通信技术基础附加.ppt

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1、1,Chapter 5 Signal Encoding Techniques,SE3221113 Lecture 21-22 Analog Data,Analog Signal,1 Signal Encoding Techniques2 Analog Data,Analog SignalAmplitude Modulation Angle Modulation3 Interpreting Signals4 Encoding Schemes,2,1 Analog Data,Analog Signals,modulate carrier frequency with analog datawhy

2、modulate analog signals?higher frequency can give more efficient transmissionpermits frequency division multiplexing(chapter 8)types of modulationAmplitudeFrequencyPhase,3,2 Analog ModulationTechniques,Amplitude ModulationFrequency ModulationPhase Modulation,4,Analog data,Analog Signals,调制过程:输入信号m(t

3、)与载波频率fc相结合,可以产生一带宽中心位于fc的信号s(t)。对数字数据,当只有模拟传输设备可用时,需要将数字数据调制转换成模拟形式。当数据已经是模拟形式时,调制的两个基本原因:一个高效的传输过程将需要较高的频率。对非波导传输,看起来不太可能传送基带信号,因为它所需要的天线半径将达许多公里。通过调制可以利用频分多路技术。,5,2.1 调幅,Amplitude Modulation调幅是最简单的调制形式。数学上该过程可以表达为:s(t)=1+na x(t)cos 2 fc t 这里cos 2 fc t 是载波信号,x(t)是输入信号(载有数据),二者归一化到单位幅值。参数 na 称为调制系数

4、,是输入信号与载波信号幅值的比值。输入信号为m(t)=na x(t),上式中的1表示直流成份,它可以防止丢失信息。该方案也被称作双边带传输载波(DSBTC),6,调幅,已调制的AM信号,调制的正弦波,7,AM 带宽,8,调幅,例:若x(t)是幅值调制信号cos 2 fm t,导出s(t)的表达式。有:s(t)=1+na cos 2 fm tcos 2 fc t利用三角衡等式,上式展开成产生的信号是原载波频率成份加上距其两边fm Hz处一对频率成份。AM是输入信号与载波信号之积。输出信号的包略是1+na x(t),只要 na 1,包略线将越过时间轴,也将丢失信息。,9,AM信号,AM信号的频谱由

5、原载波频谱加上转移至fc的输入信号。|f|fc|的部分叫高边带upper sideband,|f|fc|的部分叫低边带lower sideband。高边带和低边带都是原频带m(f)的复制,低边带是原频带的反转。例:考虑一带宽范围在300到3000Hz、用60 kHz载波调制的语音信号,产生信号的高边带是60.3至63kHz,低边带是57至59.7kHz,载波是60kHz。,10,AM信号,这里Pt是s(t)传输的所有功率,Pc是载波信号传输的功率,我们希望na 尽可能大,这样大部分信号功率可用于实际传载信息。不过,na不得大于1。,在fc处的AM及载波信号频谱,调制信号频谱,11,调幅,AM:

6、DSBTC-双边带发射载波举例:发射总功率BT=2B,若无此直流成分，AM信号的波形会怎样？,AM信号成分：载波,下边带,上边带实例：模拟载波系统,na1,输入信号与载波信号的振幅比m(t)=nax(t),12,Illustrating the amplitude modulation process：na1,13,因为每个边带含有m(t)完整的频谱,显然s(t)含有冗余的成份。一种更常用的AM变种是单边带(SSB),它的优点是只传送一个边带,消除了另一个边带和载波。这种做法的基本原理是:只需一半带宽,即BT=B,这里B是原信号的带宽,对DSBTC(双边带载波),BT=2B。因为不需要能量来发

7、送载波或者其它边带,只需较少的功率。另一种类型是压缩载波双边带(DSBSC),它滤掉载波频率只送两个边带,节省了一些功率但比DSBTC使用了更多的带宽。压缩载波的缺点是载波本来可用于同步的目的。例如,假设原模拟信号是一个对数字数据编码的ASK波形,接收器需要了解每位的起始时间,以正确地解释数据。稳定的载波提供了每个bit到达时刻的同步机制。,14,压缩载波双边带(DSBSC):,15,16,A vestigial sideband(in radio communication)is a sideband that has been only partly cut off or suppress

8、ed.The video baseband signal used in TV in countries that use NTSC or ATSC has a bandwidth of 6 MHz.To conserve bandwidth,SSB would be desirable,but the video signal has significant low frequency content(average brightness)and has rectangular synchronising pulses.The compromise is vestigial sideband

