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<TD borderColor=#6699ff><FONT
size=2>您的位置：技术园地>技术讲座-GSM蜂窝移动通信系统 </FONT></TD></TR>
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<DIV align=center><FONT size=4><B><FONT color=#000099>第三讲
GSM关键技术</FONT></B></FONT></DIV></TD></TR>
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<P> </P>
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<P class=MsoNormal
style="TEXT-JUSTIFY: inter-ideograph; TEXT-ALIGN: justify"><SPAN
lang=EN-US
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'"><FONT
size=3><B><SPAN style="mso-spacerun: yes"> </SPAN><A
name=b1>3.6 基站与移动台间的时间调整<O:P> </A></O:P></B></FONT></SPAN></P>
<P class=MsoNormal
style="TEXT-JUSTIFY: inter-ideograph; TEXT-INDENT: 21pt; TEXT-ALIGN: justify; mso-char-indent-count: 2.0; mso-char-indent-size: 10.5pt"><FONT
face=楷体_GB2312 color=#000066 size=3>由于在空中接口采用了<SPAN
lang=EN-US>TDMA技术，那么某一移动台必须在指配给它的时隙内<BR>发送，而在其余时间又必须保持寂静，否则它会干扰使用同样载频上不同时隙的<BR>另一些移动客户。</SPAN></FONT></SPAN></P>
<P class=MsoNormal
style="TEXT-JUSTIFY: inter-ideograph; TEXT-INDENT: 21pt; TEXT-ALIGN: justify; mso-char-indent-count: 2.0; mso-char-indent-size: 10.5pt"><FONT
face=楷体_GB2312 color=#000066><FONT size=3> 从图<SPAN
lang=EN-US>3－24可见，收发之间是间隔3个时隙。假如某移动台占用了时隙2（TS2），可它在呼叫期间向远离基站方向移动，因此从基站发出的信息，将会越来越迟地到达移动台。与此同时移动台的应答信息，也会越来越迟地到达基站。如果不采取措施，该时延长至使该移动台在TS2发送的信息与基站在TS3接受到的另一个呼叫信息重迭起来。所以，在呼叫进行期间，必须监视呼叫到达基站的时间，并由系统向移动台发送指令，随着移动台离开基站的距离，逐步指示移动台提前发送的时间，这就是时间的调整。<O:P>
</O:P><O:P></O:P><O:P></O:P><O:P></O:P></SPAN></FONT></SPAN></FONT><FONT
color=#000066><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></FONT><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>
<P class=MsoNormal
style="TEXT-JUSTIFY: inter-ideograph; TEXT-INDENT: 94.5pt; TEXT-ALIGN: justify; mso-char-indent-count: 9.0; mso-char-indent-size: 10.5pt"><FONT
size=3> <IMG
height=90 src="g3-6-7-8.files/g3-6-71.gif" width=303
v:shapes="_x0000_i1025"><O:P> </O:P></FONT></SPAN></P>
<P class=MsoNormal
style="TEXT-INDENT: 21pt; TEXT-ALIGN: center; mso-char-indent-count: 2.0; mso-char-indent-size: 10.5pt"
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>3-24
TCH上下行偏移<O:P> </O:P></SPAN></FONT></SPAN><FONT 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><O:P></O:P></SPAN></FONT></SPAN></FONT></P>
<P class=MsoNormal
style="TEXT-JUSTIFY: inter-ideograph; TEXT-INDENT: 21pt; TEXT-ALIGN: justify"><FONT
face=楷体_GB2312 color=#000066><FONT size=3>时间调整的提前是<SPAN
lang=EN-US>0～63个比特之间的任意值。如0个比特就表示不必调整，表明MS和BTS在一起。63个比特是调整的最大量，也就是BTS与BS之间最长距离。所以我们说，GSM系统最大覆盖范围是：<O:P>
</O:P></SPAN></FONT></SPAN></FONT></P>
<P class=MsoNormal
style="TEXT-JUSTIFY: inter-ideograph; TEXT-ALIGN: justify"><FONT
