s05-02.htm

扩频通信讲座

            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">位的</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。如果用＋</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'">，－</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'">脉冲分别表示“</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">l</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 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">和“</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">0</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">”，则在图</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">3(c)</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">中示出其波形和它相对延迟</SPAN><SPAN 
            style="FONT-SIZE: 12pt"> </SPAN><SPAN lang=EN-US 
            style="FONT-SIZE: 12pt; FONT-FAMILY: Symbol; mso-ascii-font-family: Times New Roman; mso-hansi-font-family: Times New Roman; mso-char-type: symbol; mso-symbol-font-family: Symbol">t</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'">个时片的波形。这样我们很容易求出这两个脉冲序列波形的自相关函数，如图</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">3(d)</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。自相关峰值在</SPAN><SPAN 
            lang=EN-US 
            style="FONT-SIZE: 12pt; FONT-FAMILY: Symbol; mso-ascii-font-family: Times New Roman; mso-hansi-font-family: Times New Roman; mso-char-type: symbol; mso-symbol-font-family: Symbol">t</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'">＝</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">0</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">时出现，自相关函数在</SPAN><SPAN 
            lang=EN-US 
            style="FONT-SIZE: 12pt; FONT-FAMILY: Symbol; mso-ascii-font-family: Times New Roman; mso-hansi-font-family: Times New Roman; mso-char-type: symbol; mso-symbol-font-family: Symbol">±</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt"> </SPAN><SPAN lang=EN-US 
            style="FONT-SIZE: 12pt; FONT-FAMILY: Symbol; mso-ascii-font-family: Times New Roman; mso-hansi-font-family: Times New Roman; mso-char-type: symbol; mso-symbol-font-family: Symbol">t</SPAN><SUB><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">0</SPAN></SUB><SPAN lang=EN-US 
            style="FONT-SIZE: 12pt">/2</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">范围内呈三角形。</SPAN><SPAN 
            lang=EN-US 
            style="FONT-SIZE: 12pt; FONT-FAMILY: Symbol; mso-ascii-font-family: Times New Roman; mso-hansi-font-family: Times New Roman; mso-char-type: symbol; mso-symbol-font-family: Symbol">t</SPAN><SUB><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">0</SPAN></SUB><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">为脉冲宽度。而其它延迟时，自相关函数值为－</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">1/7, </SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">即码位长的倒数取负值。</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 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">当码长取得很大时，它就越近似于图</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">3(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'">中所示的理想的随机噪声的自相关特性。自然这种码序列就被称为伪随机码或伪噪声码。由于这种码序列具有周期性，又容易产生，它就是下面即将介绍的</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">m</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">序列，成为直扩系统中常用的扩频码序列。</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'">扩频码序列除自相关性外，与其他同类码序列的相似性和相关性也很重要。例如有许多用户共用一个信道，要区分不同用户的信号，就得靠相互之间的区别或不相似性来区分。换句话说，就是要选用互相关性小的信号来表示不同的用户。两个不同信号波形</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">f(t)</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">与</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">g(t)</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">之间的相似性用互相关函数来表示：</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%" 
            align=center><SPAN lang=EN-US style="FONT-SIZE: 12pt">&nbsp;<IMG 
            height=74 src="s05-02.files/ss5-5.gif" width=450 
            border=0>&nbsp;</SPAN></P>
            <P class=MsoNormal style="TEXT-INDENT: 24pt; LINE-HEIGHT: 150%" 
            align=center><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'">如果两个信号都是完全随机的，在任意延迟时间</SPAN><SPAN 
            style="FONT-SIZE: 12pt"> </SPAN><SPAN lang=EN-US 
            style="FONT-SIZE: 12pt; FONT-FAMILY: Symbol; mso-ascii-font-family: Times New Roman; mso-hansi-font-family: Times New Roman; mso-char-type: symbol; mso-symbol-font-family: Symbol">t</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'">都不相同，则上式为</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">0</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">。如果有一定的相似性，则不完全为</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">0</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">。两个信号的互相关函数为</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">0</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">，则称之为是正交的。通常希望两个信号的互相关值越小越好，则它们越容易被区分，且相互之间的干扰也小。</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">&nbsp;<O:P> </O:P></SPAN></P>
            <P class=MsoNormal style="LINE-HEIGHT: 150%"><SPAN lang=EN-US 
            style="FONT-SIZE: 12pt"><SPAN style="mso-spacerun: yes">&nbsp; 
            </SPAN><SPAN style="COLOR: navy">5.2.1<SPAN 
            style="mso-spacerun: yes">&nbsp; </SPAN>m</SPAN></SPAN><SPAN 
            style="FONT-SIZE: 12pt; COLOR: navy; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">序列</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt; COLOR: navy"><O:P> 
            </O:P></SPAN></P>
            <P class=MsoNormal 
            style="TEXT-INDENT: 24pt; LINE-HEIGHT: 150%"><SPAN lang=EN-US 
            style="FONT-SIZE: 12pt">m</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">序列是最长线性移位寄存器序列的简称。由于</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">m</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">序列容易产生、规律性强、有许多优良的性能，在扩频通信中最早获得广泛的应用。</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'">顾名思义，</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">m</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">序列是由多级移位寄存器或其他延迟元件通过线性反馈产生的最长的码序列。在二进制移位寄存器发生器中，若</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">n</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">为级数，则所能产生的最大长度的码序列为</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">2<SUP>n</SUP></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'">位。</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 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">现在来看看如何由多级移位寄存器经线性反馈产生周期性的</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">m</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">序列。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">4(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'">为一最简单的三级移位寄存器构成的</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">m</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">序列发生器。</SPAN><SPAN