正交分頻多工已是一個被廣泛使用且有效率的傳輸技術。其是以多個窄頻載波的方式傳送訊號，使之可在多通道的環境下傳送，並藉著加入訊號保護區間，則可使傳送訊號不受符號干擾。若假設無線通訊的通道是時變的，則在此假設下會使各載波間失去正交性質，而有載波間干擾，使得系統效能降低。從分析中得知，我們並不能用提高訊雜比的方式來消除載波間干擾。因此我們要在快速時變的通道中傳送訊號，則載波間干擾勢必是要解決的問題。現今已有多種消除載波間干擾的方法，而在本篇中，我們借用一個簡單且有效的自我消除載波間干擾演算法，其利用傳送重覆訊號來降低載波間干擾。此演算法要每個傳送符號多傳送額外的訊號，因而使系統的傳輸率降低。所以我們修改系統，使得我們只需於第一個傳送符號加入額外的擴展訊號，之後其他符號不再做擴展而以原長度傳送，如此系統可增加傳輸率且也能降低符號間干擾。我們提出修改的演算法，是在接收端產生假的擴展訊號來取代原本應在傳送端額外傳送的擴展訊號，由此組合來估測通道響應，進而利用MMSE-based detection 來偵測傳送訊號。

Orthogonal frequency-division multiplexing (OFDM) transmission is widely known as an efficient communication technique. In multi-path fading channels, one OFDM symbol is composed of several narrowband sub-channel signals, and it has a long time duration. By adding guard intervals longer than the maximum channel delay between OFDM symbols, OFDM can completely remove the inter-symbol interference (ISI). In the mobile communication, time-variance of channel is a reasonable assumption. Under this assumption, the orthogonality among sub-channels may be loss. In this situation, the inter-channel interference (ICI) results and it incurs in the performance degradation of the OFDM system. From analysis we know that the ICI can’t be mitigated by increasing the signal-to-noise ration (SNR). We need to cancel or reduce the ICI if we want to improve the performance of OFDM system in the fast time-varying channel. Several ICI cancellation methods have been proposed. In this thesis, we first consider an ICI self-cancellation algorithm. This algorithm is simple and very effective to mitigate ICI. This algorithm results a good performance in a mobile communication by adding extra bits. For this algorithm, each OFDM symbol needs extends its original symbol time and this reduces the system throughput. So we modify the system to improve throughput and the modified algorithm also could effectively cancel the ICI. For the modified algorithm, we need not to extend the original symbol time to cancel the ICI except for the first symbol. We produce “pseudo extended symbols” in the receiver to replace the extended portion of an OFDM symbol in the transmitter. We estimate channel response in time domain, and finally use an MMSE-based detection method to estimate transmitted data.

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