臺灣博碩士論文加值系統

正交分頻多工（OFDM）系統有兩大主要的同步問題，其一是時間上的同步問題，其二則是載波頻率偏移量的估測問題。在一個正交分頻多工系統的接收器可以成功地解調接收到的信號之前，信號的邊界必須被清楚地區隔出來，而載波頻率偏移量也必須被估測出來，進而補償它對接收器造成的不良效應。雖然藉由週期前置（cyclic prefix）的技巧可以減輕系統對於時間同步的要求，對於載波頻率偏移量的估測錯誤則希望愈小愈好，因為任何殘留的載波頻率偏移量均會破壞載波之間的正交特性，因而產生載波間干擾。
在IEEE 802.11a無線區域網路實體層規範之中，為了系統同步的問題特別定義了一個實體層收斂程序（PLCP）。於本篇論文中，我們試圖根據此實體層收斂程序發展一完整的系統同步架構。根據此實體層收斂程序，正交分頻多工系統在傳輸一個數據包（packet）時，首先會傳送一組特定的訓練序列，名為PLCP preamble。此一特定的訓練序列由十個短重複訊號及兩個長重複訊號所構成。我們利用計算一個相關長度（correlation length）和短重複訊號相同的時間函數（timing metric）來偵測此一特定的訓練序列的位置。我們發現此特定時間函數於短重複訊號及長重複訊號之交接處會產生一急遽下滑的區間，我們經由統計此一區間的下滑模式產生此下滑區間的統計模型，利用此模型與接收到的信號比對以尋找短重複訊號及長重複訊號之交接處，藉以完成信號邊界的區隔。另外，我們依據IEEE 802.11a 中的規範，利用沒有使用的頻道來完成一低複雜度的頻率擷取（frequency acquisition）架構，此架構可以抵抗多重路徑（multipath）效應及殘餘的時間同步錯誤所造成的頻域接收信號角度旋轉。

There are two major synchronization problems of an orthogonal frequency division multiplexing (OFDM) system. One is known as frame synchronization, and the other is the estimation of the carrier frequency offset. Before an OFDM receiver can successfully demodulate the received signal, the boundary of the frame should be found and the frequency offset should be estimated and compensated. Although the frame synchronization requirement of an OFDM system is eased by the usage of the cyclic prefix, the estimation error of the carrier frequency offset should be kept as small as possible since any amount of the carrier frequency offset will destroy the orthogonality between the subcarriers and introduce intercarrier interference.
In this thesis, we propose a complete synchronization scheme, including detection of the physical layer convergence procedure (PLCP) preamble, frame synchronization, and frequency acquisition, based on the PLCP preamble field that is especially designed for synchronization in the IEEE 802.11a wireless local area network. In the proposed scheme, detection of the PLCP preamble is done by continuously monitoring the timing metrics with a correlation length that is equal to the length of each short symbol in the beginning of the PLCP preamble. Simulation results show that the probabilities of miss detection and false detection of the PLCP preamble are both under . Frame synchronization is achieved by searching the rapid degradation region of the timing metrics, which is caused by the transition from the short-symbol region to the long-symbol region of the PLCP preamble. Simulation results show that the proposed method leads to a smaller mean-squared error than the modified maximum likelihood estimator. Frequency acquisition is accomplished by the unused subcarriers specified in the PLCP preamble. The proposed acquisition scheme can combat the multipath effect and the residual time offset with significantly lower complexity than a previously described scheme.

摘要
誌謝
目錄
第一章 簡介
第二章 OFDM基本原理
第三章 IEEE 802.11a 無線區域網路之實體層規範
第四章 對已存在的同步技術之重新探討
第五章 根據 PLCP Preamble 的同步技術
第六章 結論
附錄 論文英文本

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