RF MICROELECTRONICS BEHZAD RAZAVI
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Transcript RF MICROELECTRONICS BEHZAD RAZAVI
지능형 마이크로웨이브 시스템 연구실
박 종 훈
Contents
Ch.3 Modulation and Detection
3.1 General Considerations
3.2 Analog Modulation
3.2.1 Amplitude Modulation
3.2.2 Phase and Frequency Modulation
3.3 Digital Modulation
3.3.1 Basic Concepts
3.3.2 Binary Modulation
3.3.3 Quadrature Modulation
3.4 Power Efficiency of Modulation Schemes
3.4.1 Constant-and Variable-Envelope Signals
3.4.2 Spectral Regrowth
3.5 Noncoherent Detection
3.1 General Considerations
The transmitted waveform in RF communications is
usually a high-frequency carrier modulated by the
original signal
Reason of modulations
Wired systems - Superior shielding(coaxial lines)
Wireless systems – antenna size(for resonable gain)
Must occur in a certain part of the spectrum
FCC regulations
Allows simpler detection at the receive end
3.1 General Considerations
Base band / Pass band signals
Base band – Nonzero in the vicinity of ω = 0
E.g. signal generated by a microphone or a video camera
Pass band – Nonzero in a band around a carrier
frequency ωc
3.1 General Considerations
Modulation
Converts a baseband signal to a passband counterpart
Pass band signal –
a(t), θ(t) – functions of time
Carrier signal –
Vary its amplitude or phase
ωct + θ(t) – total phase
θ(t) - excess phase
ωct + dθ/dt - total frequency
dθ/dt – excess frequency (frequency deviation)
3.1 General Considerations
Demodulation(Detection)
Inverse of modulation
Extract the original baseband signal with minimum
noise, distortion, ISI, etc.
3.1 General Considerations
Important Aspects of Modems
Quality(e.g. SNR)
Attenuation and interference in the channel
Noise at input of the detector
If the modem achieves higher tolerance of noise
Power reduced
providing longer talk time in portable device
Allowing communication over a longer distance
Bandwidth
Spectral efficiency
Power efficiency
Linear amplifier / Nonlinear amplifier
3.1 General Considerations
AWGN(Additive White Gaussian Noise) Channel
Power spectral density = N0/2
3.2 Analog Modulation
3.2.1 Amplitude Modulation
3.2.2 Phase and Frequency Modulation
3.2.1 Amplitude Modulation
Modulation
mxBB(t) : baseband signal
m : modulation index
3.2.1 Amplitude Modulation
Demodulation(Envelope detector)
SNR
3.2.1 Amplitude Modulation
Limited use in today’s wireless systems
Except for broadcast radios and the sound in television
Susceptible to noise
Highly linear power amplifier in the transmitter
High SNR at the input
3.2.2 Phase and Frequency Modulation
3.2.2 Phase and Frequency Modulation
Phase Modulation(PM)
Frequency Modulation(FM)
VCO(Voltage Controlled Oscillator)
3.2.2 Phase and Frequency Modulation
Modulator
3.2.2 Phase and Frequency Modulation
Demodulation
Demodulator
3.2.2 Phase and Frequency Modulation
Narrowband FM
3.2.2 Phase and Frequency Modulation
Narrowband FM ->
ωm increase, magnitude of the sidebands decrease
maximum frequency deviation is mAm
Low SNR
Wideband FM
Without the restriction
3.2.2 Phase and Frequency Modulation
Bessel Function
Referance – Introduction to Analog & Digital Communications 2nd
3.2.2 Phase and Frequency Modulation
Wideband FM VS Narrow FM
3.2.2 Phase and Frequency Modulation
Bandwidth(BFM)
Containing 98% of the signal power
BFM ≈2(β+1)BBB – Carson’s rule
Preemphasis and Deemphasis
Larger gain at higher freq. -> Amplifying noise at high freq.
3.2.2 Phase and Frequency Modulation
SNR Comparison
Without Preemphasis and deemphasis
With Preemphasis and deemphasis
f1 : -3dB corner frequency of the low pass filter
Typical applications : 10 to 15dB higher than 1st eqn.
