信号与系统资料：ch8 communication systems

8.0IntroductionBasicframeofacommunicationsystemsModulation:

x(t)g

y(t)Basicideasaboutthecommunicationofdiscrete-timesignalsDemodulation:

y(t)g

x(t)8.1.1SinusoidalAM(SAM)AM:(DSBAM)8.2DemodulationforSinusoidalAM8.2.1Synchronousdemodulation

Generalmodulation&synchronousdemodulation

r(t)isnotsynchronizedinphasewithc(t).ComplexexponentialAMIfx(t)ispositive&real,sinusoidalAM例2(习题8.34):下图所示的系统中利用非线性单元

实现了对信号的调制。假设x(t)带限，并且：试确定带通滤波器的参数A，wl和w

h，使得：解：因此，A=1/2，Modulationt=0:0.0001:1;ca=[cos(100*pi*t)].*[cos(2*pi*t)+1.5];plot(t,ca)t=0:0.0001:1;ca=cos(2*pi*t);plot(t,ca)t=0:0.0001:1;ca=[cos(100*pi*t)].*cos(2*pi*t);plot(t,ca)8.2.2AsynchronousdemodulationModulationK/A

=mDemodulationChargesdecrease1/etimes:

t=RCSpectrumofy(t):8.3Frequency-DivisionMultiplexingSpectraDemultiplexing射频频谱的频率分配频率范围用途传播方式信道特点30～300Hz(极低频)长波、海底通信兆米波穿透导电大地与海水0.3～3kHz(音频）数据终端、电话铜导线3～30kHz(甚低频）导航、电话……地表面波低损耗、小衰落……30～300kHz(低频）工业通信、导航…大多为地表面波，微衰落、高大气噪声1MHz调幅广播103~107GHz光通信光纤、大气低损耗(红外线、可见光、紫外线)（通讯频率范围不同，复用与解复用技术不同）用于光通信的波分复用与解复用器

——阵列波导光栅(AWG,arrayedwaveguidegrating)相邻阵列波导长度差几十微米FDM~WDM(DWDM)在ISI网站检索AWG，按年份数显示2005(282)1998(68)1969(6)1978(3)2003(265)1997(50)1984(6)1979(3)2004(258)1996(38)1990(6)1980(3)2006(253)1995(18)1975(5)1983(3)2008(243)1971(9)1989(5)1970(2)2007(240)1994(9)1992(5)1973(2)2002(222)1974(8)1993(5)1977(2)2001(150)1985(8)1972(4)1981(2)2009(130)1987(7)1976(4)1982(2)2000(121)1988(7)1991(4)1986(2)1999(82)8.4Single-sidebandsinusoidalAM—single-sidebandmodulation(SSB)retainingtheuppersideband.totalbandwidthofy(t)is22wMretainingthelowersideband.SystemstoimplementSSBgUsing90°phase-shiftnetwork:retaininglowersideband:retaininguppersideband:8.5AMWithAPulse-trainCarrier8.5.1Modulationofapulse-traincarrier

8.5.2Time-divisionmultiplexingconditiontorecoveringx(t)例3：脉冲串载波的幅度调制可按下图建模，其输出是q(t)。习题(8.30)(a)设x(t)是带限信号，如图所示。确定并画出R(j

w)和Q(j

w)。解：(a)(b)求最大的D值，使得通过滤波器后有y(t)=x(t)。(b)H(jw)的主瓣:wD/2=p,w=2p/Dp/T,D

2T解：(c)确定并画出使y(t)=x(t)的补偿滤波器M(j

w)。8.6Pulse-AmplitudeModulation(PAM)8.6.1PAmodulatedsignals8.6.2Intersymbolinterference(码间干扰)inPAMsystemsintersymbolinterferencephenomenonSmearingcanresultfrom:Bandwidthconstraintsofthechannel;Phasedispersioncausedbynon-constantgroupdelay;Avoidsmearing8.6.3Digitalpulse-amplitude

&pulse-codeModulationDigitalsystems(DS):SignalsinPAM

gsignalsinDSSignalsinDS

g

abinarynumber8.7SinusoidalFrequencyModulation(SFM)Phasemodulation(PM)Frequencymodulation(FM)relationshipbetweenthePM&FM

InstantaneousfrequencyFrequencyPhaseFMPM系统增益因子8.7.1NarrowbandfrequencymodulationDefinitionSimplifyofthey(t)inNFMSpectrumofy(t)

带宽同,短距离,抗干扰其中cos(msin(wt))具有周期性x=0:0.0001:0.2;b=sin(20*pi*x);figure(1),plot(x,b)y=cos(5*b);figure(2),plot(x,y)一般的频率调制信号：是具有基波频率的周期信号8.7.2Widebandfrequencymodulation

都是具有基波频率为

wm的周期信号TheirFTgimpulsetrain——kwm

谱线幅度~傅立叶级数系数gBessel函数谱线~±wc

+n

wm,n=0,±1,±2,…总带宽B

2mwm=2Dw

2kfA>>wm8.7.3Periodicsquare-wavemodulatingsignal已调信号的频谱:B=2Dw

=2ADemodulation:TrackingthephaseorfrequencyofthemodulatedsignalAMsignalsSummary:ModulatedsignalBandwidthDemodulationCharacte