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<TD borderColor=#6699ff><FONT size=2>您的位置：技术园地>技术讲座-扩频通信</FONT></TD></TR>
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<DIV align=center><FONT size=4><B><SPAN
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color=#000080 size=5>第五讲
直接序列扩频系统(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>扩频码序列</SPAN></FONT></B></FONT></P>
<P class=MsoNormal
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lang=EN-US style="FONT-SIZE: 12pt; COLOR: navy">5.2.1<SPAN
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style="FONT-SIZE: 12pt; COLOR: navy; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">码序列的相关性</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt; COLOR: navy"><O:P>
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<P class=MsoNormal
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style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">在扩展频谱通信中需要用高码率的窄脉冲序列。这是指扩频码序列的波形而言。并未涉及码的结构和如何产生等问题。</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: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">那么究竟选用什么样的码序列作为扩频码序列呢</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">? </SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">它应该具备哪些基本性能呢</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">? </SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">现在实际上用得最多的是伪随机码，或称为伪噪声</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">(PN)</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">码。</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: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">这类码序列最重要的特性是具有近似于随机信号的性能。因为噪声具有完全的随机性，也可以说具有近似于噪声的性能。但是，真正的随机信号和噪声是不能重复再现和产生的。我们只能产生一种周期性的脉冲信号来近似随机噪声的性能，故称为伪随机码或</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">PN</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">码。</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: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">为什么要选用随机信号或噪声性能的信号来传输信息呢？许多理论研究表明，在信息传输中各种信号之间的差别性能越大越好。这样任意两个信号不容易混淆，也就是说，相互之间不易发生干扰，不会发生误判。理想的传输信息的信</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">号形式应是类似噪声的随机信号，因为取任何时间上不同的两段噪声来比较都不会完全相似。用它们代表两种信号，其差别性就最大。</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: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">在数学上是用自相关函数来表示信号与它自身相移以后的相似性的。随机信号的自相关函数的定义为下列积分：</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt"><O:P> </O:P></SPAN></P>
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height=149 src="s05-02.files/ss5-3.gif" width=450
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align=center><SPAN lang=EN-US
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<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: 宋体; 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: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">为信号的时间函数，</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: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">为时间延迟。</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: 宋体; 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: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">与其相对延迟的</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: 宋体; 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: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">来比较</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">:<O:P> </O:P></SPAN></P>
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