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<TD borderColor=#6699ff><FONT
size=2>您的位置：技術(shù)園地>技術(shù)講座-GSM蜂窩移動(dòng)通信系統(tǒng) </FONT></TD></TR>
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<DIV align=center><FONT size=4><B><FONT color=#000099>第三講
GSM關(guān)鍵技術(shù)</FONT></B></FONT></DIV></TD></TR>
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<P class=MsoNormal
style="TEXT-JUSTIFY: inter-ideograph; TEXT-ALIGN: justify"><SPAN
lang=EN-US
style="FONT-FAMILY: 宋體; mso-hansi-font-family: 'Times New Roman'"><FONT
size=3><B><SPAN style="mso-spacerun: yes"> </SPAN><A
name=b1>3.6 基站與移動(dòng)臺(tái)間的時(shí)間調(diào)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=楷體_GB2312 color=#000066 size=3>由于在空中接口采用了<SPAN
lang=EN-US>TDMA技術(shù)，那么某一移動(dòng)臺(tái)必須在指配給它的時(shí)隙內(nèi)<BR>發(fā)送，而在其余時(shí)間又必須保持寂靜，否則它會(huì)干擾使用同樣載頻上不同時(shí)隙的<BR>另一些移動(dòng)客戶。</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=楷體_GB2312 color=#000066><FONT size=3> 從圖<SPAN
lang=EN-US>3－24可見，收發(fā)之間是間隔3個(gè)時(shí)隙。假如某移動(dòng)臺(tái)占用了時(shí)隙2（TS2），可它在呼叫期間向遠(yuǎn)離基站方向移動(dòng)，因此從基站發(fā)出的信息，將會(huì)越來越遲地到達(dá)移動(dòng)臺(tái)。與此同時(shí)移動(dòng)臺(tái)的應(yīng)答信息，也會(huì)越來越遲地到達(dá)基站。如果不采取措施，該時(shí)延長至使該移動(dòng)臺(tái)在TS2發(fā)送的信息與基站在TS3接受到的另一個(gè)呼叫信息重迭起來。所以，在呼叫進(jìn)行期間，必須監(jiān)視呼叫到達(dá)基站的時(shí)間，并由系統(tǒng)向移動(dòng)臺(tái)發(fā)送指令，隨著移動(dòng)臺(tái)離開基站的距離，逐步指示移動(dòng)臺(tái)提前發(fā)送的時(shí)間，這就是時(shí)間的調(diào)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: 宋體; 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: 宋體; 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: 宋體; mso-hansi-font-family: 'Times New Roman'"><FONT
face=楷體_GB2312 color=#000066 size=3>圖<SPAN lang=EN-US>3-24
TCH上下行偏移<O:P> </O:P></SPAN></FONT></SPAN><FONT face=楷體_GB2312
color=#000066><SPAN
style="FONT-FAMILY: 宋體; 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=楷體_GB2312 color=#000066><FONT size=3>時(shí)間調(diào)整的提前是<SPAN
lang=EN-US>0～63個(gè)比特之間的任意值。如0個(gè)比特就表示不必調(diào)整，表明MS和BTS在一起。63個(gè)比特是調(diào)整的最大量，也就是BTS與BS之間最長距離。所以我們說，GSM系統(tǒng)最大覆蓋范圍是：<O:P>
</O:P></SPAN></FONT></SPAN></FONT></P>
<P class=MsoNormal
style="TEXT-JUSTIFY: inter-ideograph; TEXT-ALIGN: justify"><FONT
face=楷體_GB2312 color=#000066 size=3><SPAN lang=EN-US
style="FONT-FAMILY: 宋體; 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: 宋體">m</SPAN><SPAN
lang=EN-US
style="FONT-FAMILY: 宋體; 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: 宋體">´</SPAN><SPAN
lang=EN-US
style="FONT-FAMILY: 宋體; 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: 宋體">´</SPAN><SPAN
lang=EN-US
style="FONT-FAMILY: 宋體; 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: 宋體">´</SPAN><SPAN
lang=EN-US
style="FONT-FAMILY: 宋體; 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=楷體_GB2312 color=#000066><SPAN lang=EN-US
style="FONT-FAMILY: 宋體; 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: 宋體">m</SPAN>s：每個(gè)比特的時(shí)長。<O:P>
</O:P></SPAN></FONT></FONT></P>
<P class=MsoNormal
style="TEXT-JUSTIFY: inter-ideograph; TEXT-ALIGN: justify"><FONT
