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
波束成形（Beamforming）
在 802.11n 之前，大多數的存取點預設配備了全向型天線（Omnidirectional Antenna）。
「全向型（Omnis）」的輻射在所有方向都是相等的，其覆蓋面通常被描繪成一個圓
圈，而存取點就在其中心。全向型天線的成本之一，是預定送往接收器的訊號將均勻地
散布在各個方向，即使接收器僅位於這些方向之一。我發現闡述的最好方式，就是回
想約翰 • 沃納梅克的名言—廣告費有一半都被浪費了。麻煩的是，沒人知道是哪一半
被浪費了。波速成形就是此 802.11 難題的答案，它使天線陣列的能量集中在客戶端設
備的方向。透過使用相同的能量，但是集中在接收器的方向，就有可能增加在接收器
的訊雜比。擁有更高的訊雜比，就能夠使用更有效率的編碼方式，因此得到更快的速
度。圖 4-1 展示了波束成形的基本想法。一個傳送器（Transmitter）也稱為
波束成形
器
（
Beamformer
），能夠「集中（Focuses）」或「導引（Steers）」能量，因此在單一方
向傳輸將變得更快，而朝向的接收器（Receiver）也稱為
波束接收器
（
Beamformee
）。
（Beamformer 與 Beamformee 兩個術語在瞭解一個需要訊框交換的顯性頻道量測協定上
非常有用）此圖描繪了一個經典波束成形的權衡。欲形成往一個方向的波束，結果往往
成為一個 8 字形的覆蓋樣式，如圖所示。
全向型
波束成形
圖 4-1 波束成形概念上的過程
在本質上，波束成形是一種可電力導引的天線。當傳送到一個給定的接收器時，天線將 ...
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