802.11a傳輸端減少非線性效應之訊號預先處理

Transmitter Signal Precessing Techniques to Reduce Nonlinear Effects in 802.11a Systems

申家玲(Chia-Ling Shen)

指導教授 ： 李穎

元智大學/工程學院/電機工程學系/碩士(2002年)

摘要

802.11a無線區域網路規格採用OFDM訊號。由於OFDM有很高的功率峰值對平均值比(Peak-to-Average power Ratio；簡稱PAPR)，所以易受無線通訊系統傳輸端非線性扭曲失真的影響。非線性效應會降低傳輸訊號品質，提升帶內訊號(inband signals)之錯誤率，並造成帶外互調(outband intermodulation)，形成鄰近頻帶干擾。我們探討兩種減少非線性效應之訊號預先處理技術：(1)進行波峰修整(peakwindowing)。(2)進行預先矯正(predistortion)。遵照802.11a實體層規格，藉由功率頻譜求出滿足802.11a傳輸頻譜遮罩之最少的功率反減值(output backoff，OBOmask)，可了解兩方法在抑制帶外干擾的表現。也藉由錯誤率曲線了解兩方法在提升接收訊號品質的表現。本研究對peakwindowin及predistortion各自提出改良，並綜合比較兩種方法效能。模擬結果顯示單獨使用波峰修整(peakwindowing)效能有限，並無法有效提升訊號品質，也無法有效抑制帶外干擾 ; 單獨使用預先矯正(predistortion)性能極佳 ; 波峰修整(peakwindowing)與預先矯正(predistortion)互相搭配使用，性能亦佳。

關鍵字

802.11a ； 波峰修整 ； 預先矯正 ； 功率峰值對平均值比 ； 非線性效應

並列摘要

The IEEE 802.11a standard adopts Orthogonal Frequency Division Multiplexing(OFDM) signals.OFDM signal exhibits large peak-to-average power ratio(PAPR), and thus is vulnerable towards transmitter nonlinearity. The nonlinear effects will lead to the increase of inband signal error rate and outband intermodulations. We discuss two techniques to reduce the nonlinearity : (1) peak windowing , (2)predistortion. We compute minimum output backoff (OBOmask) required for the transmit signal power spectrum to satisfy the IEEE 802.11a spectrum mask. OBOmask reflects outband suppression performance. We also evalute the BER curves that reflect inband received signal quality. Simulation results show that using peak windowing alone has limited effect in promoting signal quality and suppressing outband intermodulations. Using predistortion alone performs very well. Using peak windowing and predistortion together perform well, too.

並列關鍵字

802.11a ； peak windowing ； predistortion ； PAPR ； nonlinear effects

參考文獻

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