will be
discussed in chapter 8)
• Types of modulation
– Amplitude Modulation
– Angel Modulation
– Frequency Modulation
– Phase Modulation
Amplitude Modulation
• cos2πfct = carrier
• x(t) = input signal
• na = modulation index (< 1)
– Ratio of amplitude of input signal to carrier
• Double sideband transmitted carrier (DSBTC)
Amplitude Modulation
Spectrum of AM
AM Power
• Transmitted power
• Pt = total transmitted power in s(t)
• Pc = transmitted power in carrier
Single Sideband (SSB)
• Variant of AM is single sideband (SSB)
– Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required
– Less power is required
• Disadvantages
– Poor performance in fading channels
Angle Modulation
• Frequency modulation
– Derivative of the phase is proportional to
modulating signal
• Phase modulation
– Phase is proportional to modulating signal
Analog
Modulation
Required Reading
• Stallings chapter 5