October 2012
Intermediate to advanced
160 pages
2h 49m
Chinese
Content preview from 802.11n技術手冊
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第三章
以降低硬體實作的複雜度。
4
當使用迴旋編碼時，直到實體層速率超過 300 Mbps 前，可
以使用一個單編碼器。
比起先前 802.11 所使用的迴旋編碼，LDPC 編碼是一種高性能的錯誤更正碼。LDPC 的
運作是將資料劃分為區塊，並增加冗餘位元至每個區塊，以期能夠恢復傳輸錯誤。在
802.11n 中，LDPC 使用了與迴旋編碼相同的編碼速率，如果 LDPC 編碼速率為 R=2/3，
每三個位元中，即有一個是編碼程序增加的冗餘位元。相較於迴旋編碼，LDPC 的實作
需要更複雜的電路（因此更耗電）。對於 LDPC 的支援是選用的，但已經變得更加普遍
了。LDPC 的作業對於終端使用者而言是透明的，因為 MAC 層是採用透明的方式來交
換有關 LDPC 能力的資訊，因此它也僅在傳輸雙方都支援的情形下方可使用。
防護間隔（Guard Interval，GI）
防護間隔應為最大多重路徑延遲時間的四倍，儘管這不是一個法定的規則，但它是
OFDM 系統設計師所採用的經驗法則。
5
802.11 在設計時，設計師們對於延遲時間採用
了一個 200 ns（奈秒）的保守值，並選擇使用 800 ns 的防護間隔。實作的經驗顯示，大
多數室內環境甚至很少會達到 100 ns 的延遲，而且往往是接近 50 至 75 ns。為了擠出無
線電鏈結更多的效能提升，802.11n 包含了一個短防護間隔的選項，可以將防護間隔減
少到 400 ns。是否可以成功的使用短防護間隔需要依據多重路徑的傳播，這需要進階的
射頻分析工具回報。一般情況下 ...
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