信號與系統(tǒng)英文課件：Chapter 8 Communication Systems

1、8.0 Introduction 8 Communication SystemsA Typical Communication System convertortransmitting systemchannelreceiving systemspeechsignalmodulationimagesignalconvertorspeechimagedemodulationModulation: Embedding an information-bearing signal into a second signal.Demodulation: Extracting the information

2、-bearing signal. 8 Communication SystemsModulation methods:AM-Amplitude ModulationFM-Frequency ModulationPM-Phase ModulationAM/wc-AM/with carrier帶載波帶載波AM/sc-AM/suppressed carrier抑制載波抑制載波在抑制載波的正弦幅度調(diào)制中有雙邊帶、單邊帶、在抑制載波的正弦幅度調(diào)制中有雙邊帶、單邊帶、 殘留邊帶：殘留邊帶： DSB-Double side-band SSB-Single side-band VSB-Vestigial si

3、de-bandA very common form of AM is sinusoidal amplitude modulation.8.1 Complex Exponential and Sinusoidal Amplitude ModulationModulating system model:x(t)c(t)y(t)=x(t)c(t)x(t) - modulating signalc(t) - Carrier signaly(t) - modulated signal 8 Communication Systems( )x tcosct( )cos()ccc ttFor the sinu

4、soidal amplitude modulation:8.1.1 Amplitude Modulation with Complex Exponential CarrierExponential carrier:For convenience, let c=0, so)()(cctjetctjcetc)(Output signal(modulated signal):tjcetxty)()(1()()*()2()2()()( ()ccY jX jC jC jY jX j (1) Modulation Theory 8 Communication Systems 8 Communication

5、 Systems( )( )( )cos( )sincjtccy tx t ex ttjx tt(2) Implementation 8 Communication Systemsreal valued signal8.1.2 Amplitude Modulation with Sinusoidal signalFor convenience, choose c=0, sottxtctxtyccos)()()()()(21)(21)()()()()(*)(21)(ccccjjXjjXjYjCjCjXjY 8 Communication Systems 8 Communication Syste

6、mscMx(t) is band limited by M 8 Communication SystemscM8.2 Demodulation for Sinusoidal AM8.2.1 Synchronous demodulation 8 Communication Systems)2(41)2(41)(21)(ccjjXjjXjXjWIn frequency domain:211( )( ) ( )( )cos( )( )cos222ccw ty t c tx ttx tx ttIn time domain:(1) Demodulation process 8 Communication

7、 Systems2McMW 8 Communication Systems( )( )cos() cos()11( )cos()( )cos(2)22cccccccccw tx tttx tx ttWhen , it is referred to as synchronous demodulation.cc(2) Synchronous problemconstant8.2.2 Asynchronous demodulation (Envelope Demodulation)Amplitude-modulated signal:tAtxtyccos)()( 8 Communication Sy

8、stemsmax( )Ax t 8 Communication Systems15 8 Communication Systems8.3 Frequency-Division MultiplexingModulation16 8 Communication SystemsDemodulation17 8 Communication Systems8.4 Single-Sideband Sinusoidal AMDouble-sideband modulationLower sidebandUpper sideband18)( j )( j 21)j (cdncdn0YYXSynchronous

9、 Demodulation 8 Communication SystemsSingle-Sideband Sinusoidal Amplitude Modulation19：Band-pass filter01BB帶通濾波器)(txcosct)()(tyDcccc+(j )H)(tyS 8 Communication Systems20In frequency:)cos()()(Dttxtyc)( j )( j 21)j (ccDXXY)j ()j ()j (DSHYY 8 Communication Systems21)sgn(j)j (HLower sideband modulationU

10、pper sideband modulation)sgn(j)j (H)(jH)cos (ct)(htx)(1ty)(2ty)sin(ct)(tyS)(tx 8 Communication Systems22)( j )( j 21)j (cc1XXY)sgn()j (j)j ()j ()j (hXHXX)( j )( j j21sin)()j (chchch2XXttxFY)j ()j ()j (21SYYY)sgn(j)(jH)cos (ct)(htx)(1ty)(2ty)sin(ct)(tyS)(txIn frequency: 8 Communication Systems23利用希爾伯

11、特變換下邊帶幅度調(diào)制的頻譜利用希爾伯特變換下邊帶幅度調(diào)制的頻譜24)100cos()j (AtFX)100()100(Example: We know that x(t) X(jw).1.Please determine and depict the frequency spectrum at the points of A、B、C、D and y(t);2. What is the relation between y(t) and x(t)? 25)j (*)j (21)j (ABXXX)100()100(21XXExample: We know that x(t) X(jw).1.Ple

12、ase determine and depict the frequency spectrum at the points of A、B、C、D and y(t);2. What is the relation between y(t) and x(t)? 26)j ()j ()j (1BCHXXExample: We know that x(t) X(jw).1.Please determine and depict the frequency spectrum at the points of A、B、C、D and y(t);2. What is the relation between y(t) and x(t)? 27)100()100(21)j (CCDXXXExample: We know that x(t) X(jw).1.Please determine and depict the frequency spectrum at the points of A、B、C、D and y(t);2. What is the relation between y(t) and x(t)? 28)j ()j ()j (2DHXY)j (41)j (XY)(41)(txtyExample: We know th