正交分頻多工為現今適用於寬頻多媒體資料的重要傳輸技術，不僅已被許多無線通訊系統採用，更逐漸被導入寬頻以及無線行動通訊等應用之中。然而，此系統存在一主要問題，即其系統效能對於頻率偏移效應十分敏感。頻率偏移通常產生於當傳送與接收兩端的本地震盪器頻率無法完美同步時，或是都卜勒效應造成通道響應具有時變現象時，載波之間的正交特性將因此受到破壞而產生載波間干擾，進而降低整體的系統效能。近期文獻提出將訊號分別通過兩條路徑傳輸，並在接收端合併以進行解調後，藉以達到消除載波間干擾的方法，主要包含共軛消除(CC)與相位旋轉共軛消除(PRCC)。此類方法在頻率偏移較小且恆定時固然可提供不錯的載波間干擾消除能力；然而，若應用在時變通道，甚至在頻率偏移較大的環境中，它們往往無法提供令人滿意的系統效能。
在此篇論文中，我們提出一個在正交分頻多工系統中使用共軛傳輸方法的適應性接收機設計，以實現使載波間干擾自我消除之效用。我們的方法利用適應性正規化BLMS演算法，可在接收端透過相位調整追蹤時變通道中的頻率偏移變化。相較於PRCC方法，我們不僅在接收端免除了回傳頻率偏移資訊至傳送端的需求，更藉由即時更新共軛傳輸方法中的相位旋轉值，而增加了系統所能容忍之頻率偏移範圍。根據電腦模擬結果，我們驗證了在時變通道下，本篇論文的方法可達到較CC與PRCC方法更好的位元錯誤率以及載波對干擾功率比。此外，在考慮所使用的頻率偏移估測值有誤差存在的情況下，本論文提出的方法亦能得到比既有方法更穩定的系統效能。

Orthogonal frequency division multiplexing (OFDM) is a wideband transmission technique capable of high-rate multimedia services and has been widely adopted in wireless communication systems. A major problem in OFDM systems is that system performances are highly sensitive to frequency offsets. The mismatch of local oscillators at the transmitter and the receiver causes single frequency offset in the received signal, while a time-varying channel results in a spread of frequency shifts. Each type of frequency offset destroys the orthogonality among subcarriers and induces intercarrier interference (ICI), which could limit overall system performance. To deal with the ICI problems induced from the frequency offset in OFDM systems, two-path parallel cancellation schemes known as conjugate cancellation (CC) and general phase rotated conjugate cancellation (PRCC) were proposed recently. However, since the frequency offset considered in these schemes is modeled as time-invariant, they cannot provide satisfactory performance under time-varying channels, especially those with high frequency offsets.
In this thesis, we have proposed an adaptive receiver design based on the conjugate transmission for ICI self-cancellation in OFDM systems. The proposed scheme utilizes an adaptive normalized BLMS algorithm to adjust the artificial phase rotation factor with frequency offset variations which usually occur under time-varying channels. Unlike PRCC, the proposed receiver is able to employ the frequency offset estimate directly, rather than feed it back to the transmitter and result in an overhead of signaling. Besides, with the adaptive phase rotation design, the tolerable range of frequency offsets can be maximized. Computer simulation results show that the proposed scheme outperforms the CC scheme and the PRCC scheme under time-varying channels in terms of both BER and CIR performances. The robustness of the proposed scheme is also shown to be superior than that of previous works when the frequency offset estimation error is considered.

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