正交分頻多工為現今適用於高速多媒體傳輸服務的重要技術，其不僅具有優越的抗多重路徑干擾能力，更有極佳的頻譜效益，因此已被許多主流通訊標準採用並作為核心的傳輸技術。此技術的主要問題之一在於其傳輸效能相較於其他系統更容易受到當頻率偏移產生時，因正交性被破壞所導致的載波間干擾所影響。頻率偏移通常來自於當傳送器與接收器兩端的本地震盪器頻率間無法避免的同步誤差，或在移動環境中產生的都卜勒偏移效應。上述情況中，後者由於無法單純藉由頻率同步方式克服載波間干擾，因此成為較具挑戰性的研究方向。
近期文獻提出了使用共軛傳輸之載波間干擾消除方法，如共軛消除法(CC, conjugate cancellation)與相位旋轉共軛消除法(PRCC, phase rotated conjugate cancellation)，其主要觀念在於若將一訊號分別通過兩條獨立路徑進行共軛傳送，當在接收端藉由特殊方式合併兩條路徑之信號時，既能達到使這些路徑各自的載波間干擾相互消除之效果，亦可獲得多樣性增益，進而提升信號接收品質；其中，PRCC由於額外使用了一個最佳化的相位旋轉，因此可提供相較於CC更好的效能。基於上述方法，Wang更進一步著眼在頻率偏移隨時間變化之環境，提出了使用共軛傳輸機制的適應性接收機設計，以改善在PRCC方法中，相位旋轉無法及時對應頻率偏移變化而造成的效能下降問題。然而，在例如高速移動時造成的快速時變通道中，反映在兩條不同路徑的頻率偏移可能因此產生差異，如此一來，前述的適應性接收機將無法提供有效的載波間干擾消除能力。
在此篇論文中，我們將檢視並分析現有之載波間干擾消除方法在頻率偏移具有高速時變性的環境下的系統效能。接著將針對Wang的方法，提出一個改良式適應性接收機設計。根據電腦模擬結果，我們驗證了在時變通道下，本篇論文所提出的新型適應性接收機可達到較先前方法更好的位元錯誤率以及載波對干擾功率比。此外，我們也驗證了當所使用的通道資訊有誤差存在時，所提出的方法可確實提供穩定的系統效能。

Orthogonal frequency division multiplexing (OFDM) has become one of the most
promising techniques to afford high-rate multimedia services due to its superior multipath
resistance and flexible modulation properties. A major disadvantage of OFDM is its high
sensitivity to frequency synchronization errors or the Doppler effect, which would induce
amplitude attenuation as well as intercarrier interference (ICI). Recently, two-path
diversity transmission schemes, known as conjugate cancellation (CC) and phase rotated
conjugate cancellation (PRCC), were proposed for ICI cancellation in OFDM systems.
Although these schemes have advantages such as good performance, backward
compatibility with the existing OFDM systems, and low receiver complexity, they might
not provide satisfactory performance under channels where the frequency offset changes
with time. To overcome this problem, Wang et al. proposed an adaptive receiver for
OFDM systems using conjugate transmission, which can outperform CC and PRCC at
many aspects. Nevertheless, there would still be some impairments in this adaptive
receiver if the frequency offsets seen in both paths were not identical, which often occurs
in fast time-varying channels.
In this thesis, we not only investigate the existing two-path transmission schemes
for ICI cancellation in OFDM systems, including the benefits and drawbacks under
different channel environments, but also propose a modified version of Wang et al.’s
ii
adaptive receiver. The proposed scheme introduces two phase rotations to the two
transmission paths, instead of using only one common phase rotation for both paths in
Wang et al.’s scheme, where the two phase rotations are adaptively updated using the
normalized block least mean-squared algorithm. Computer simulation results show that
the proposed scheme outperforms CC, PRCC, and Wang et al.’s work under time-varying
channels, especially for those cases with high-order modulation. It is also demonstrated
that the proposed scheme is robust against channel estimation errors.

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