October 2012
Intermediate to advanced
160 pages
2h 49m
Chinese
Content preview from 802.11n技術手冊
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第四章
時空區塊編碼（STBC）
時空區塊編碼可以在當無線電鏈路數目超過空間串流數目時使用。在效用方面，STBC
能夠透過兩道空間串流來發送兩次相同的資料串流，因此當接收器在其中一道空間串流
中遺漏了一個資料區塊，在解碼第二道空間串流的資料時將有第二次機會。STBC 獲取
了來自 MIMO 的增益，並且幾乎全部使用於增加傳輸範圍。一個單一編碼串流必須使用
兩個無線電鏈路來進行傳輸，這意謂著一個採用 STBC 傳輸的 2×2 MIMO 設備，以單
串流為作業模式較有效率。802.11 介面包含一個降速（Step-down）演算法，用以選取
更慢且更可靠的傳輸速率。當資料頻道過於衰弱，無法對每個無線電支援一個完整串流
時，可以使用 STBC。在這類的環境中，STBC 是值得以 50％傳輸速度的損失來換取。
當一個具備 STBC 功能的 AP，主要服務的對象是遠距離的單串流設備時，STBC 絕對是
值得考慮的。
低密度同位檢查（LDPC）
直到 802.11n 為止，所有基於 OFDM 的實體層使用迴旋碼作為前向錯誤更正（FEC）
碼。概念上，迴旋編碼的運作是隨著時間「抹除（Smearing out）」錯誤，因此如果鄰近
的位元足夠好的情形下，單一位元的錯誤是可以被更正的。在 802.11 定義為一個選項的
低密度同位檢查碼，也是採用類似的方式運作，但相較於迴旋編碼，LDPC 也提供了編
碼增益。根據頻道模型，模擬顯示 LDPC 增加的訊雜比在 1.5 至 3 dB 之間。
4
LDPC 作業的編碼字數為
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