本論文針對802.16的OFDM系統來探討通道估測的問題。其中所考慮的通道模型包括延遲擴展與都普勒功率頻譜密度。因為利用preamble估測通道的方式不能用在資料傳輸，所以必須在每個OFDM symbol被接收時，重新作通道估測。在MMSE的通道估測中，接收端必須要得到某些通道的統計特性，而在先前的許多論文中已經針對在接收端已經知道通道統計特性的情況下，且在一個碼框的時間內，考慮非時變的通道來探討通道估測。本論文假設在接收端不知道通道統計特性的情況下，使用decision-directed(DD) MMSE的通道估測方式，且其允許使用在資料傳輸中。其利用每個碼框前的preamble來做通道估測，且用data-aided的估測方式來計算通道的統計特性。
　　在本論文中已對DD-MMSE的估測方式，在許多的通道中做模擬。模擬結果顯示在延遲擴展較短時，可以得到較佳的系統效能。同時從模擬結果也可以得知，在較高的都普勒頻率時，DD-MMSE的估測方式不會受到其影響而使系統效能衰減。

In this thesis, channel estimation for 802.16 OFDM systems is studied. The channel models considered in this thesis have both delay and Doppler power spectrum. In these cases, preamble-based channel estimation does not allow data transmission; hence a new estimate is required every time an OFDM symbol is received. For minimum-mean-square-error (MMSE) estimation, certain channel statistics are required for operation. Previous research papers have dealt with this topic under the assumptions of known channel statistics and time-invariant impulse response during a data frame. The decision-directed (DD) MMSE estimator allow data transmission even if channel statistics are not available to the user. The estimator employs the preamble of every frame and data-aided estimation for computation of the channel statistics.
Simulation results are provided for DD-MMSE channel estimation in several channel conditions. A good performance if the channel delay spread is short. On the other hand, the simulation results also show that the performance of DD-MMSE channel estimation is not degraded by high Doppler frequencies.

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