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<TD borderColor=#6699ff><FONT size=2>您的位置：技術園地>技術講座-擴頻通信</FONT></TD></TR>
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<DIV align=center><FONT color=#000080 size=5><SPAN
style="FONT-FAMILY: 宋體; mso-ascii-font-family: Times New Roman; mso-hansi-font-family: Times New Roman"><B>第二講
擴展頻譜通信的基本概念</B></SPAN></FONT><FONT size=4></FONT></DIV></TD></TR>
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style="FONT-SIZE: 12pt; mso-spacerun: yes"> </SPAN><FONT
size=4><B><SPAN lang=EN-US style="mso-spacerun: yes"> </SPAN><FONT
color=#800080><SPAN lang=EN-US><A name="2.1 擴展頻譜通信的定義">2.1
</A></SPAN><A name="2.1 擴展頻譜通信的定義"><SPAN
style="FONT-FAMILY: 宋體; mso-ascii-font-family: Times New Roman; mso-hansi-font-family: Times New Roman">擴展頻譜通信的定義</SPAN></A></FONT></B></FONT><SPAN
lang=EN-US style="FONT-SIZE: 12pt"><O:P> </O:P></SPAN></P>
<P class=MsoNormal
style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; TEXT-INDENT: 24pt; LINE-HEIGHT: 150%"><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">所謂擴展頻譜通信，可簡單表述如下：<FONT
face=楷體_GB2312
color=#008080>“擴頻通信技術是一種信息傳輸方<BR>式，其信號所占有的頻帶寬度遠大于所傳信息必需的最小帶寬；頻帶的擴展是通過一個獨立的碼序列來完成，用編碼及調制的方法來實現的，與所傳信息數據無關；在接收端則用同樣的碼進行相關同步接收、解擴及恢復所傳信息數據”。</FONT></SPAN></P>
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<P class=MsoNormal
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lang=EN-US style="FONT-SIZE: 12pt"><O:P></O:P></SPAN></P>
<P class=MsoNormal
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style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">這一定義包含了以下三方面的意思：</SPAN><SPAN
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<P class=MsoNormal
style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; TEXT-INDENT: 24pt; LINE-HEIGHT: 150%"><B><FONT
color=#0080c0><SPAN lang=EN-US
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">一、</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">信號的頻譜被展寬了。</SPAN></FONT></B><SPAN
lang=EN-US style="FONT-SIZE: 12pt"><O:P> </O:P></SPAN></P>
<P class=MsoNormal
style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; 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: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">我們知道，傳輸任何信息都需要一定的帶寬，稱為信息帶寬。</SPAN></P>
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style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">例如人類的語音的信息帶寬為</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">300Hz --- 3400Hz</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">，電視圖像信息帶寬為數</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">MHz</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">。為了充分利用頻率資源，通常都是盡量采用大體相當的帶寬的信號來傳輸信息。在無線電通信中射頻信號的帶寬與所傳信息的帶寬是相比擬的。如用調幅信號來傳送語音信息，其帶寬為語音信息帶寬的兩倍；電視廣播射頻信號帶寬也只是其視頻信號帶寬的一倍多。這些都屬于窄帶通信。</SPAN></P>
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style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">一般的調頻信號，或脈沖編碼調制信號，它們的帶寬與信息帶寬之比也只有幾到十幾。擴展頻譜通信信號帶寬與信息帶寬之比則高達</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">100 --- 1000</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">，屬于寬帶通信。</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt"><O:P> </O:P></SPAN></P>
<P class=MsoNormal
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style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">為什么要用這樣寬的頻帶的信號來傳輸信息呢</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">? </SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">這樣豈不太浪費寶貴的頻率資源了嗎</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">?</SPAN></P>
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lang=EN-US style="FONT-SIZE: 12pt"> <O:P> </O:P></SPAN><SPAN
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style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"><FONT
color=#0080c0><B>二、</B></FONT></SPAN><FONT color=#0080c0><B><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">采用擴頻碼序列調制的方式來展寬信號頻譜。</SPAN></B></FONT><SPAN
lang=EN-US style="FONT-SIZE: 12pt"><O:P> </O:P></SPAN></P>
<P class=MsoNormal
style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; TEXT-INDENT: 12pt; LINE-HEIGHT: 150%; mso-char-indent-count: 1.0; mso-char-indent-size: 12.0pt"><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">我們知道，在時間上有限的信號，其頻譜是無限的。例如很窄的脈沖信號，其頻譜則很寬。信號的頻帶寬度與其持續時間近似成反比。</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">1</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">微秒的脈沖的帶寬約為</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">1MHz</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">。因此，如果用限窄的脈沖序列被所傳信息調制，則可產生很寬頻帶的信號。</SPAN></P>
<P class=MsoNormal
style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; TEXT-INDENT: 12pt; LINE-HEIGHT: 150%; mso-char-indent-count: 1.0; mso-char-indent-size: 12.0pt"><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">如下面介紹的直接序列擴頻系統就是采用這種方法獲得擴頻信號。這種很窄的脈沖碼序列，其碼速率是很高的，稱為擴頻碼序列。這里需要說明的一點是所采用的擴頻碼序列與所傳信息數據是無關的，也就是說它與一般的正弦載波信號一樣，絲毫不影響信息傳輸的透明性。擴頻碼序列僅僅起擴展信號頻譜的作用。</SPAN></P>
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lang=EN-US style="FONT-SIZE: 12pt"><O:P></O:P></SPAN></P>
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<B><FONT color=#0080c0>三、在接收端用相關解調來解擴</FONT></B></SPAN></P>
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</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">正如在一般的窄帶通信中，已調信號在接收端都要進行解調來恢復所傳的信息。在擴頻通信中接收端則用與發送端相同的擴頻碼序列與收到的擴頻信號進行相關解調，恢復所傳的信息。換句話說，這種相關解調起到解擴的作用。即把擴展以后的信號又恢復成原來所傳的信息。這種在發端把窄帶信息擴展成寬帶信號，而在收端又將其解擴成窄帶信息的處理過程，會帶來一系列好處。弄清楚擴頻和解擴處理過程的機制，是理解擴頻通信本質的關鍵所在。</SPAN></P>
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color=#800080 size=4><B><SPAN lang=EN-US><A
name="2.2 擴頻通信的理論基礎">2.2<SPAN style="mso-spacerun: yes">
</SPAN></A></SPAN><A name="2.2 擴頻通信的理論基礎"><SPAN
style="FONT-FAMILY: 宋體; mso-ascii-font-family: Times New Roman; mso-hansi-font-family: Times New Roman">擴頻通信的理論基礎</SPAN></A></B></FONT><SPAN
lang=EN-US style="FONT-SIZE: 12pt"><O:P> </O:P></SPAN></P>
<P class=MsoNormal
style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; TEXT-INDENT: 24pt; LINE-HEIGHT: 150%"><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋體; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">長期以來，人們總是想法使信號所占領譜盡量的窄，以充分利用十分寶貴的頻譜資源。為什么要用這樣寬頻帶的信號來傳送信息呢</SPAN><SPAN
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