多重輸入輸出正交分頻多工系統之同步設計研究

Abstract

由於能有效處理頻率選擇性衰減的問題，正交分頻多工技術成為無線通訊技術中被廣泛使用的技術。高速資料傳送及高品質的通訊服務已成為無線通訊發展的主要挑戰之一，而多傳輸天線及多接收天線系統設計是有效克服這些問題的方法。然而，正交分頻多工技術最主要的缺點在於對頻率及時間偏移的敏感。在本篇論文裡，我們設計一套全面的同步系統，適用於多天線正交分頻多工的架構。此外，使用近似於IEEE 802.1n 的設定來進行模擬。這個同步流程的設計包括封包偵測，頻率偏移的同步技術，符元時間偏移估測及時脈同步設計。在封包偵測的部份，我們把自相關運算的輸出值和訊號強度門檻做比較來偵測出封包的開端[44]。在符元時間偏移估測部分，我們先使用兩個接續窗口方法進行粗略的符元時間偏移估測，再用所提出的簡化直接時間維度精確符元時間同步(STDFS)方法去微調得到更精確的符元時間偏移估測。由於 IEEE 802.11n 先導訊號的特色，我們提出平滑方法利用其特色藉由平均自相關運算輸出值去加強頻率偏移的估測。時脈偏移估測部分，我們藉由只運算最為分離的引導訊號去降低運算複雜度。至於時脈偏移補償部分，經由模擬結果，我們建議在IEEE 802.11n 系統使用Lagrange 內插器較為適合。

Since the ability to deal with the frequency-selective fading channel, orthogonal frequency division multiplexing (OFDM) is a popular method for wireless communication. The development of wireless communication for high date rate and high-quality service is becoming one of the major challenging targets in wireless communication. Multiple-input multiple-output (MIMO) OFDM technique is an efficient solution for these targets. However, a major drawback of OFDM is the high sensitivity to frequency and timing offset. We propose an overall synchronization scheme suitable for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems in this thesis. Moreover, we conduct the simulations in IEEE 802.11n-like setting. The synchronization schemes considered include frame timing detection, carrier frequency offset synchronization, symbol timing estimation and sampling clock offset synchronization. In the frame detection, we detect the beginning of a frame by comparing the auto-correlation outputs to the signal power threshold [44]. In the symbol timing synchronization, we perform the coarse symbol timing estimation first with the double-sliding-window method [45], then adjust the symbol timing by the proposed simplified direct time-domain fine symbol timing synchronization (STDFS) method to get a more accurate symbol timing estimation than the current techniques. Owing to the features of the IEEE 802.11n preambles, we propose a smoothed method which utilizes the features to enhance the carrier frequency offset estimation by averaging the available auto-correlation outputs. In the sampling clock offset estimation, we reduce the computation complexity by only computing the most apart pilots. As for the sampling clock compensation, we find that Lagrange interpolator is suitable for IEEE 802.11n systems from simulation results.