face=楷體_GB2312 color=#000066><FONT size=3><SPAN
style="mso-spacerun: yes">
</SPAN>63：時(shí)間調(diào)整的最大比特?cái)?shù)。<O:P> </O:P></FONT></SPAN></FONT></P>
<P class=MsoNormal
style="TEXT-JUSTIFY: inter-ideograph; TEXT-ALIGN: justify"><FONT
face=楷體_GB2312 color=#000066 size=3><SPAN
style="mso-spacerun: yes"> </SPAN>3
</SPAN>10<SUP>8</SUP>m／s ：電波速度。<O:P> </O:P></SPAN></FONT></P>
<P class=MsoNormal
style="TEXT-JUSTIFY: inter-ideograph; TEXT-ALIGN: justify"><FONT
face=楷體_GB2312 color=#000066><FONT size=3><SPAN
style="mso-spacerun: yes">
</SPAN>其覆蓋半徑是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=楷體_GB2312 color=#000066><FONT size=3>當(dāng)一個(gè)特定連接建立時(shí)，<SPAN
lang=EN-US>BTS不斷測量自己脈沖時(shí)隙與收到的MS時(shí)隙之間的時(shí)間偏移量。基于這個(gè)測量，它可以向MS提供要求的時(shí)間提前量，并在SACCH上以每秒2次的頻度通知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: 宋體; mso-hansi-font-family: 'Times New Roman'"><FONT
size=3><A name=b2>3.7<SPAN style="mso-spacerun: yes">
</SPAN>話音編碼<O:P> </A></O:P></FONT></SPAN></B></P>
<P class=MsoNormal><SPAN lang=EN-US
style="FONT-FAMILY: 宋體; mso-hansi-font-family: 'Times New Roman'"><FONT
size=3><SPAN style="mso-spacerun: yes">
</SPAN><FONT face=楷體_GB2312
color=#000066>由于GSM系統(tǒng)是一種全數(shù)字系統(tǒng)，話音或其它信號(hào)都要進(jìn)行數(shù)字化處理，因而第一步要把話音模擬信號(hào)轉(zhuǎn)換成數(shù)字信號(hào)（即1和0的組合）。<O:P>
</O:P></FONT></FONT></SPAN></P>
<P class=MsoNormal><SPAN lang=EN-US
style="FONT-FAMILY: 宋體; mso-hansi-font-family: 'Times New Roman'"><FONT
size=3><SPAN style="mso-spacerun: yes">
</SPAN><FONT face=楷體_GB2312
color=#000066>我們對PCM編碼比較熟悉，它是采用A律波形編碼，分為3步：<O:P>
</O:P></FONT></FONT></SPAN></P>
<P class=MsoNormal><FONT face=楷體_GB2312 color=#000066><FONT
size=3><SPAN style="mso-spacerun: yes"> </SPAN>---
采樣。在某瞬間測量模擬信號(hào)的值。采樣速率8kHz/s。<O:P> </O:P></FONT></SPAN></FONT></P>
<P class=MsoNormal><FONT face=楷體_GB2312 color=#000066><FONT
size=3><SPAN style="mso-spacerun: yes"> </SPAN>---
量化。對每個(gè)樣值用8個(gè)比特的量化值來表示對應(yīng)的模擬信號(hào)瞬間值，即為樣值指配256（2<SUP>8</SUP>）個(gè)不同電平值中的一個(gè)。<O:P>
</O:P></FONT></SPAN></FONT></P>
<P class=MsoNormal style="TEXT-INDENT: 21pt"><FONT face=楷體_GB2312
color=#000066><FONT size=3>---
編碼。每個(gè)量化值用8個(gè)比特的二進(jìn)制代碼表示，組成一串具有離散特性的數(shù)字信號(hào)流。<O:P>
</O:P></FONT></SPAN></FONT><FONT face=楷體_GB2312><FONT
size=3><O:P></O:P></FONT></SPAN></FONT><FONT face=楷體_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=楷體_GB2312
color=#000066>用這種編碼方式，數(shù)字鏈路上的數(shù)字信號(hào)比特速率為<SPAN lang=EN-US>64kbit／s<SPAN
style="mso-spacerun: yes"> </SPAN>（8kbit／s
</SPAN></FONT></SPAN><FONT face=楷體_GB2312
color=#000066>8）。如果GSM系統(tǒng)也采用此種方式進(jìn)行話音編碼，那么每個(gè)話音信道是64kbit／s，8個(gè)話音信道就是512kbit／s。考慮實(shí)際可使用的帶寬，GSM規(guī)范中規(guī)定載頻間隔是200kHz。因此要把它們保持在規(guī)定的頻帶內(nèi)，必需大大地降低每個(gè)話音信道的編碼的比特率，這就要靠改變話音編碼的方式來實(shí)現(xiàn)。<O:P>
</O:P></SPAN></FONT></FONT></P>
<P class=MsoNormal
style="TEXT-JUSTIFY: inter-ideograph; TEXT-INDENT: 21pt; TEXT-ALIGN: justify"><FONT
face=楷體_GB2312 color=#000066><FONT size=3>聲碼器編碼可以是很低的速率（可以低于<SPAN
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