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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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</SPAN></A></SPAN><A name="7.1 为什么提出混合式扩频系统？"><SPAN
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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'">直接序列扩展频谱系统的优点与局限</SPAN><SPAN
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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'">直接利用码片速率极高的伪随机序列对信息比特流进行调制和利用相关<BR>接收方法进行解调的这种直接扩展频谱系统具有很多优点。但是也有不足之处。</SPAN><SPAN
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color=#ff9933 size=2><B> 直接序列扩频的优点是</B></FONT>：</SPAN><SPAN
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直扩信号的功率谱密度低，具有隐蔽性和低的截获概率，从而抗侦察；抗截获的能力强；另外，功率污染小，即对其它系统引起的电磁环境污染小，以利于多种系统共存。</SPAN><SPAN
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直扩伪随机序列的伪随机性和密钥量使信息具有保密性，即系统本身具有加密的能力。因为用伪随机序列对信息比特流进行扩展频谱，就相当于对信息的加密；而所拥有的码型不同的伪随机序列的数目，就相当于密钥量。当不知道直扩系统所采用的码型时，就无法破译。</SPAN><SPAN
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利用直扩伪随机序列码型的正交性，可构成直接序列扩展频谱码分多址系统。在这样的码分多址系统中，每个通信站址分配一个地址码</SPAN><SPAN
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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'">一种伪随机序列</SPAN><SPAN
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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'">，利用地址码的正交性通过相关接收来识别出来自不同站址的信息。</SPAN><SPAN
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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'">码分多址系统中的用户是共享频谱资源的。</SPAN><SPAN
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直接扩展频谱系统具有抗宽带干扰、抗多频干扰及单频干扰的能力。这是因为直接扩展频谱系统具有很高的处理增益，对有用信号进行相关接收，对干扰</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">信号进行频谱扩展使其大部分的干扰功率被接收机中频带通滤波器所滤除的原因。</SPAN><SPAN
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利用直接扩展频谱信号的相关接收，它具有抗多径效应的能力。当直扩伪随机序列的码片宽度</SPAN><SPAN lang=EN-US
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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'">小于多径时延时，利用相关接收可以消除多径时</SPAN><SPAN
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