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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 color=#000080 size=5><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: Times New Roman; mso-hansi-font-family: Times New Roman"><B>第二讲
扩展频谱通信的基本概念</B></SPAN></FONT><FONT size=4></FONT></DIV></TD></TR>
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size=4><B><SPAN lang=EN-US style="mso-spacerun: yes"> </SPAN><FONT
color=#800080><SPAN lang=EN-US><A name="2.1 扩展频谱通信的定义">2.1
</A></SPAN><A name="2.1 扩展频谱通信的定义"><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: Times New Roman; mso-hansi-font-family: Times New Roman">扩展频谱通信的定义</SPAN></A></FONT></B></FONT><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'">所谓扩展频谱通信，可简单表述如下：<FONT
face=楷体_GB2312
color=#008080>“扩频通信技术是一种信息传输方<BR>式，其信号所占有的频带宽度远大于所传信息必需的最小带宽；频带的扩展是通过一个独立的码序列来完成，用编码及调制的方法来实现的，与所传信息数据无关；在接收端则用同样的码进行相关同步接收、解扩及恢复所传信息数据”。</FONT></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'">这一定义包含了以下三方面的意思：</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、</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">信号的频谱被展宽了。</SPAN></FONT></B><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'">我们知道，传输任何信息都需要一定的带宽，称为信息带宽。</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'">例如人类的语音的信息带宽为</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">300Hz --- 3400Hz</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">，电视图像信息带宽为数</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">MHz</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">。为了充分利用频率资源，通常都是尽量采用大体相当的带宽的信号来传输信息。在无线电通信中射频信号的带宽与所传信息的带宽是相比拟的。如用调幅信号来传送语音信息，其带宽为语音信息带宽的两倍；电视广播射频信号带宽也只是其视频信号带宽的一倍多。这些都属于窄带通信。</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'">一般的调频信号，或脉冲编码调制信号，它们的带宽与信息带宽之比也只有几到十几。扩展频谱通信信号带宽与信息带宽之比则高达</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">100 --- 1000</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">，属于宽带通信。</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'">为什么要用这样宽的频带的信号来传输信息呢</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'">这样岂不太浪费宝贵的频率资源了吗</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">?</SPAN></P>
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lang=EN-US style="FONT-SIZE: 12pt"> <O:P> </O:P></SPAN><SPAN
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color=#0080c0><B>二、</B></FONT></SPAN><FONT color=#0080c0><B><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">采用扩频码序列调制的方式来展宽信号频谱。</SPAN></B></FONT><SPAN
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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'">我们知道，在时间上有限的信号，其频谱是无限的。例如很窄的脉冲信号，其频谱则很宽。信号的频带宽度与其持续时间近似成反比。</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">1</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">微秒的脉冲的带宽约为</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">1MHz</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">。因此，如果用限窄的脉冲序列被所传信息调制，则可产生很宽频带的信号。</SPAN></P>
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style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; TEXT-INDENT: 12pt; LINE-HEIGHT: 150%; mso-char-indent-count: 1.0; 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'">如下面介绍的直接序列扩频系统就是采用这种方法获得扩频信号。这种很窄的脉冲码序列，其码速率是很高的，称为扩频码序列。这里需要说明的一点是所采用的扩频码序列与所传信息数据是无关的，也就是说它与一般的正弦载波信号一样，丝毫不影响信息传输的透明性。扩频码序列仅仅起扩展信号频谱的作用。</SPAN></P>
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<B><FONT color=#0080c0>三、在接收端用相关解调来解扩</FONT></B></SPAN></P>
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</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">正如在一般的窄带通信中，已调信号在接收端都要进行解调来恢复所传的信息。在扩频通信中接收端则用与发送端相同的扩频码序列与收到的扩频信号进行相关解调，恢复所传的信息。换句话说，这种相关解调起到解扩的作用。即把扩展以后的信号又恢复成原来所传的信息。这种在发端把窄带信息扩展成宽带信号，而在收端又将其解扩成窄带信息的处理过程，会带来一系列好处。弄清楚扩频和解扩处理过程的机制，是理解扩频通信本质的关键所在。</SPAN></P>
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color=#800080 size=4><B><SPAN lang=EN-US><A
name="2.2 扩频通信的理论基础">2.2<SPAN style="mso-spacerun: yes">
</SPAN></A></SPAN><A name="2.2 扩频通信的理论基础"><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: Times New Roman; mso-hansi-font-family: Times New Roman">扩频通信的理论基础</SPAN></A></B></FONT><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'">长期以来，人们总是想法使信号所占领谱尽量的窄，以充分利用十分宝贵的频谱资源。为什么要用这样宽频带的信号来传送信息呢</SPAN><SPAN
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