Asstt. Professor Adeel Akram

What is signal encoding?

 In communications systems, the altering of the characteristics of a signal to make the signal more suitable for an intended application, such as optimizing the signal for transmission  Modifying the signal spectrum, increasing the information content, providing error detection and/or correction, and providing data security  A single coding scheme usually does not provide more than one or two specific capabilities.  Different codes have different sets of advantages and disadvantages.

Reasons for Choosing Encoding Techniques

 Digital data, digital signal  Equipment less complex and less expensive than digital to-analog modulation equipment  Analog data, digital signal  Permits use of modern digital transmission and switching equipment

Reasons for Choosing Encoding Techniques

 Digital data, analog signal  Some transmission media will only propagate analog signals  E.g., Fax/Modem  Analog data, analog signal   Analog data in electrical form can be transmitted easily and cheaply Done with voice transmission over voice-grade lines

Signal Encoding Criteria

 What determines how successful a receiver will be in interpreting an incoming signal?

 Signal-to-noise ratio   Data rate Bandwidth  An increase in data rate increases bit error rate  An increase in SNR decreases bit error rate  An increase in bandwidth allows an increase in data rate

Comparing Encoding Schemes

 Signal interference and noise immunity  Performance in the presence of noise  Cost and complexity  The higher the signal rate to achieve a given data rate, the greater the cost

Digital Data to Analog Signals

 Keying is a form of modulation where the modulating signal takes one of two or more values at all times. For example: "on" or "off“  The name derives from the Morse code key used for telegraph signaling  Amplitude-shift keying (ASK)  Amplitude difference of carrier frequency  Frequency-shift keying (FSK)  Frequency difference near carrier frequency  Phase-shift keying (PSK)  Phase of carrier signal shifted

Amplitude-Shift Keying

 One binary digit represented by presence of carrier, at constant amplitude  Other binary digit represented by absence of carrier

s



A

cos 0  2 

f c t

 binary binary 1 0  where the carrier signal is Acos(2πf

c

t)

Amplitude-Shift Keying

 Susceptible to sudden gain changes  Inefficient modulation technique  On voice-grade lines, used up to 1200 bps  Used to transmit digital data over optical fiber

Binary Frequency-Shift Keying (BFSK)

 Two binary digits represented by two different frequencies near the carrier frequency

s

  

A A

cos cos   2 

f

1

t

2 

f

2

t

  binary 1 binary 0  where f 1 and f 2 are offset from carrier frequency f

c

opposite amounts by equal but

Binary Frequency-Shift Keying (BFSK)

 Less susceptible to error than ASK  On voice-grade lines, used up to 1200bps  Used for high-frequency (3 to 30 MHz) radio transmission  Can be used at higher frequencies on LANs that use coaxial cable

Phase-Shift Keying (PSK)

 Two-level PSK (BPSK)  Uses two phases to represent binary digits

s

 

A A

cos cos   2 2  

f f c t c t

    binary 1 binary 0 

A

cos

A

 2 cos   2

f c t



f



c t

 binary 1 binary 0

Phase-Shift Keying (PSK)

 Differential PSK (DPSK)  Phase shift with reference to previous bit   Binary 0 – signal burst of same phase as previous signal burst Binary 1 – signal burst of opposite phase to previous signal burst

Phase-Shift Keying (PSK)

 Four-level PSK (QPSK)  Each element represents more than one bit

s

      

A A A A

cos  cos  cos  cos  2 2 2 2    

f f f c c f c c t t t t

     4 3  4 3  4  4 11 01 00 10

Quadrature Amplitude Modulation

 QAM is a combination of ASK and PSK  Two different signals sent simultaneously on the same carrier frequency

s



d

1 cos 2 

f c t



d

2   sin 2 

f c t

Quadrature Amplitude Modulation

Analog Data to Analog Signal

 Modulation of digital signals  When only analog transmission facilities are available, digital to analog conversion required  Modulation of analog signals  A higher frequency may be needed for effective transmission

Modulation Techniques

 Amplitude modulation (AM)  Angle modulation  Frequency modulation (FM)  Phase modulation (PM)

Outline  Cellular Concept  Cellular Architecture  Frequency Reuse  Multiple Access Methods  FDMA, TDMA, and CDMA  In particular, we focus on CDMA.

Different Generations

 1G  analog  2G  digital  3G  higher data rate for multimedia applications

1G Cellular Systems

 Many Different Standards:  AMPS (US)   NMT (Northern Europe) TACS (Europe)    Spectrum  NTT (Japan) many others...

around 800 and 900 MHz

Frequency Division Duplex (FDD) Forward Link mobile Reverse Link base station Two separate frequency bands forward and reverse links.

are used for Typically, 25 MHz in each direction.

AMPS: 824-849 MHz (forward) 869-894 MHz (reverse)

Frequency Division Multiple Access (FDMA)  The spectrum of each link (forward or reverse) is further divided into frequency bands  Each station assigned fixed frequency band idle idle idle

Number of User Channels in AMPS  Bandwidth allocated to each user in each link (forward or reverse) is 30 KHz.

 No. of user channels = Total bandwidth / user bandwidth = 25 MHz / 30 kHz  = 833 Is it enough?

Frequency Reuse

Radio coverage, called a cell.

f

The same frequency can be reused in different cells, if they are far away from each other

f

Cellular Architecture MS – Mobile Station BSC – Base Station Controller MSC or MTSO– Mobile Switching Center MS PSTN – Public Switched Telephone Network BSC MSC PSTN segmentation of the area into cells

Geometric Representation

 Cells are commonly represented by hexagons .