9、 modulation.In vestigial sideband the full upper sideband of bandwidth W2=4 MHz is transmitted,but only W1=1.25 MHz of the lower sideband is transmitted,along with a carrier.The absence of the lower sideband components at high frequencies must be compensated for,and this is done by the RF and IF fil

10、ters.,退化边带(VSB-vestigial sideband)技术就利用了一个边带和一衰减功率的载波。,17,Filtering scheme for the generation of VSB modulated wave.,18,Magnitude response of VSB filter;only the positive-frequency portion is shown.,19,2.2 Angle Modulation角度调制,调频FM和调相PM技术都是角度调制的特殊情况。被调制的信号可以表示成:s(t)=Ac cos 2 fc t+(t)对相位调制,相位正比于将调制的信

11、号m(t):(t)=np m(t)，其中 np 是相位调制系数。对频率调制,相位的导数正比于将调制的信号m(t):(t)=nf m(t)，其中 nf 是频率调制系数。,20,载波调制正弦波信号调幅(DSBTC)波调相波调频波,cos(2 fm t),cos(2 fc t),21,例:如果(t)是相位调制信号 np cos 2 fm t,导出的表达式s(t),设Ac=1。可直接看出:s(t)=cos 2 fc t+np cos 2 fm t 与载波信号的瞬时相位偏差是 np cos 2 fm t。信号的相位角将随载波信号的未调制值简单地按正弦方式变化,相位偏差峰值等于 np。以上等式可以用Bes

12、sel三角衡等式展开:这里,Jn(np)是一类第n阶Bessel函数,22,例:仅为了方便起见,如果选取(t)是频率调制信号-nf sin 2 fm t。这样:与载波信号的瞬时相位偏差是-nf sin 2 fm t。信号的频率将随载波信号的未调制值简单地按正弦方式变化,频率偏差峰值等于 nf 弧度/秒。FM信号等式和PM信号等式有同样的形式,只不过用F/fm 代替np。,Frequency Modulation Consider a sinusoidal modulating signal defined bym(t)=Amcos(2pfmt)The instantaneous frequen

13、cy of the resulting FM signal equals fi(t)=fc+kf Amcos(2pfmt)=fc+Df cos(2pfmt)where Df=kfAm The frequency deviation Df represents the maximum departure of the instantaneous frequency of the FM signal from the carrier frequency fc.,For an FM signal,the frequency deviation Df is proportional to the am

14、plitude of the modulating signal and is independent of the modulation frequency.,The ratio of the frequency deviation Df to the modulation frequency fm,is commonly called the modulation index of the FM signal:b=Df/fm andi(t)=2pfct+bsin(2pfmt),The parameter b represents the phase deviation of the FM

15、signal,the maximum departure of the angle i(t)from the angle 2pfct of the unmodulated carrier;b is measured in radians.,The FM signal itself is given by s(t)=Accos2pfct+bsin(2pfmt)Depending on the modulation index b,we may distinguish two cases of FM:Narrowband FM,for which b is small compared to on

16、e radian.Wideband FM,for which b is large compared to one radian.,26,与AM一样,FM和PM产生的信号带宽中心位于fc上。不过,可以看到它们的带宽幅值是完全不同的幅度调制是一个线性过程，对一频率为fm的正弦信号进行调制,产生频率是将载波信号fc与调制的信号fm相加和相减。对AM:BT=2B，B是被调制信号的带宽。对FM和PM，s(t)中存在有位于fc+fm、fc+2fm等处的频率成份。一般,理论上要求有无限宽的频带传送FM或PM信号；实际上可以用Carson经验公式用于估计带宽:BT=2(+1)B。这里：再写FM的公式为:BT

17、=2F+2B所以FM和PM比AM需要更宽的带宽。,27,Angle Modulation角度调制,FM和PM信号的形状是十分相象的。的确,如果没有调制函数的知识,是不可能区分它们的。差值F的极值是:(Hz)这里Am是m(t)的最大值,这样增加m(t)的幅度将增加F,换言之将增加带宽BT。这与AM是完全不同的,AM里调制电平变化将影响AM信号的功率,但不会影响它的带宽。,28,29,Bell 调制解调器,30,31,Summary,looked at signal encoding techniques:digital data,digital signalanalog data,digital

18、 signaldigital data,analog signalanalog data,analog signal,32,You should now be able to:modulate carrier frequency with analog data,vs.FSKwhy modulate analog signals?higher frequency can give more efficient transmissiontypes of modulationAmplitudeFrequencyPhase,33,Introduction Reading:,数据与计算机通信，第5.4节。,Coursework:5.24,5.26,