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<TD borderColor=#6699ff><FONT size=2>您的位置：技術園地>技術講座-擴頻通信</FONT></TD></TR>
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<DIV align=center><FONT size=4><B><SPAN
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color=#000080 size=5>第五講
直接序列擴頻系統(tǒng)(2)</FONT></SPAN></B></FONT></DIV></TD></TR>
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<P class=MsoNormal
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color=#800080><B><FONT size=4><SPAN lang=EN-US>5.2<SPAN
style="mso-spacerun: yes"> </SPAN></SPAN></FONT><SPAN
lang=EN-US><FONT size=4></FONT></SPAN><FONT size=4><SPAN
lang=EN-US>擴頻碼序列</SPAN></FONT></B></FONT></P>
<P class=MsoNormal
style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; LINE-HEIGHT: 150%"><SPAN
lang=EN-US style="FONT-SIZE: 12pt; COLOR: navy">5.2.1<SPAN
style="mso-spacerun: yes"> </SPAN></SPAN><SPAN
style="FONT-SIZE: 12pt; COLOR: navy; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">碼序列的相關性</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt; COLOR: navy"><O:P>
</O:P></SPAN></P>
<P class=MsoNormal
style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; TEXT-INDENT: 24pt; LINE-HEIGHT: 150%"><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">在擴展頻譜通信中需要用高碼率的窄脈沖序列。這是指擴頻碼序列的波形而言。并未涉及碼的結(jié)構和如何產(chǎn)生等問題。</SPAN></P>
<P class=MsoNormal
style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; TEXT-INDENT: 24pt; LINE-HEIGHT: 150%"><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">那么究竟選用什么樣的碼序列作為擴頻碼序列呢</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">? </SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">它應該具備哪些基本性能呢</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">? </SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">現(xiàn)在實際上用得最多的是偽隨機碼，或稱為偽噪聲</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">(PN)</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">碼。</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt"><O:P> </O:P></SPAN></P>
<P class=MsoNormal
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style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">這類碼序列最重要的特性是具有近似于隨機信號的性能。因為噪聲具有完全的隨機性，也可以說具有近似于噪聲的性能。但是，真正的隨機信號和噪聲是不能重復再現(xiàn)和產(chǎn)生的。我們只能產(chǎn)生一種周期性的脈沖信號來近似隨機噪聲的性能，故稱為偽隨機碼或</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">PN</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">碼。</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt"><O:P> </O:P></SPAN></P>
<P class=MsoNormal
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style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">為什么要選用隨機信號或噪聲性能的信號來傳輸信息呢？許多理論研究表明，在信息傳輸中各種信號之間的差別性能越大越好。這樣任意兩個信號不容易混淆，也就是說，相互之間不易發(fā)生干擾，不會發(fā)生誤判。理想的傳輸信息的信</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">號形式應是類似噪聲的隨機信號，因為取任何時間上不同的兩段噪聲來比較都不會完全相似。用它們代表兩種信號，其差別性就最大。</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt"><O:P> </O:P></SPAN></P>
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style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">在數(shù)學上是用自相關函數(shù)來表示信號與它自身相移以后的相似性的。隨機信號的自相關函數(shù)的定義為下列積分：</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt"><O:P> </O:P></SPAN></P>
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align=center><SPAN lang=EN-US style="FONT-SIZE: 12pt"> <IMG
height=149 src="s05-02.files/ss5-3.gif" width=450
border=0> </SPAN></P>
<P class=MsoNormal
style="TEXT-INDENT: 24pt; LINE-HEIGHT: 150%; mso-char-indent-count: 2.0; mso-char-indent-size: 12.0pt"
align=center><SPAN lang=EN-US
style="FONT-SIZE: 12pt"><O:P></O:P></SPAN></P>
<P class=MsoNormal
style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; TEXT-INDENT: 24pt; LINE-HEIGHT: 150%; mso-char-indent-count: 2.0; mso-char-indent-size: 12.0pt"><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">式中</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt"> f(t)</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">為信號的時間函數(shù)，</SPAN><SPAN
lang=EN-US
style="FONT-SIZE: 12pt; FONT-FAMILY: Symbol; mso-ascii-font-family: Times New Roman; mso-hansi-font-family: Times New Roman; mso-char-type: symbol; mso-symbol-font-family: Symbol">t</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">為時間延遲。</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt"><O:P> </O:P></SPAN></P>
<P class=MsoNormal
style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; TEXT-INDENT: 24pt; LINE-HEIGHT: 150%; mso-char-indent-count: 2.0; mso-char-indent-size: 12.0pt"><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">上式的物理概念是</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">f(t)</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">與其相對延遲的</SPAN><SPAN
lang=EN-US
style="FONT-SIZE: 12pt; FONT-FAMILY: Symbol; mso-ascii-font-family: Times New Roman; mso-hansi-font-family: Times New Roman; mso-char-type: symbol; mso-symbol-font-family: Symbol">t</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt"> </SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">的</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">f( t - </SPAN><SPAN lang=EN-US
style="FONT-SIZE: 12pt; FONT-FAMILY: Symbol; mso-ascii-font-family: Times New Roman; mso-hansi-font-family: Times New Roman; mso-char-type: symbol; mso-symbol-font-family: Symbol">t</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">)</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">來比較</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">:<O:P> </O:P></SPAN></P>
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