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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'">延的影响，因而直接扩展频谱系统具有抗多径干扰的能力。</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'">利用直接扩展频谱信号可实现精确的测距定位。直接扩展频谱系统除可进行通信外，还可利用直接扩展频谱信号的发送时刻与反回时刻的时间差，测出目标物的距离。因此，在同时具有通信和导航能力的综合信息系统中显示了直接扩展频谱系统的优势。</SPAN><SPAN
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直接扩展频谱系统适用于数字话音和数据信息的传输。这是由于扩频系统本身是数字系统所决定的。</SPAN><SPAN lang=EN-US
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color=#ff9933 size=2>其局限性在于：</FONT></B></SPAN><SPAN lang=EN-US
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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'">直接序列扩展频谱系统是二个宽带系统，虽然可与窄带系统电磁兼容；但不能与其建立通信。另外，对模拟信源</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'">如话音</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'">需作预先处理</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'">如语声编码</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'">后，才可按入直扩系统。</SPAN><SPAN
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在直接扩展频谱系统的接收机存在明显的远近效应。所谓远近效应是指大功率的信号</SPAN><SPAN lang=EN-US
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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'">近处的电台</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'">抑制小功率信号</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'">远端的电台</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'">的现象。对此，需要在系统中采用自动功率控制以保证远端和近端电台到达接收机的有用信号是同等功率的。这一点，增加了直接扩展频谱系统在移动通信环境中应用的复杂性。</SPAN><SPAN
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直接扩展频谱系统的处理增益受限于码片</SPAN><SPAN lang=EN-US
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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'">速率和信源的比特率，即码片速率的提高和信源比特率的下降都存在困难。处理增益受限，意味着抗干扰能力受限，多址能力受限。</SPAN></P>
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<P class=MsoNormal
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lang=EN-US style="FONT-SIZE: 12pt">7.1.2<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'">跳频系统的扰点与局限</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'">利用伪随机序列指令码对系统的载波频率进行控制的跳频系统也具有其独特之处和局限性。</SPAN></P>
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size=2><B> 其优点是：</B></FONT></SPAN><FONT
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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'">跳频图案的伪随机性和跳频图案的密钥量使跳频系统具有保密性。即使是模拟话音的跳频通信，只要敌方不知道所使用的跳频图案就具有一定的保密的能力。当跳频图案的密钥足够大时，具有抗截获的能力。</SPAN><SPAN
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由于载波频率是跳变的，具有抗单频及部分带宽干扰的能力。当跳变的频率数目足够多时，跳频带宽足够宽时，其抗干扰能力是很强。</SPAN><SPAN
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利用载波频率的快速跳变，具有频率分集的作用，从而使系统具有抗多径衰落的能力。条件是跳变的频率间隔具要大于相关带宽。</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'">利用跳频图案的的正交性可构成跳频码分多址系统，共享频谱资源，并具有承受过载的能力。</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'">跳频系统为瞬时窄带系统，能与现有的窄带系统兼容通信。</SPAN></P>
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