多重輸入多重輸出之正交分頻多工系統有一個主要的缺點，是其輸出信號會產生相對高的功率峰均比使得訊號經過功率放大器時產生非線性失真。雖然已有許多降低其功率峰均比的技術被提出，但運算複雜度高。而且這些技術大部分不適用於使用空間-頻域區塊碼的系統。
在本論文中，我們針對空間-頻域區塊碼之多重輸入多重輸出正交分頻多工系統提出低複雜度功率峰均比的降低方法。我們首先藉由對輸入訊號乘上不同的旋轉向量來產生多個彼此可視為統計獨立的訊號，再運用空間-頻域區塊碼的線性特性分別把這些訊號分成多個子訊號。這些子訊號在經過快速傅立葉轉換後，我們再利用快速傅立葉轉換的特性在時域中進行等同於頻域的空間-頻域編碼，並藉由選擇屬於不同組的子訊號進行編碼以產生更多的候選訊號。在所提出的方法中，我們只須執行少量的快速傅立葉轉換就可在時域中產生大量彼此相關性小的候選訊號，進而選出具有最低功率峰均比的訊號作為傳輸訊號。此外我們更設定一個門檻值來避免產生不必要的候選訊號以降低在選擇最佳候選訊號的運算複雜度。經由模擬和比較結果我們可看出;相較於其他可適用於空間-頻域區塊碼的功率峰均比降低技術，我們所提出的方法可以低的複雜度達到不錯的功率峰均比降低的效能。

One inherent drawback associated with multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems is the high peak-to-average power ratio (PAPR) at the transmitter’s output, and this usually causes undesirable nonlinear distortions. There have been a number of PAPR reduction techniques proposed for MIMO-OFDM systems, but their computational complexity is too high to be useful for practical applications. Besides, most of them cannot be used for MIMO-OFDM systems with space frequency block coding (SFBC).
In this thesis, we propose a low-complexity PAPR reduction scheme for SFBC-based MIMO-OFDM systems. We first multiply the input sequence by a set of phase rotation vectors respectively and then utilize the linear property of SFBC to decompose each resulting sequence into several sub-sequences. After computing the inverse fast Fourier transform (IFFT) to convert each frequency-domain sub-sequence into a time-domain signal, we utilize the IFFT properties to do equivalent SFBC encoding operations in the time-domain for generating candidate signal sets, where the one with the lowest maximum PAPR is selected for transmission. Based on the proposed approach, we can obtain a large number of candidate signal sets by computing only a few IFFTs. Moreover, we reduce the complexity of selecting the optimal candidate signal set by setting a threshold to avoid generating unnecessary candidate signal sets. Simulation results show that, with lower computational complexity, the proposed scheme has comparable PAPR reduction performance to existing schemes for SFBC-based MIMO-OFDM systems.

摘 要
目 錄
第一章 簡介
第二章 功率峰均比以及其相關降低技術
第三章 應用於空間-頻域區塊編碼之 MIMO-OFDM系統之低複雜度功率峰均比降低技術
第四章 模擬結果及比較
第五章 結論
附錄 論文英文本

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