展頻通訊目前在無線通訊領域中被廣泛的研究，不外乎它可以有效的利用頻寬，抗多路經干擾，及增加用戶隱私的特性。這類系統最有挑戰的地方是如何更有效率利用頻寬，及抗多通道的頻域選擇性；這些性質將會決定誰可以成為未來的通訊系統主流。而多載子的分碼多工存取處理(MC-CDMA)就是一種可以提高傳輸容量的方法。
多載子的分碼多工存取處理結合了OFDM和CDMA的技術，將可用的頻寬切成許多小段，每個訊號先經過頻域編碼，在對應到不同的正交子通道。這樣做可以把每段子通道視為平的而可以克服通道選擇性。快速傅利葉轉換及反傅利葉轉換可視為其調遍及解調變的方法並且不會增加太多複雜性。他特殊的一個能力就是在前面插入週期性的保護區塊，這區塊使得線性的摺積可視為旋性的摺積並可克服碼際干擾。

一般來說，MC-CDMA並未解決通道等化的問題，只不過將其轉移到頻域上。當不同間的載子正交姓被破壞時，多存取干擾將導致性能衰減。為了保持其正交性，通道必須預估並且等化。之前的論文通常假設通道已經完全知道或是正確的檢測訊號可以回授給估測器，在時間及頻域軸上以導引信號作通道預估，或是利用RLS估測通道再以MMSE接收。但在真實的環境中，完美的信號檢測是永遠無法達到。

這篇論文，我們以廣用的Jakes模型來考慮較真實的通道，估測器並不知道完美的信號檢測，因次下個時間的通道估測必須倚靠之前的信號回授。頻域上的卡門濾波器用來預估通道，子通道追蹤器演算法及增強型MMSE接收器經由考慮預估通道的誤差。這方法可正確並直接預估通道，並用來設計強健型等化器。使其不會因為訊號回授造成嚴重的誤差累積，並可提高傳輸效率。

Although multi-carrier code division multiple access (MC-CDMA) is highly promising for high-capacity wireless local communications, the conventional channel estimation with coherent detection methods cannot provide accurate channel information for equalization design as the fading rate (Doppler Spread) of the channel increases.
This work presents a frequency-domain channel estimation method, which is also capable of estimating the statistic properties of channel estimation error of MC-CDMA in Rayleigh fading channel.

The statistic properties of channel estimation error are useful for repairing in robust equalization design and can be estimated by using the system dynamic via Kalman filter technique in frequency domain.

Following channel estimation, a subcarrier channel-tracking algorithm combined with enhanced MMSE receiver is used for robust equalization design with consideration of channel estimation error to prevent the severe error propagation under fast fading channel and interference.

The performance analysis considering the effect of channel-estimation error is derived. Simulation results indicate that the proposed channel estimation can track the channel fluctuation well and the proposed enhanced MMSE method can achieve more robust equalization of MC-CDMA communication systems under the multipath Rayleigh fading channel and narrowband interferences.

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