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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 style="TEXT-JUSTIFY: inter-ideograph; TEXT-ALIGN: justify"><SPAN
lang=EN-US
style="FONT-SIZE: 12pt; mso-bidi-font-size: 10.0pt; mso-hansi-font-family: Times New Roman"><B>3.2
时分多址技术（TDMA）<O:P> </O:P></B></SPAN></P>
<P
style="TEXT-JUSTIFY: inter-ideograph; TEXT-INDENT: 24pt; TEXT-ALIGN: justify; mso-char-indent-count: 2.0; mso-char-indent-size: 10.5pt"><SPAN
style="FONT-SIZE: 12pt; mso-bidi-font-size: 10.0pt; mso-hansi-font-family: Times New Roman"><FONT
face=楷体_GB2312
color=#000066>多址技术就是要使众多的客户公用公共通信信道所采用的一种技术。实现多<BR>址的方法基本上有三种，即采用频率、时间或码元分割的多址方式，人们通常<BR>称它们为频分多址（<SPAN
lang=EN-US>FDMA）、时分多址（TDMA）和码分多址（CDMA）。在传统的无线电广播中，均采用频分多址（FDMA<SPAN
style="mso-spacerun: yes">
</SPAN>）方式，每个广播信道都有一个频点，如果你要收听某一广播信道，则必须把你的收音机调谐到这一频点上。模拟蜂窝移动系统也采用了此技术，某一小区中的某一客户呼叫占用了一个频点，即一个信道（实际上是占用两个，因为是双向连接，即双工通信），则其</SPAN>它呼叫就不能再占用。</FONT></SPAN><FONT
face=楷体_GB2312 color=#000066><SPAN lang=EN-US
style="FONT-SIZE: 12pt; mso-bidi-font-size: 10.0pt; mso-hansi-font-family: Times New Roman"> </SPAN></FONT><SPAN
lang=EN-US
style="FONT-SIZE: 12pt; mso-bidi-font-size: 10.0pt; mso-hansi-font-family: Times New Roman"><O:P>
</O:P></SPAN></P>
<P
style="TEXT-JUSTIFY: inter-ideograph; TEXT-INDENT: 24pt; TEXT-ALIGN: justify; mso-char-indent-count: 2.0; mso-char-indent-size: 10.5pt"><SPAN
style="FONT-SIZE: 12pt; mso-bidi-font-size: 10.0pt; mso-hansi-font-family: Times New Roman"><FONT
face=楷体_GB2312 color=#000066>在<SPAN
lang=EN-US>GSM中，无线路径上是采用时分多址（TDMA）方式。每一频点（频道或叫载频TRX）上可分成8个时隙，每一时隙为一个信道，因此，一个TRX最多可有8个移动客户同时使用，见图3－6所示。<O:P>
</O:P></SPAN></FONT><FONT face=楷体_GB2312><SPAN
lang=EN-US><O:P></O:P></SPAN></FONT><FONT face=楷体_GB2312><SPAN
lang=EN-US><O:P></O:P></SPAN></FONT><SPAN
lang=EN-US><O:P></O:P></SPAN></SPAN></P>
<P
style="TEXT-JUSTIFY: inter-ideograph; TEXT-INDENT: 96pt; TEXT-ALIGN: justify; mso-char-indent-count: 8.0; mso-char-indent-size: 10.5pt"
align=left><SPAN lang=EN-US
style="FONT-SIZE: 12pt; mso-bidi-font-size: 10.0pt; mso-hansi-font-family: Times New Roman">
<IMG height=133 src="g3-2.files/g3-2.H8.gif" width=305
v:shapes="_x0000_i1025"><O:P> </O:P></SPAN></P>
<P
style="TEXT-INDENT: 24pt; TEXT-ALIGN: center; mso-char-indent-count: 2.0; mso-char-indent-size: 10.5pt"
align=center><SPAN
style="FONT-SIZE: 12pt; mso-bidi-font-size: 10.0pt; mso-hansi-font-family: Times New Roman"><FONT
face=楷体_GB2312 color=#000066>图<SPAN lang=EN-US>3－6
