正交分頻多工系統可以實現高頻譜效率及高速資料傳輸的需求，如數位音訊廣播、數位視訊廣播和數位高畫質電視，是一在未來相當具有發展性的技術。然，正交分頻多工系統在無線行動通訊上會因時變通道環境而失去各子載波彼此間的正交性，產生載波之間的互相干擾，造成錯誤地板現象。我們展示在雙重選擇性通道環境裡，由於在時變環境中，所收的訊號裡最主要的能量組成之信號干擾比會隨多重路徑衰減通道環境其延遲展延的長度增加而呈指數衰減，其意味著將造成更嚴重的載波之間互相干擾。 在本篇論文研究，我們提出一個最大似然序列估計的設計，充分的利用到頻率選擇性和時間分集來降低載波之間的互相干擾。 藉由更進一步的回饋和剩餘的載波之間互相干擾消除機制，再操作第二階段最大似然序列估計之後，我們所提方法之位元錯誤率相當接近匹配濾波器極限。模擬結果指出我們的方法在雙重選擇性通道環境裡有強健的表現。 我們根據等效頻域通道矩陣裡對角元素彼此間的關連性，推導一線性最小均方誤差的通道估測方法來搭配最大似然序列估計機制，使系統可以在操作上具即時性，不會有延遲；從模擬裡亦可看到，我們的方法在位元錯誤率上也比其他同樣基於領航訊號輔助來做減少載波之間互相干擾的方法有較好的表現和更有效率的頻帶利用。

For orthogonal frequency division multiplexing (OFDM) systems in wireless mo-bile applications, time-variant and multipath fading channels lead to the orthogonality between subcarriers destroyed and introduces intercarrier interference (ICI), resulting in an error floor. We show that severer ICI resulted from doubly selective channels, since signal-to-interference ratio (SIR) of main components of the power of each re-ceived signal in time-variant channels decreases exponentially with the length of de-lay spread increasing. In this study, we propose a novel MLSE scheme to fully exploit the frequency selectivity and temporal diversity to suppress ICI. With further decision feedback and residual ICI cancellation mechanism, performance of our proposed method is very close to the matched-filter (MF) bound. Simulation results indicate that our method is robust to doubly selective channels. We also derive an LMMSE channel estimation method to couple with the MLSE scheme on per state, which op-erates without latency and shows a better performance and more efficient channel bandwidth efficiency than other pilot-based ICI mitigation method from simulations.

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