face=楷体_GB2312 color=#000066 size=3><SPAN lang=EN-US
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'"><SPAN
style="mso-spacerun: yes">
</SPAN>3.7</SPAN><SPAN lang=EN-US
style="FONT-FAMILY: Symbol; mso-hansi-font-family: Times New Roman; mso-char-type: symbol; mso-symbol-font-family: Symbol; mso-ascii-font-family: 宋体">m</SPAN><SPAN
lang=EN-US
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'">s
</SPAN><SPAN lang=EN-US
style="FONT-FAMILY: Symbol; mso-hansi-font-family: Times New Roman; mso-char-type: symbol; mso-symbol-font-family: Symbol; mso-ascii-font-family: 宋体">´</SPAN><SPAN
lang=EN-US
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'">
63 </SPAN><SPAN lang=EN-US
style="FONT-FAMILY: Symbol; mso-hansi-font-family: Times New Roman; mso-char-type: symbol; mso-symbol-font-family: Symbol; mso-ascii-font-family: 宋体">´</SPAN><SPAN
lang=EN-US
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'"> 3
</SPAN><SPAN lang=EN-US
style="FONT-FAMILY: Symbol; mso-hansi-font-family: Times New Roman; mso-char-type: symbol; mso-symbol-font-family: Symbol; mso-ascii-font-family: 宋体">´</SPAN><SPAN
lang=EN-US
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'">
10<SUP>8</SUP>m／s = 70km<O:P> </O:P></SPAN></FONT></P>
<P class=MsoNormal
style="TEXT-JUSTIFY: inter-ideograph; TEXT-ALIGN: justify"><FONT
face=楷体_GB2312 color=#000066><SPAN lang=EN-US
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'"><SPAN
style="mso-spacerun: yes"><FONT size=3>
</FONT></SPAN><FONT size=3>3.7</FONT></SPAN><FONT size=3><SPAN
lang=EN-US
style="FONT-FAMILY: Symbol; mso-hansi-font-family: Times New Roman; mso-char-type: symbol; mso-symbol-font-family: Symbol; mso-ascii-font-family: 宋体">m</SPAN>s：每个比特的时长。<O:P>
</O:P></SPAN></FONT></FONT></P>
<P class=MsoNormal
style="TEXT-JUSTIFY: inter-ideograph; TEXT-ALIGN: justify"><FONT
face=楷体_GB2312 color=#000066><FONT size=3><SPAN
style="mso-spacerun: yes">
</SPAN>63：时间调整的最大比特数。<O:P> </O:P></FONT></SPAN></FONT></P>
<P class=MsoNormal
style="TEXT-JUSTIFY: inter-ideograph; TEXT-ALIGN: justify"><FONT
face=楷体_GB2312 color=#000066 size=3><SPAN
style="mso-spacerun: yes"> </SPAN>3
</SPAN>10<SUP>8</SUP>m／s ：电波速度。<O:P> </O:P></SPAN></FONT></P>
<P class=MsoNormal
style="TEXT-JUSTIFY: inter-ideograph; TEXT-ALIGN: justify"><FONT
face=楷体_GB2312 color=#000066><FONT size=3><SPAN
style="mso-spacerun: yes">
</SPAN>其覆盖半径是35km。<O:P> </O:P></FONT></SPAN></FONT></P>
<P class=MsoNormal
style="TEXT-JUSTIFY: inter-ideograph; TEXT-INDENT: 26.25pt; TEXT-ALIGN: justify"><FONT
face=楷体_GB2312 color=#000066><FONT size=3>当一个特定连接建立时，<SPAN