频分多址和时分多址方式</SPAN></FONT></SPAN><FONT face=楷体_GB2312
color=#000066><SPAN lang=EN-US
style="FONT-SIZE: 12pt; mso-bidi-font-size: 10.0pt; mso-hansi-font-family: Times New Roman"> <O:P>
</O:P></SPAN></FONT></P>
<P
style="TEXT-INDENT: 24pt; TEXT-ALIGN: center; mso-char-indent-count: 2.0; mso-char-indent-size: 10.5pt"
align=center><FONT face=楷体_GB2312 color=#000066><SPAN lang=EN-US
style="FONT-SIZE: 12pt; mso-bidi-font-size: 10.0pt; mso-hansi-font-family: Times New Roman">
<O:P></O:P></SPAN></FONT></P>
<P
style="TEXT-JUSTIFY: inter-ideograph; TEXT-INDENT: 24pt; TEXT-ALIGN: justify; mso-char-indent-count: 2.0; mso-char-indent-size: 10.5pt"><FONT
face=楷体_GB2312 color=#000066><SPAN
style="FONT-SIZE: 12pt; mso-bidi-font-size: 10.0pt; mso-hansi-font-family: Times New Roman">图中所示（<SPAN
lang=EN-US>a为FDMA，b为TDMA）是一个方向的情况，在相反方向上必定有一组对应的频率（FDMA）／时隙（TDMA）。</SPAN></SPAN><SPAN
lang=EN-US
style="FONT-SIZE: 12pt; mso-bidi-font-size: 10.0pt; mso-hansi-font-family: Times New Roman"> <O:P>
</O:P></SPAN></FONT></P>
<P
style="TEXT-JUSTIFY: inter-ideograph; TEXT-INDENT: 24pt; TEXT-ALIGN: justify; mso-char-indent-count: 2.0; mso-char-indent-size: 10.5pt"><FONT
face=楷体_GB2312 color=#000066><SPAN lang=EN-US
style="FONT-SIZE: 12pt; mso-bidi-font-size: 10.0pt; mso-hansi-font-family: Times New Roman">TDMA系统具有如下特性：<O:P>
</O:P></SPAN></FONT></P>
<P
style="TEXT-JUSTIFY: inter-ideograph; TEXT-INDENT: 24pt; TEXT-ALIGN: justify; mso-char-indent-count: 2.0; mso-char-indent-size: 10.5pt"><FONT
face=楷体_GB2312 color=#000066><SPAN lang=EN-US
style="FONT-SIZE: 12pt; mso-bidi-font-size: 10.0pt; mso-hansi-font-family: Times New Roman">(1)
每载频多路。如前所述，TDMA系统形成频率时间矩阵，在每一频率上产生多个时隙，这个矩阵中的每一点都是一个信道，在基站控制分配下，可为任意一移动客户提供电话或非话业务。 </SPAN></FONT></P>
<P
style="TEXT-JUSTIFY: inter-ideograph; TEXT-INDENT: 24pt; TEXT-ALIGN: justify; mso-char-indent-count: 2.0; mso-char-indent-size: 10.5pt"><FONT
face=楷体_GB2312 color=#000066><SPAN lang=EN-US
style="FONT-SIZE: 12pt; mso-bidi-font-size: 10.0pt; mso-hansi-font-family: Times New Roman; mso-char-type: symbol; mso-symbol-font-family: Wingdings; mso-ascii-font-family: 宋体"> (2)
</SPAN><SPAN lang=EN-US
style="FONT-SIZE: 12pt; mso-bidi-font-size: 10.0pt; mso-hansi-font-family: Times New Roman">突发脉冲序列传输。移动台信号功率的发射是不连续的，只是在规定的时隙内发射脉冲序列。 </SPAN></FONT></P>
<P
style="TEXT-JUSTIFY: inter-ideograph; TEXT-INDENT: 24pt; TEXT-ALIGN: justify; mso-char-indent-count: 2.0; mso-char-indent-size: 10.5pt"><FONT
face=楷体_GB2312 color=#000066><SPAN lang=EN-US
