正交分頻多工系統能滿足高速和高品質的需求，因而被廣泛使用在現在和未來的無線通訊系統中。但是頻率偏移對於正交分頻多工系統有很明顯的影響。這是由於載波頻率間隔縮短使得載波間正交特性容易受到傳送和接收兩端震盪器頻率不匹配以及都卜勒效應的影響而被破壞，因此系統效能會受到載波間干擾而降低。為了達到更好的系統效能，利用兩條平行路徑消除載波間干擾的方法被提出了。這種稱作共軛消除的方法在頻率偏移小的時候能有效改善載波對干擾能量比的效能，然而在頻率偏移大的時候則無法提供令人滿意的效能。除此之外，在頻率偏移大於0.25時甚至會比傳統正交分頻多工系統的效能差。
在此篇論文中，我們提出了修正式共軛傳輸的方法和增強的方法。不像傳統共軛消除方法的第二條路徑的訊號是第一條路徑的訊號作共軛所得到的輸出，我們提出修正的第二條路徑的訊號是第一條路徑的訊號乘上j之後再作共軛所得到的輸出。為了驗證我們所提出來的方法的系統效能，我們提供了載波對干擾能量比和位元錯誤率的電腦模擬結果。經由模擬結果，可以知道我們提出的方法在頻率偏移大的時候仍然能提供不錯的系統效能。除此之外，若我們能得知簡略的頻率偏移資訊的話，我們可以利用增強的方法來更進一步改善系統效能。

Orthogonal frequency division multiplexing (OFDM) has become one of the promising techniques for future wireless communication systems to achieve high-speed and high-quality digital transmission and reception. However, this technique usually has high sensitivity to frequency offset which is often caused by the Doppler shift and/or the mismatch between oscillators in the transmitter and receiver. Such frequency offset distorts the orthogonality among the subcarriers and then introduces intercarrier interference (ICI) which lowers the overall system performance. To achieve better performance, a two-path parallel cancellation scheme known as conjugate cancellation (CC) was proposed recently. Although this scheme can effectively improve the carrier-to-interference ratio (CIR) performance at low frequency offset situations, it cannot provide satisfactory performance when the frequency offset is high. Furthermore, the CIR performance of the CC scheme is even worse than a regular OFDM system when the frequency offset is higher than 0.25.
In this thesis, we propose a modified two-path conjugate transmission scheme along with an enhanced method for intercarrier interference cancellation in OFDM systems. Unlike conventional CC scheme where the second-path’s signal is formed by the conjugate of the first one, the proposed scheme employs the conjugate of the first-path signal multiplied by j as the second path’s signal. Simulation results show that the proposed scheme can achieve better CIR and bit error rate (BER) performance than the CC scheme at high frequency offset situations. Moreover, if coarse frequency offset information is available, the enhanced method can further improve the system performance.

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