1
Chapter 8
Communication Systems
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Chapter 8 Communication Systems
Communication Channel
信道
Source
Signals
Modulator
调制器
Modulated
Signals
Be best suited for transmitting
3
Chapter 8 Communication Systems
Modulation,The general process of embedding an information
-bearing signal into a second signal.
Extracting the information-bearing signal.Demodulation,
Modulation,
Amplitude Modulation (AM)
Frequency Modulation (FM)
Phase Modulation (PM)
Modulating
Signal
Carrier
Signal
Modulated
Signaltctxtytx
tc
Amplitude Modulation
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Chapter 8 Communication Systems
§ 8.1 Complex Exponential and Sinusoidal Amplitude Modulation
1,cjtc t e
2,c o s cc t t
t
tx
t
ty
Envelope
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Chapter 8 Communication Systems
1,Suppose the modulator and demodulator are not
synchronized in phase.
① Complex Exponential Carrier
§ 8.2.1 Synchronous Demodulation
ccjte
tx
ty
Communication
Channel
Transmitter
Modulation
Receiver
Synchronous Demodulation
LPFtxtw
djte
2,Suppose the modulator and demodulator are not
synchronized in frequency.
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Chapter 8 Communication Systems
② Sinusoidal Carrier
cctc o s
tx
ty
Communication
Channel
Transmitter
Modulation
Receiver
Synchronous Demodulation
LPFtxtw
cos d t
1,Suppose the modulator and demodulator are not
synchronized in phase.
2,Suppose the modulator and demodulator are not
synchronized in frequency.
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§ 8.3 Frequency-Division Multiplexing（频分复用）
Chapter 8 Communication Systems
t1cos?
tx1
t2cos?
tx2
ty1BPF1?
ty2BPF2?

tn?cos
txntynBPF
ty
n?
jX1
M 0?M?
M 0?M?
jX 2
M 0?M?
jX n
2?2 1?1 0?
jY
n?n

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Chapter 8 Communication Systems
Quadrature Multiplexing（正交多路复用）
tc?cos
tx1
tc?sin
tx2
tr
tc?sin
tc?cos
LPF
LPF
ty1
ty2
M?
M?
Communication
Channeltr
tw1
tw2
txty 11?
txty 22?
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Chapter 8 Communication Systems
§ 8.4 Single-Sideband Sinusoidal Amplitude Modulation
（单边带）
M 0?
jX
M?
tc?cos
txty
c 0?
jY
c?
M?2M?2
Upper
sideband UppersidebandLower sideband
Double Sideband Modulation (DSB)
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Chapter 8 Communication Systems
Retain the upper sideband
c 0?c?
M?
Upper
sideband Uppersideband
M?
c 0?c?
Lower
sideband
M?M?
Retain the lower sideband
Single Sideband Modulation (SSB)
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Chapter 8 Communication Systems
M 0?
jX
M? M 0
jX p
M?
j?
j
Figure 8.21 SSB system using a 90o phase-shift network.




0 j
0 j-
jH
tx
ty
tc?sin
ty2
ty1
txp
90o phase-shift
cos ct?
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Chapter 8 Communication Systems
cpcp jjXjjjXjjY 2 12 12
ttxty cp?s i n2?
M 0
jX p
M?
j?
j
c 0
jY1
c?
2/1
c
c?Lower Sideband
jY
2/1?
2/1
jY2
c c?
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Chapter 8 Communication Systems
Homework:
8.1 8.3 8.22 8.28