多天線正交頻域多工系統在通道的偵測上比單天線的系統要來得複雜得多，主要是因為多根天線傳送資料時，接收端收到的訊號是整個疊加在一起的，所以即使知道收到的訊號為何，也無法輕易的估測出通道的效應，若是系統應用在行動通訊上，由於通道會在短時間內改變，通道的偵測就更加困難。然而，期望值最大化演算法能利用疊代的計算，把多重的訊號各自分開，大大地降低運算的複雜度，在此篇論文中，我們將探討在時變通道中此演算法配合更正錯誤碼的整體效能。

　For MIMO OFDM systems, channel estimation schemes have been mostly based on pilot-assisted approaches, assuming a quasi-static fading model that allows the channel to be constant for a block of symbols and change independently to a new realization. Using detected symbols as a pilot symbol is a general way to tracking time varying channel, but error propagation might happen. In this thesis, we exploit error control coding to correct detected symbols. With EM algorithm, we can derive a simple and effective way to track time varying channel in MIMO OFDM systems.

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