s05-05.htm

关于扩频的一些好资料

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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'">一般多采用有二值或三值相关特性的码作为地址码。同时还需要有一定的数量。</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">Gold</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">码就可以作为地址码来用，它既有较优良的相关特性，也有足够的数量可供选。</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'">其二是要注意克服“远－近”问题。</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'">所谓“远一近”问题指的是距离近的用户的信号强，它会干扰距离远的弱信号的接收。解决的办法是采用自动功率控制，自动调节各用户的发射功率，使达到接收机时各用户信号功率基本相等，也就是满足接收机输入端等功率的条件，才能正确地区分有用信号。</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'">其三是同时通话的用户数，决定于整个网内的噪声水平。</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'">因此，直扩码分多址系统是一种噪声受限的系统。随着用户数的增加，通信质量逐渐变坏。</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">&nbsp;</SPAN></P>
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            lang=EN-US style="FONT-SIZE: 12pt; COLOR: #333399">5.5.4<SPAN 
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            style="FONT-SIZE: 12pt; COLOR: #333399; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">直扩系统的抗多径干扰性能</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'">多径信道就是发射机和接收机之间电波传播的路径不止一条。例如由于大气层的反射和折射，以及由于建筑物等对电波的反射都是形成多径信道的原因。不</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">同的传播路径使电波在幅度上衰减不同，到达时间的延迟也不同。</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'">直扩系统能够同步锁定在最强的直达路径的电波上。其它有延迟到达的电波，由于相关解扩的作用，只起到噪声干扰的作用。这就是利用</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'">码的自相关特性，只要延迟超过半个</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'">码时片，其相关值就很小，可作为噪声来对待。另外，如果采用不同时延的匹配滤波器，把多径信号分离出来，类似梳状滤波器</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">(RAKE)</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">的作用那样，还可以变害为利，将这些多径信号在相位上对齐相加，起到增加接收信号能量的作用。</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'">因此，直扩系统是一种有效的抗多径干扰的通信系统。</SPAN></P>
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            color=#000080><SPAN lang=EN-US style="FONT-SIZE: 12pt">5.5.5<SPAN 
            style="mso-spacerun: yes">&nbsp; </SPAN></SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">直扩测距定时系统</SPAN></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'">直扩系统的发展是从测距开始的。电磁波在空间是以固定的光速传播的。如果测定了电波传播的时间，也就测定了距离。</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'">用直扩信号来测取和定时有独特的优点。当采用一个较长周期的</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'">码序列作为发射信号、用它于目的地反射回来或转发回来的</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'">码序列的相位进行比较，即比较两个码序列相差的时片数，就可以看出其时间差，也就能换算出发射机与目的地之间的距离。不难把码片选得很窄，即码的钟速率很高，则可以高精度的测距与定时，基本的分辨率即一个码片。</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'">此外，有了精确的测距的定时系统，不难形成一个精确的定位系统；按照简单的几何关系，已知两个点的位置</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'">座标</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'">和距离，及其在某一平面上分别与第三点的距离，也就能确定第三点的座标位置。</SPAN><SPAN 
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