style="FONT-SIZE: 12pt; mso-bidi-font-size: 10.0pt; mso-hansi-font-family: Times New Roman; mso-char-type: symbol; mso-symbol-font-family: Wingdings; mso-ascii-font-family: 宋体"> (3)
</SPAN><SPAN lang=EN-US
style="FONT-SIZE: 12pt; mso-bidi-font-size: 10.0pt; mso-hansi-font-family: Times New Roman">传输速率高，自适应均衡。每载频含有时隙多，则频率间隔宽，传输速率高，但数字传输带来了时间色散，使时延扩展量加大，则务必采用自适应均衡技术。 </SPAN></FONT></P>
<P
style="TEXT-JUSTIFY: inter-ideograph; TEXT-INDENT: 24pt; TEXT-ALIGN: justify; mso-char-indent-count: 2.0; mso-char-indent-size: 10.5pt"><FONT
face=楷体_GB2312 color=#000066><SPAN lang=EN-US
style="FONT-SIZE: 12pt; mso-bidi-font-size: 10.0pt; mso-hansi-font-family: Times New Roman; mso-char-type: symbol; mso-symbol-font-family: Wingdings; mso-ascii-font-family: 宋体"> (4)
</SPAN><SPAN lang=EN-US
style="FONT-SIZE: 12pt; mso-bidi-font-size: 10.0pt; mso-hansi-font-family: Times New Roman">传输开销大。由于TDMA分成时隙传输，使得收信机在每一突发脉冲序列上都得重新获得同步。为了把一个时隙和另一个时隙分开，保护时间也是必须的。因此，TDMA系统通常比FDMA系统需要更多的开销。 <O:P>
</O:P></SPAN></FONT></P>
<P
style="TEXT-JUSTIFY: inter-ideograph; TEXT-INDENT: 21pt; TEXT-ALIGN: justify"><FONT
face=楷体_GB2312 color=#000066>(5) <SPAN lang=EN-US
style="FONT-SIZE: 12pt; mso-bidi-font-size: 10.0pt; mso-hansi-font-family: Times New Roman">对于新技术是开放的。例如当话音编码算法的改进而降低比特速率时，TDMA系统的信道很容易重新配置以接纳新技术。</SPAN></FONT></P>
<P
style="TEXT-JUSTIFY: inter-ideograph; TEXT-INDENT: 21pt; TEXT-ALIGN: justify"><FONT
face=楷体_GB2312 color=#000066><SPAN lang=EN-US
style="FONT-SIZE: 12pt; mso-bidi-font-size: 10.0pt; mso-hansi-font-family: Times New Roman; mso-char-type: symbol; mso-symbol-font-family: Wingdings; mso-ascii-font-family: 宋体">(6)</SPAN><SPAN
lang=EN-US
style="FONT-SIZE: 12pt; mso-bidi-font-size: 10.0pt; mso-hansi-font-family: Times New Roman">
共享设备的成本低。由于每一载频为许多客户提供业务，所以TDMA系统共享设备的每客户平均成本与FDMA系统相比是大大降低了。</SPAN></FONT></P>
<P
style="TEXT-JUSTIFY: inter-ideograph; TEXT-INDENT: 21pt; TEXT-ALIGN: justify"><FONT
face=楷体_GB2312 color=#000066><SPAN lang=EN-US
style="FONT-SIZE: 12pt; mso-bidi-font-size: 10.0pt; mso-hansi-font-family: Times New Roman; mso-char-type: symbol; mso-symbol-font-family: Wingdings; mso-ascii-font-family: 宋体">(7)</SPAN><SPAN
lang=EN-US
style="FONT-SIZE: 12pt; mso-bidi-font-size: 10.0pt; mso-hansi-font-family: Times New Roman"><SPAN
style="mso-char-type: symbol; mso-symbol-font-family: Wingdings; mso-ascii-font-family: 宋体">
</SPAN>移动台较复杂。它比FDMA系统移动台完成更多的功能，需要复杂的数字信号处理。</SPAN></FONT></P>
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