lang=EN-US>BTS不断测量自己脉冲时隙与收到的MS时隙之间的时间偏移量。基于这个测量，它可以向MS提供要求的时间提前量，并在SACCH上以每秒2次的频度通知MS。<O:P>
</SPAN></FONT></SPAN></FONT><FONT size=3><SPAN
lang=EN-US></SPAN></FONT></SPAN><FONT
size=3><O:P></O:P></FONT></SPAN></P>
<P class=MsoNormal
style="TEXT-JUSTIFY: inter-ideograph; TEXT-INDENT: 26.25pt; TEXT-ALIGN: justify"><FONT
size=3><SPAN lang=EN-US></SPAN></FONT></SPAN><FONT
size=3><O:P></O:P></FONT></SPAN></P>
<P class=MsoNormal
style="TEXT-INDENT: 21.15pt; mso-char-indent-count: 2.0; mso-char-indent-size: 10.55pt"><B><SPAN
lang=EN-US
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'"><FONT
size=3><A name=b2>3.7<SPAN style="mso-spacerun: yes">
</SPAN>话音编码<O:P> </A></O:P></FONT></SPAN></B></P>
<P class=MsoNormal><SPAN lang=EN-US
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'"><FONT
size=3><SPAN style="mso-spacerun: yes">
</SPAN><FONT face=楷体_GB2312
color=#000066>由于GSM系统是一种全数字系统，话音或其它信号都要进行数字化处理，因而第一步要把话音模拟信号转换成数字信号（即1和0的组合）。<O:P>
</O:P></FONT></FONT></SPAN></P>
<P class=MsoNormal><SPAN lang=EN-US
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'"><FONT
size=3><SPAN style="mso-spacerun: yes">
</SPAN><FONT face=楷体_GB2312
color=#000066>我们对PCM编码比较熟悉，它是采用A律波形编码，分为3步：<O:P>
</O:P></FONT></FONT></SPAN></P>
<P class=MsoNormal><FONT face=楷体_GB2312 color=#000066><FONT
size=3><SPAN style="mso-spacerun: yes"> </SPAN>---
采样。在某瞬间测量模拟信号的值。采样速率8kHz/s。<O:P> </O:P></FONT></SPAN></FONT></P>
<P class=MsoNormal><FONT face=楷体_GB2312 color=#000066><FONT
size=3><SPAN style="mso-spacerun: yes"> </SPAN>---
量化。对每个样值用8个比特的量化值来表示对应的模拟信号瞬间值，即为样值指配256（2<SUP>8</SUP>）个不同电平值中的一个。<O:P>
</O:P></FONT></SPAN></FONT></P>
<P class=MsoNormal style="TEXT-INDENT: 21pt"><FONT face=楷体_GB2312
color=#000066><FONT size=3>---
编码。每个量化值用8个比特的二进制代码表示，组成一串具有离散特性的数字信号流。<O:P>
</O:P></FONT></SPAN></FONT><FONT face=楷体_GB2312><FONT
size=3><O:P></O:P></FONT></SPAN></FONT><FONT face=楷体_GB2312><FONT
size=3><O:P></O:P></FONT></SPAN></FONT><FONT
size=3><O:P></O:P></FONT></SPAN></P>
<P class=MsoNormal
style="TEXT-JUSTIFY: inter-ideograph; TEXT-INDENT: 21pt; TEXT-ALIGN: justify"><FONT
size=3><FONT face=楷体_GB2312
color=#000066>用这种编码方式，数字链路上的数字信号比特速率为<SPAN lang=EN-US>64kbit／s<SPAN
style="mso-spacerun: yes"> </SPAN>（8kbit／s
</SPAN></FONT></SPAN><FONT face=楷体_GB2312
color=#000066>8）。如果GSM系统也采用此种方式进行话音编码，那么每个话音信道是64kbit／s，8个话音信道就是512kbit／s。考虑实际可使用的带宽，GSM规范中规定载频间隔是200kHz。因此要把它们保持在规定的频带内，必需大大地降低每个话音信道的编码的比特率，这就要靠改变话音编码的方式来实现。<O:P>
</O:P></SPAN></FONT></FONT></P>
<P class=MsoNormal
style="TEXT-JUSTIFY: inter-ideograph; TEXT-INDENT: 21pt; TEXT-ALIGN: justify"><FONT
face=楷体_GB2312 color=#000066><FONT size=3>声码器编码可以是很低的速率（可以低于<SPAN
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