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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><A name="第五讲? 直接序列扩频系统">第五讲
直接序列扩频系统</A>(1)</FONT></SPAN></B></FONT></DIV></TD></TR>
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<P class=MsoNormal style="LINE-HEIGHT: 150%"><FONT color=#800080
size=4><B><SPAN lang=EN-US><A name="5.1 直扩系统的组成与原理">5.1<SPAN
style="mso-spacerun: yes"> </SPAN></A></SPAN><A
name="5.1 直扩系统的组成与原理"><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: Times New Roman; mso-hansi-font-family: Times New Roman">直扩系统的组成与原理</SPAN><SPAN
lang=EN-US><SPAN style="mso-spacerun: yes">
</SPAN></SPAN></A></B></FONT><SPAN lang=EN-US
style="FONT-SIZE: 12pt"><O:P></O:P></SPAN></P>
<P class=MsoNormal style="LINE-HEIGHT: 150%"><SPAN lang=EN-US
style="FONT-SIZE: 12pt; mso-spacerun: yes"><FONT
color=#000080>5.1.1 组成与原理 </FONT>
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<P class=MsoNormal style="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">
</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">前面已经说过：所谓直接序列</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">(DS)</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">扩频，就是直接用具有高码率的扩频码序列在发端去扩展信号的频谱。而在收端，用相同的扩频码序列去进行解扩，把展</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">宽的扩频信号还原成原始的信息。图</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">5</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">－</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'">为直扩系统的组成与原理框图。</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt"><O:P> </O:P></SPAN></P>
<P class=MsoNormal style="LINE-HEIGHT: 150%" align=center><SPAN
lang=EN-US style="FONT-SIZE: 12pt"> <IMG height=333
src="s05.files/ss5-1.gif" width=500 border=0><O:P> </O:P></SPAN></P>
<P class=MsoNormal style="LINE-HEIGHT: 150%; TEXT-ALIGN: center"
align=center><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">图</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">5</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">－</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">1<O:P> </O:P></SPAN></P>
<P class=MsoNormal
style="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'">在图</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">5</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">－</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">1(a)</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">中，假定发送的是一个频带限于</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">fin</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">以内的窄带信息。将此信息在信息调制器中先对某一副载额</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">fo</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">进行调制</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'">例如进行调幅或窄带调频</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'">，得到一中心频率为</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">fo</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">而带宽为</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">2fin</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">的信号，即通常的窄带信号。一般的窄带通信系统直接将此信号在发射机中对射频进行调制后由天线辐射出去。</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt"><O:P> </O:P></SPAN></P>
<P class=MsoNormal
style="TEXT-INDENT: 24pt; LINE-HEIGHT: 150%; mso-char-indent-count: 2.0; mso-char-indent-size: 12.0pt"><SPAN
lang=EN-US style="FONT-SIZE: 12pt"><O:P></O:P></SPAN></P>
<P class=MsoNormal
style="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'">但在扩展频谱通信中还需要增加一个扩展频谱的处理过程。常用的一种扩展频谱的方法就是用一高码率</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">fc</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">的随机码序列对窄带信号进行二相相移键控调制见图</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">5</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">－</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">1(b)</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">中发端波形。二相相移键控相当于载波抑制的调幅双边带信号。选择</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">fc </SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">＞＞</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt"> fo</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">＞</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt"> fin</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">。这样得到了带宽为</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">2fc</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">的载波抑制的宽带信号。这一扩展了频谱的信号再送到发射机中去对射频</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">f<SUB>T</SUB></SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">进行调制后由天线辐射出去。</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt"><O:P> </O:P></SPAN></P>
<P class=MsoNormal
style="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'">信号在射频信道传输过程中必然受到各种外来信号的干扰。因此，在收端，进入接收机的除有用信号外还存在干扰信号。假定干扰为功率较强的窄带信号，宽带有用信号与干扰信号同时经变频至中心频率为中频</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">f<SUB>I</SUB></SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">输出。不言而喻，对这一中频宽带信号必须进行解扩处理才能进行信息解调。</SPAN><SPAN
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'">解扩实际上就是扩频的反变换，通常也是用与发端相同的调制器，并用与发端完全相同的伪随机码序列对收到的宽带信号再一次进行二相相移键控。</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt"><O:P> </O:P></SPAN></P>
<P class=MsoNormal
style="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'">从图</SPAN><SPAN
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