在正交分頻多工 (OFDM) 系統中，載波間干擾 (ICI) 是影響系統效能的重要因素；這是由於載波頻率間隔 (subcarrier spacing) 窄以及允許頻譜重疊，使得載波間的正交特性 (orthogonality) 易受傳送、接收兩端震盪器不匹配及都卜勒 (Doppler) 效應所破壞的緣故。 傳統的載波間干擾自動消除技術 (ICI self-cancellation scheme) 不需透過頻率偏移補償或頻域等化器 (FEQ)等，即可大幅降低載波間干擾，但卻必須付出頻譜效率 (spectral efficiency) 減半的代價。而利用線性相關碼 (correlative code) 壓抑載波間干擾的技術則不會降低頻譜效率，但是在壓抑頻譜間干擾的效能上卻不夠顯著，而且可能造成錯誤跨OFDM符號傳遞。
在此篇研究中，基於修改線性相關碼壓抑載波間干擾的技術，我們提出了一個新的壓抑載波間干擾的技術，其中主要的不同在於，我們提出的方法中，最後一個子載波上的訊號是與同一個OFDM符號中的第一個子載波上的訊號一起編碼，取代了傳統系統中與下一個OFDM符號中的第一個子載波上的訊號一起編碼，以使其更能抵抗載波間干擾，並且改善錯誤跨OFDM符號傳遞的問題。和原本的系統相比較，我們所提出之方法能只需額外付出些微的硬體複雜度更有效的壓抑載波間干擾，並且防止了錯誤跨OFDM符號傳遞的問題。為了驗證系統效能，我們不僅提供了載波能量對干擾能量比(CIR)的理論推導，也提供了數個電腦模擬結果，兩者都顯示了我們的方法能更有效的壓抑載波間干擾。另外，位元錯誤率 (bit error rate) 的模擬則顯示了修正後的方法因為不會有錯誤跨OFDM符號傳遞的問題而得到了改善。

In orthogonal frequency division multiplexing (OFDM) systems, the property of spectrum overlapping in an orthogonal way makes the system very sensitive to the frequency offset, which is usually introduced by the mismatch of oscillators in the transmitter and receiver or the Doppler effect. It is well known that the intercarrier interference (ICI) caused by the frequency offset may significantly degrade the system performance. Conventional ICI self-cancellation schemes can mitigate the ICI effect dramatically without doing frequency offset compensation, but the expense of halving spectral efficiency makes them less valuable. On the other hand, ICI can be suppressed by performing correlative coding in the frequency domain without halving spectral efficiency; however, this scheme usually cannot provide satisfactory performance in terms of the carrier-to-interference ratio (CIR) and the detection errors generated in the receiver would be propagated through OFDM symbols.
In this thesis, we propose a new ICI suppression scheme that can be regarded as an improved version of the approach based on correlative coding; the main difference is that, for the proposed scheme, the data of the last subcarrier in an OFDM symbol is encoded with that of the first subcarrier in the same OFDM symbol, instead of the data of the first subcarrier in the subsequent OFDM symbol used for the original approach. Due to the cyclic nature, the modified correlative encoding procedure in the frequency domain can be easily realized by a specific time-domain windowing process. As compared to the original correlative coding scheme, the modified approach achieves better performance for ICI suppression and prevents error propagation through OFDM symbols, with a slight increase in the computational complexity at the transmitter but no increase at the receiver. To verify the effectiveness of the proposed scheme, we not only derive the theoretical CIR, but also present a number of computer simulation results, where both of them match pretty well.

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