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face=楷体_GB2312 color=#000066 size=3>话音编码器和信道编码器将每一<SPAN
lang=EN-US>20ms话音数字化并编码，提供456个比特。首先对它进行内部交织，即将456个比特分成8帧，每帧57比特，见图3-31所示。<O:P>
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face=楷体_GB2312 color=#000066 size=3>图<SPAN lang=EN-US>3-31<SPAN
style="mso-spacerun: yes"> </SPAN>GSM 20ms话音编码交织<O:P>
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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
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style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'"><FONT
size=3><SPAN lang=EN-US><O:P></O:P></SPAN></FONT></SPAN></P>
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face=楷体_GB2312 color=#000066 size=3>如果将同一<SPAN
lang=EN-US>20ms话音的2组57比特插入到同一普通突发脉冲序列中(见图3-32)，那么该突发脉冲串丢失则会导致该20ms的话音损失25％的比特，显然信道编码难以恢复这么多丢失的比特。因此必须在两个话音帧间再进行一次交织，即块间交织。</SPAN></FONT><FONT
size=3><SPAN lang=EN-US><O:P> </O:P></SPAN></FONT></SPAN></P>
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style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'"><FONT
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<IMG height=98 src="g3-9.files/g3-9.H9.gif" width=344
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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>图<SPAN lang=EN-US>3-32<SPAN
style="mso-spacerun: yes"> </SPAN>普通突发脉冲串</SPAN></FONT><FONT
size=3><SPAN lang=EN-US><O:P> </O:P></SPAN></FONT></SPAN></P>
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align=center><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'"><FONT
size=3><SPAN lang=EN-US><O:P></O:P></SPAN></FONT></SPAN></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>把每<SPAN
lang=EN-US>20ms话音456比特分成的8帧为一个块，假设有A、B、C、D四块，见图3-33所示，在第一个普通突发脉冲串中，两个57比特组分别插入A块和D块的各1帧(插入方式如图3-34所示，这就是二次交织)，这样一个20ms的话音8帧分别插入8个不同普通突发脉冲序列中，然后一个一个突发脉冲序列发送，发送的突发脉冲序列首尾相接处不是同一话音块，这样即使在传输中丢失一个脉冲串，只影响每一话音比特数的12.5％，而这能通过信道编码加以校正。<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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<IMG height=79 src="g3-9.files/g3-9.H10.gif" width=343
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face=楷体_GB2312 color=#000066 size=3>图<SPAN lang=EN-US>3-33<SPAN
style="mso-spacerun: yes"> </SPAN>话音信道编码<O:P>
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lang=EN-US><O:P></O:P></SPAN></FONT><FONT face=楷体_GB2312
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size=3> <O:P> </O:P></FONT></SPAN></P>
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size=3><IMG height=300
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face=楷体_GB2312 color=#000066 size=3>图<SPAN lang=EN-US>3-34 二次交织<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
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lang=EN-US
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'"><FONT
size=3> <O:P> </O:P></FONT></SPAN></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>二次交织经得住丧失一整个突发脉冲串的打击，但增加了系统时延。因此，在<SPAN
lang=EN-US>GSM系统中，移动台和中继电路上增加了回波抵消器，以改善由于时延而引起的通话回音。</SPAN></FONT></SPAN></P>
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