通信系统原理chapter4

1、通信系统统原理四川大学电气信息学院1模拟信号数字化传输模拟信号编码传输模拟信号编码传输脉冲编码调制（PCM）主要由采样（抽样）、量化、编码三个步骤组成。2若从调制有观点看，PCM的编码过程：编码后的传输方式：直接的数字基带传输；对载波调制后的数字调制传输。3PCM系统组成：4采样与脉冲模拟调制一、采样目的：将时间上连续的模拟信号变为时间上离散的采样信号。问题：在接收端，能否根据这些离散脉冲序列无失真地恢复出原连续信号？答案：Nyquist采样定理（一）低通信号的采样频带受限的低通信号的Nyquist采样定理：5再设：采样所用的脉冲信号是单位冲激序列：则，采样后的信号可以表示为：结论：为了无失真

2、地恢复原模拟信号，要求采样频率必须等于或高于奈氏频率6（二）带通信号的采样分析：7故，采样后的信号频谱如下：8WWHHS3)5 . 1 (2212其采样后的频谱如下图所示：91011二、模拟信号脉冲调制利用正弦载波，可以有正弦波调幅（AM）、调频（FM）、调相（PM）。类似地， 利用脉冲序列作为载波，可以有：脉幅调制（PAM）、脉宽调制（PDM或PWM）、脉位调制（PPM）12量化与编码一、量化与编码的概念（一）量化的概念必须对采样值作进一步处理：量化电平之间的间隔称为量化间隔（量化阶距）：kkqqxxxxkk11量化间隔相等时，称为均匀量化；不相等时，称为非均匀量化。13（二）编码概述在实际

3、的数字系统中，PCM广泛使用的二进制码有三种。自然二进制码（NBC）折叠二进制码（FBC）格雷二进制码（RBC）14三种编码各有优缺点：（1）自然二进制码优点：编码操作简便，译码器结构简单。缺点：若较高位编码在传送中发生误码，将产生较大的误差。（2）格雷码优点：误码成相邻码字的概率最大，因而误码噪声较小。缺点：译码时不能逐位独立进行，需要转换为自然二进制码后再译码。（3）折叠码优点：1. 编码电路简单；2. 如果在传输过程中出现误码，对小信号影响较小。缺点：大信号时误码对折叠码影响较大。语音信号出现小信号的概率大。15二、线性（均匀）量化与量化信噪比（一）量化特性量化特性曲线是阶梯形曲线，根据

4、有无零量化电平，又可分别为：上图所示为均匀中升量化特性曲线。可以看出：16（二）量化噪声与量化信噪比对于均匀量化：maxmax)()()(2222xxqqqdxxPxxxxEeE17由于有M个量化间隔，因此，可以把积分区域分割成M个子区域：1)()(212iixxiqMiqdxxPxx又因为：121222212dePeiMiq18用量化信噪比衡量量化器的质量，定义：当量化级数M很大时，量化器输出信号曲线与输入信号曲线很接近：1920minmaxlg20 xRdB电话信号动态范围很大，一般在：40dB50dB而对电话信号的信噪比要求大于25dB，若采用均匀量化，编码位数将不低于11位。编码位数愈

5、多，信息速率就愈高，要求传输带宽愈宽。212223可见：语音信号取小信号的概率大。而由于均匀量化时：解决办法：非均匀量化就具有这样的量化特性。24数字语音PCM（非均匀量化）量化间隔不相等的量化称为非均匀量化非均匀量化。量化阶距随输入信号的大小而变化：非均匀量化的实质：25在发送端采用压缩器：对输出进行均匀量化，相当于对输入进行非均匀量化。接收端采用扩张器：对于语音信号，CCITT建议采用两种压缩特性：我国使用A律26PCM系统的噪声性能影响PCM系统性能的主要噪声：因此系统的输出信噪比输出信噪比为：qeqoeqoPCMooNNNSNNSNS127keqoPCMooPNSNS2241/28差分

6、脉码调制（DPCM）一、DPCM的基本原理DPCM与PCM的区别：PCM是对信号采样值进行量化、编码后传输。DPCM是对信号采样值与信号预测值的差值进行量化、编码后传输。DPCM系统原理图：29DPCM优点：30DPCM缺点：较易受到传输线路噪声的干扰。数学模型分析：因此，可定义DPCM的量化信噪比为：31因此，如果比特率相同，则DPCM比PCM的量化信噪比高：qDPCMSNRSNR可见：DPCM的关键在预测预测！预测越好，信号与预测的差值就越小，数据压缩的效果就越好。32语音信号可以采用线性预测法：结论：DPCM的抗噪声性能优于PCM，优越的程度决定于预测增益的大小，预测增益越大，抗抗噪声性

7、能越好。33增量调制（DM或 M）一、增量调制的基本特点是脉冲编码调制的一种特例。是继PCM后出现的又一种语音信号的编码方法。与PCM相比，DM编译码简单，抗误码性能好。34二、过载特性为避免过载，应满足条件：sTdttdxmax)(35三、关于DM系统的量化噪声DM系统的量化信噪比很低，为了使DM有一定的质量，需抽样间隔TS很小，即要求：msff2经分析可知，在单一音信号不过载的情况下，其输出量化信噪比为：3283msqoffNS36自适应差分脉码调制（ADPCM）有自适应系统的DPCM称为自适应差分脉码调制。实际使用的ADPCM系统为32Kbit/s37时分复用（TDM）一.数字信号在传输

8、过程中一般都采用时分复用（TDM）方式TDM：将传输时间划为若干个互不重叠的时隙，相互独立的多路信号顺序地占用各自的时隙，合成一个复用信号，在同一信道中传输。这样形成的TDM信号称为帧。38二.TDM与FDM的区别：TDM：各路信号在时域上分开，在频域上混叠。FDM：各路信号在频域上分开，在时域上混叠。TDM信号的形成和分离都可通过数字电路实现，比FDM信号使用调制器和滤波器要简单。三.数字复接系列采用TDM的PCM数字电话系统，称为数字复接系列。系列形成原则：先把一定路数的数字电话信号复合成一个标准的数据流，称为基群。然后再用数字复接技术将基群复合成更高速的数据信号。在数字复接技术中，按传输

9、速率的不同，分别称为基群、二次群、三次群、四次群等。每一种群路既可以传送数字电话信号，也可以用来传送其它相同速率的数字信号，如电视信号、数据信号、FDM信号的群路编码信号等。39四.准同步数字系列（PDH）现有的四次群以下数字复接系列称为：准同步数字系列（PDH, plesiochronous digital hierachy）。40五.同步数字系列（SDH, synchronous digital hierarchy）SDH是随着光纤通信的发展，为满足大容量高速传输的要求，在美国提出的同步光纤网（SONET）建议的基础上，由CCITT的建议形成：41424344454647Pictures

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