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<IMG height=105 src="g3-6-7-8.files/g3-6-73.gif" width=384
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align=center><FONT face=楷体_GB2312 color=#000066><SPAN lang=EN-US
style="FONT-FAMILY: 宋体; mso-hansi-font-family: Times New Roman"><FONT
size=3> <O:P> </FONT></SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: Times New Roman"><FONT
size=3>图<SPAN lang=EN-US>3-26<SPAN style="mso-spacerun: yes">
</SPAN>数字信息传输方框图<O:P>
</O:P><O:P></O:P></SPAN></FONT></SPAN></FONT></P>
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</SPAN>移动通信的传输信道属变参信道，它不仅会引起随机错误，而更主要的是造成突发错误。随机错误的特点是码元间的错误互相独立，即每个码元的错误概率与它前后码元的错误与否是无关的。突发错误则不然，一个码元的错误往往影响前后码元的错误概率。或者说，一个码元产生错误，则后面几个码元都可能发生错误。因此，在数字通信中，要利用信道编码对整个通信系统进行差错控制。差错控制编码可以分为分组编码和卷积编码两类。<O:P>
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</SPAN>分组编码的原理框图见图3-27。分组编码是把信息序列以k个码元分组，通过编码器将每组的k元信息按一定规律产生r个多余码元(称为检验元或监督元)，输出长n＝k十r的一个码组。因此，每个码组的r个检验元仅与本组的信息元有关而与别组无关。分组码用(n，k)表示，n表示码长，k表示信息位数目，R＝k／n称为分组编码的效率，也称编码率或码率。<O:P>
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face=楷体_GB2312 color=#000066><SPAN lang=EN-US
style="FONT-FAMILY: 宋体; mso-hansi-font-family: Times New Roman"><FONT
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<IMG height=81 src="g3-6-7-8.files/g3-6-74.gif" width=356
v:shapes="_x0000_i1028"><O:P> </O:P></FONT></SPAN></FONT></P>
<P class=MsoNormal style="TEXT-ALIGN: center" align=center><FONT
face=楷体_GB2312 color=#000066><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: Times New Roman"><FONT
size=3>图<SPAN lang=EN-US>3-27<SPAN style="mso-spacerun: yes">
</SPAN>分组编码<O:P> </O:P><O:P></O:P></SPAN></FONT></SPAN></FONT></P>
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</SPAN>卷积编码的原理框图见图3-28。卷积编码就是将信息序列以ko个码元分段，通过编码器输出长为no的一段码段。但是该码的no
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ko个检验码不仅与本段的信息元有关，而且也与其前m段的信息元有关，故卷积码用(no，ko，m)表示，称No＝(2n十1)no为卷积编码的编码约束长度。与分组编码一样，卷积编码的编码效率也定义为R＝ko／no，对于具有良好纠、检错性能并能合理而又简单实现的大多数卷积码，总是ko＝l或是(no
- ko)＝l，也就是说它的编码效率通常只有l／5，1／4，1／3，1／2，2／3，3／4，4／5……。<O:P>
</O:P></FONT></SPAN></FONT><FONT face=楷体_GB2312><SPAN lang=EN-US
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size=3><O:P></O:P></FONT></SPAN></FONT><FONT face=楷体_GB2312><SPAN
lang=EN-US
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size=3><O:P></O:P></FONT></SPAN></FONT><SPAN lang=EN-US
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<IMG height=81 src="g3-6-7-8.files/g3-6-75.gif" width=337
v:shapes="_x0000_i1029"><O:P> <O:P> </O:P></FONT></SPAN></P>
<P class=MsoNormal style="TEXT-ALIGN: center" align=center><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: Times New Roman"><FONT
face=楷体_GB2312 color=#000066 size=3>图<SPAN lang=EN-US>3-28<SPAN
style="mso-spacerun: yes"> </SPAN>卷积编码<O:P>
</O:P></SPAN></FONT><FONT face=楷体_GB2312 size=3><SPAN
lang=EN-US><O:P></O:P></SPAN></FONT><FONT face=楷体_GB2312
size=3><SPAN lang=EN-US><O:P></O:P></SPAN></FONT><FONT size=3><SPAN
lang=EN-US><O:P></O:P></SPAN></FONT></SPAN></P>