3.3 Digital Modulation
ASK, PSK, FSK
Analog parameters
signal quality, spectral efficiency, and power efficiency
Digital parameter
BER(bit error rate)
Average number of erroneous bits observed at the output of the detector
divided by the total number of bits received in a unit time
3.3.1 Basic Concepts
Binary and M-ary Signaling
Binary waveform(Digital baseband signal)
bn : ‘bit’ value in the time interval
Multilevel(M-ary signaling)
Bandwidth relaxed
bn : ‘symbol’ value in the time interval
3.3.1 Basic Concepts
Basic Functions ( e.g. FSK )
Digitally modulated waveforms
3.3.1 Basic Concepts
Signal Constellations
3.3.1 Basic Concepts
Cartesian minimum
distance : Relate to the bit
error rate
3.3.1 Basic Concepts
Optimum Detection
Since the baseband signal is digital, the detector output
must be sampled every bit period to determine the
received value
Problem of Noise
3.3.1 Basic Concepts
Solution
Use of filter
Sampling is synchronized such that the peak value of the pulse
is sensed, the output SNR is high
3.3.1 Basic Concepts
Noise components that vary significantly in a period of
Tb tend to average out
3.3.1 Basic Concepts
Matched Filter
Pulse p(t) that is corrupted by additive white noise, there
exists an optimum filter that maximizes the SNR at the
sampling instant
3.3.1 Basic Concepts
Maximum value at t = Tb
3.3.1 Basic Concepts
Ep : energy of the signal
P(t) : voltage quantity
Optimum detection of modulated signals
where x(t) = p(t) + n(t). If p(t) is zero outside the interval [0 Tb],
then
3.3.1 Basic Concepts
Coherent and Noncoherent Detection
Detection schemes that require phase synchronization
3.3.1 Basic Concepts
This circuit employs two narrowband filters
3.3.1 Basic Concepts
Definition of Bandwidth
Containing 99% signal power
3.3.2 Binary Modulation
BPSK(Binary PSK)
BFSK(Binary FSK)
ASK is rarely used in RF applications
3.3.2 Binary Modulation
PDF for binary data with additive noise
3.3.2 Binary Modulation
BPSK
3.3.2 Binary Modulation
BFSK
3.3.2 Binary Modulation
BPSK VS BFSK
Bit energy in BFSK must be twice that in BPSK
Minimum distance between the points in the
constellation is greater in BPSK
BPSK has a 3-dB advantage over BFSK
3.3.2 Binary Modulation
Quadrature Modulation
To subdivide a binary data stream into pairs of two bits
3.3.2 Binary Modulation
Categories
QPSK(Quadrature Phase Shift Keying)
Offset QPSK(OQPSK)
π/4-QPSK
MSK(Minimum Shift Keying)
GMSK(Gaussian MSK)
3.3.2 Binary Modulation
QPSK
Important drawback of QPSK is large phase changes
3.3.2 Binary Modulation
OQPSK
3.3.2 Binary Modulation
Phase step is only ±90˚
BER and spectrum of OQPSK are identical to those of QPSK
critical drawback
It doew not lend itself to differential encoding
Differential encoding plays an important role in noncoherent
receivers, the most popular type in today’s RF applications
3.3.2 Binary Modulation
3.3.2 Binary Modulation
Since no two consecutive points are from the same
constellation
Maximum phase step is 135˚, 45 ˚ less than QPSK
BER are identical to those of QPSK
3.3.2 Binary Modulation
MSK
Continuous phase modulation
Rectangular pulse leading to a wide spectrum and presenting
difficulties in the design of power amplifiers
3.3.2 Binary Modulation
The smooth phase transition in MSK lower the signal power
in the sidelobes of the spectrum
But at the cost of widening the main lobe
Decay proportional to
3.3.2 Binary Modulation
GMSK(Gaussian MSK)
The phase change is made smoother than MSK
α increase -> narrower the spectrum -> ISI increase
3.3.2 Binary Modulation
3.4 Power Efficiency of Modulation Schemes
3.4.1 Constant- and Variable-Envelope Signals
3.4.2 Spectral Regrowth
3.4.1 Constant- and Variable-Envelope Signals
The Spectrum grows when a variable-envelope signal
passes through a nonlinear system
A(t) vary with time : variable-envelope
3.4.2 Spectral Regrowth
Spectral Regrowth
If the PA exhibits significant nonlinearity, then the
shape of xI(t) and xQ(t) is not preserved and the
spectrum is not limited to the desired bandwidth
FM and FSK waveform have no abrupt phase change and
exhibit a constant envelope, they can be amplified by
means of nonlinear Pas with no spectral regrowth.
Trade-off between spectral efficiency and power
efficiency
As the signal bandwidth is more limited -by filtering or pulse
shaping- the power amplifier must achieve a higher linearity
so as to avoid spectral regrowth
3.5 Noncoherent Detection
Coherent FSK Detection
Provide the highest SNR.
Require that the phase of the local oscillator in the receiver
Noncoherent Detection
1.5dB greater than that in coherent FSK detection
Lower complexity
3.5 Noncoherent Detection
DPSK(Differential Phase Shift Keying)
3dB higher than coherent PSK