s03.htm

扩频通信讲座

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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'">扩频通信提高了抗干扰性能，但付出了占用频带宽的代价。</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'">如果让许多用户共用这一宽频带，则可大为提高频带的利用率。由于在扩频通信中存在扩频码序列的扩频调制，充分利用各种不同码型的扩频码序列之间优良的自相关特性和互相关特性，在接收端利用相关检测技术进行解扩，则在分配给不同用户码型的情况下可以区分不同用户的信号，提取出有用信号。这样一来，在一宽频带上许多对用户可以同时通话而互不干扰。</SPAN></P>
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            size=4><A name="3.5  抗多径干扰"><B><FONT color=#800080><SPAN 
            lang=EN-US>3.5<SPAN style="mso-spacerun: yes">&nbsp; 
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            style="FONT-FAMILY: 宋体; mso-ascii-font-family: Times New Roman; mso-hansi-font-family: Times New Roman">抗多径干扰</SPAN></FONT></B></A></FONT><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'">在无线通信的各个频段，长期以来，多径干扰始终是一个难以解决的问题之一。在以往的窄带通信中，采用两种方法来提高抗多径干扰的能力：</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'">一是把最强的有用信号分离出来，排除其他路径的干扰信号，即采用分集／</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">接收技术；</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'">二是设法把不同路径来的不同延迟、不同相位的倍号在接收端从时域上对齐</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">相加，合并成较强的有用信号，即采用梳状滤波器的方法。</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'">这两种技术在扩频通信中都易于实现。利用扩频码的自相关特性，在接收端</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'">从多径信号中提取和分离出最强的有用信号，或把多个路径来的同一码序列的波形相加合成，这相当于梳状滤波器的作用。另外，采用频率跳变扩频调制方式的扩频系统中，由于用多个频率的信号传送同一个信息，实际上起到了频率分集的作用。</SPAN></P>
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            size=4><A name="3.6  能精确地定时和测距"><B><FONT color=#800080><SPAN 
            lang=EN-US>3.6<SPAN style="mso-spacerun: yes">&nbsp; 
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            style="FONT-FAMILY: 宋体; mso-ascii-font-family: Times New Roman; mso-hansi-font-family: Times New Roman">能精确地定时和测距</SPAN></FONT></B></A></FONT><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'">我们知道电磁波在空间的传播速度是固定不变的光速。人们自然会想到如果能够精确测量电磁波在两个物体之间传播的时间，也就等于测量两个物体之间的距离。</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'">在扩频通信中如果扩展频谱很宽，则意味着所采用的扩频码速率很高，每个码片占用的时间就很短。当发射出去的扩频信号在被测物体反射回来后，在接收端解调出扩频码序列，然后比较收发两个码序列相位之差，就可以精确测出扩频信号往返的时间差，从而算出二者之间的距离。测量的精度决定于码片的宽度，也就是扩展频谱的宽度。码片越窄，扩展的频谱越宽，精度越高。</SPAN></P>
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            size=4><A name="3.7  适合数字话音和数据传输，以及开展多种通信业务"><B><FONT 
            color=#800080><SPAN lang=EN-US>3.7<SPAN 
            style="mso-spacerun: yes">&nbsp; </SPAN></SPAN><SPAN 
            style="FONT-FAMILY: 宋体; mso-ascii-font-family: Times New Roman; mso-hansi-font-family: Times New Roman">适合数字话音和数据传输，以及开展多种通信业务</SPAN></FONT></B></A></FONT><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'">扩频通信一般都采用数字通信、码分多址技术，适用于计算机网络，适合于数据和图象传输。</SPAN></P>
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            size=4><A name="3.8  安装简便，易于维护"><B><FONT color=#800080><SPAN 
            lang=EN-US>3.8<SPAN style="mso-spacerun: yes">&nbsp; 
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            style="FONT-FAMILY: 宋体; mso-ascii-font-family: Times New Roman; mso-hansi-font-family: Times New Roman">安装简便，易于维护</SPAN></FONT></B></A></FONT><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'">扩频通信设备是高度集成，采用了现代电子科技的尖端技术，因此，十分可靠、小巧，大量运用后成本低，安装便捷，易于推广应用。</SPAN></P></TD>
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