 Why hexagon?  How about circle?

 How about square, or triangle?

Hexagon vs Circles

 Notice how the circles below would leave gaps in our layout. Still, why hexagons and not triangles or rhomboids?

Hexagonal Cells

Cell site and Cell

 The cell site is a location or a point, the cell is a wide geographical area  Cells site covers a portion or a sector of each cell, not the whole thing. Antennas from other cell sites cover the other portions. The covered area, if you look closely, resembles a sort of rhomboid In reality, the cell is the

red hexagon

Channel Reuse

 The total number of channels are divided into N groups.

 N is called reuse factor .

 Each cell is assigned one of the groups.

 The same group can be reused by two different cells provided that they are sufficiently far apart .

Example: N = 7

Reuse Distance

 How far apart can two users share the same channel?

 It depends on whether signal quality is acceptable or not.

 The larger the distance between the two users, the better the signal quality.

 How to measure signal quality?

Nyquist Bandwidth

 Given a bandwidth of B, the highest signal transmission rate for binary signals (two voltage levels) is: 

C = 2B



Ex: B=3100 Hz; C=6200 bps

 With multilevel signaling  C = 2B log 2 

M

M = number of discrete signal or voltage levels

Signal Quality

 The signal quality depends on the ratio between signal power and interference (noise) power .

I S

 

i S I i

Interference from the i-th interfering BS.

 This is called signal-to (SNR or SIR).

noise (interference) ratio

Signal-to-Noise Ratio

 Ratio of the power in a signal to the power contained in the noise that’s present at a particular point in the transmission  Typically measured at a receiver  Signal-to-noise ratio (SNR, or S/N) (

SNR

) dB  10 log 10 signal power noise power  A high SNR means a high-quality signal, low number of required intermediate repeaters  SNR sets upper bound on achievable data rate



Shannon Capacity Formula

Equation:

C



B

log 2  1  SNR   Represents theoretical maximum that can be achieved  In practice, only much lower rates achieved  Formula assumes white noise (thermal noise)  Impulse noise is not accounted for  Attenuation distortion or delay distortion not accounted for

Classifications of Transmission Media

 Transmission Medium  Physical path between transmitter and receiver  Guided Media  Waves are guided along a solid medium  E.g., copper twisted pair, copper coaxial cable, optical fiber  Unguided Media   Provides means of transmission but does not guide electromagnetic signals Usually referred to as wireless transmission  E.g., atmosphere, outer space

Propagation Model

 The received signal power depends on the distance between the transmitter and the receiver

P r



P

0  

d d

0     

P 0 d 0

is the power received at a reference distance    is called the path loss exponent Typically, 2 ≤  ≤ 6 *

Typical values of

α

Table : Path Loss Exponents for Different Environments Propagation Environment Free Space Urban Area Shadowed Urban Area In-Building Line-of-Sight Obstructed In Building Obstructed In Factory Path Loss Exponent

2 2.7 to 3.5

3 to 5 1.6 to 1.8

4 to 6 2 to 3 As shown in Table typical values for the path loss exponent are between 2 to 6

2G Cellular Systems

 Four Major Standards:  GSM ( European )  IS-54 (later becomes IS-136 , US)  JDC (Japanese Digital Cellular)  IS-95 ( CDMA , US )

Example: GSM

 Frequency Band  935-960, 890-915 MHz  Two pieces of 25 MHz band (same as AMPS)  AMPS has 833 user channels  How about GSM ?

Time Division Multiple Access (TDMA)

 The mobile users access the channel in round robin fashion .

 Each station gets one slot in each round .

Slots 2, 5 and 6 are idle

FDMA/TDMA, example GSM 960 MHz f 124 935.2 MHz 915 MHz 1 124 20 MHz 200 kHz 1 890.2 MHz 1 2 3 7 8 Each freq. carrier is divided into 8 time slots.

t

Number of channels in GSM

 Freq. Carrier: 200 kHz  TDMA: 8 time slots per freq carrier  No. of carriers = 25 MHz / 200 kHz = 125  No. of user channels = 125 * 8 = 1000

Capacity Comparison

 Reuse factor   7 for AMPS 3 for GSM ( why smaller reuse factor?

)  What’s the capacity of GSM relative to AMPS?

A. one half of AMPS B. the same C. 3 times larger D. 10 times larger

Answer

 AMPS  reuse factor = 7  no. of users / cell = 833 / 7 = 119  GSM  reuse factor = 3  no. of users / cell = 1000 / 3 = 333  almost 3 times larger than AMPS!

Multiple Access Methods

Three major types:  Frequency Division Multiple Access (FDMA)  Time Division Multiple Access (TDMA)  Code Division Multiple Access (CDMA)   Frequency hopping (FH-CDMA) Direct sequence (DS-CDMA)

Frequency-Time Plane

Frequency Partition of signal space into time slots and frequency bands Time

FDMA

Frequency Time Different users transmit at different frequency bands simultaneously

TDMA

Frequency Time Different users transmit at different time slots Each user occupy the whole freq. spectrum

Frequency Hopping CDMA

Frequency Time At each successive time slot, the frequency band assignments are reordered Each user employs a code that dictates the frequency hopping pattern

Assignment

 Write note on 3G Mobile technology  Write note on 3.5G Mobile technology  Write note on 3.75G Mobile technology  Write note on 4G Mobile technology  Give an Overview of GSM network Architecture  Difference between CDMAOne and CDMA2000

Questions

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