지능형 마이크로웨이브 시스템 연구실
박 종 훈
Contents
 Ch.3 Modulation and Detection
 3.1 General Considerations
 3.2 Analog Modulation
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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 –
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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)


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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
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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)
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 Frequency Modulation(FM)
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
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
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3.2.2 Phase and Frequency Modulation
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Narrowband FM ->
ωm increase, magnitude of the sidebands decrease


maximum frequency deviation is mAm
Low SNR
 Wideband FM

Without the restriction
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
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
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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)
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bn : ‘bit’ value in the time interval
 Multilevel(M-ary signaling)
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Bandwidth relaxed
bn : ‘symbol’ value in the time interval
3.3.1 Basic Concepts
 Basic Functions ( e.g. FSK )
 Digitally modulated waveforms
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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
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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
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
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
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
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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
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

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

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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