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color=#000066>在GSM系统中，上述两种编码方法均在使用。首先对一些信息比特进行分组编码，构成一个</FONT></SPAN><FONT
face=楷体_GB2312 color=#000066><SPAN lang=EN-US
style="mso-ascii-font-family: 宋体">“</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: Times New Roman">信息分组十奇偶<SPAN
lang=EN-US>(检验)比特</SPAN></SPAN><SPAN lang=EN-US
style="mso-ascii-font-family: 宋体">”</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: Times New Roman">的形式，然后对全部比特做卷积编码，从而形成<SPAN
class=name>编码比特。这两次编码适用于话音和数据二者，但它们的编码方案略有差异。采用</SPAN></SPAN><SPAN
class=name lang=EN-US
style="mso-ascii-font-family: 宋体">“</SPAN><SPAN class=name
style="FONT-FAMILY: 宋体; mso-hansi-font-family: Times New Roman">两次</SPAN><SPAN
class=name lang=EN-US
style="mso-ascii-font-family: 宋体">”</SPAN><SPAN class=name
style="FONT-FAMILY: 宋体; mso-hansi-font-family: Times New Roman">编码的好处是：在有差错时，能校正的校正<SPAN
lang=EN-US>(利用卷积编码特性)，能检测的检测(利用分组编码特性)。</SPAN></SPAN></FONT><SPAN
class=name
style="FONT-FAMILY: 宋体; mso-hansi-font-family: Times New Roman"><SPAN
lang=EN-US><O:P> </O:P><O:P></O:P></SPAN></SPAN></FONT></P>
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</SPAN><FONT face=楷体_GB2312
color=#000066>GSM系统首先是把话音分成20ms的音段,这20ms的音段通过话音编码器被数字化和话音编码，产生260个比特流，并被分成：<O:P>
</O:P></FONT></FONT></SPAN></P>
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</SPAN><FONT face=楷体_GB2312 color=#000066>·50个最重要比特<O:P>
</O:P></FONT></FONT></SPAN></P>
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face=楷体_GB2312 color=#000066><SPAN
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size=3>·</FONT><SPAN class=name><FONT class=name face=楷体_GB2312
color=#000066><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: Times New Roman"><FONT
size=3><SPAN lang=EN-US>132个重要比特</SPAN></FONT></SPAN></FONT><FONT
size=3><SPAN lang=EN-US><O:P>
</O:P></SPAN></FONT></SPAN></SPAN></FONT></P>
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size=3>·<SPAN lang=EN-US><SPAN class=name>78个不重要比特<O:P>
</O:P></SPAN></SPAN></FONT></SPAN></FONT></P>
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style="FONT-FAMILY: 宋体; mso-hansi-font-family: Times New Roman"><FONT
face=楷体_GB2312 color=#000066 size=3>如图<SPAN
lang=EN-US>3-29，对上述50个比特添加上3个奇偶检验比特(分组编码)，这53个比特同132个重要比特与4个尾比特一起卷积编码，比率1：2，因而得378个比特，另外78个比特不予保护。<O:P></O:P><O:P></O:P></SPAN></FONT><FONT
color=#000066 size=3><SPAN lang=EN-US><O:P></O:P></SPAN></FONT><FONT
size=3><SPAN lang=EN-US><O:P> </O:P></SPAN></FONT></SPAN></P>
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lang=EN-US
style="FONT-FAMILY: 宋体; mso-hansi-font-family: Times New Roman"><FONT
size=3> <O:P>
<IMG height=96 src="g3-6-7-8.files/g3-6-76.gif" width=353
v:shapes="_x0000_i1030"><O:P> </O:P></FONT></SPAN></P>
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align=center><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: Times New Roman"><FONT
face=楷体_GB2312 color=#000066 size=3>图<SPAN lang=EN-US>3-29<SPAN
style="mso-spacerun: yes"> </SPAN>GSM数字话音的信道编码
</SPAN></FONT></SPAN></P>
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