線性相位變化降低正交分頻多工訊號功率峰均比

PAPR Reduction of OFDM Signal by Linear Phase Variation

謝侑伸(Yu-Shen Hsieh)

指導教授 ： 李穎

元智大學/工程學院/通訊工程學系/碩士(2004年)

摘要

正交分頻多工(OFDM)技術已被廣泛使用於現今通訊系統中，但其高PAPR的特性使放大器功率效率低落，因此如何有效降低OFDM訊號的PAPR為一研究主題。本論文提出以線性相位變化法降低OFDM訊號的PAPR：頻域上對各子區塊加上線性相位對應到時域上的操作，是利用循環平移的概念，將各子區塊的時域波形高峰值錯開。觀念與傳統的部份傳輸序列(PTS)不同，部分傳輸序列是對每個子區塊的所有子載波加上相同的相位變化。對線性相位變化、常數相位變化與頻帶保留進行子載波數8、52、128的模擬，發現線性相位變化法可以得到最多的PAPR降低量。此外，將上述三種方法對子載波數8，分為兩個大小相等的子區塊進行窮盡搜尋，以布林函數基底分解後，發現只有線性相位變化法可以得到所有長度8的低PAPR序列。以802.11a模擬訊號顯示，線性相位變化法在PAPR CCDF=10-3約降低PAPR 4.1dB時，符合頻譜遮罩、EVM規格的發射訊號所需功率反減值約可減少0.86dB，的確可提升功率效率。

關鍵字

正交分頻多工 ； 功率峰均比 ； 線性相位 ； 循環平移

並列摘要

OFDM technique has been employed widely in communication systems nowadays. Its’ high peak to average power ratio (PAPR) results in low efficiency of the amplifier, therefore, reducing the PAPR for OFDM signal has become a topic for research. PAPR reduction for OFDM signal by linear phase variations is proposed in this thesis. Applying linear phase variations to subblocks of subcarriers in frequency domain is equivalent to applying cyclic shifts in time domain and can avoid peak overlapping. The concept is different from the conventional partial transmit sequence (PTS) techniques in that PTS applie constant phase variations to subblocks.

Linear phase variation achieves the best PAPR reduction performance in our simulation comparing PAPR reduction with linear phase variation, constant phase variation and tone reservation for 8, 52, and 128-subcarrier QPSK-OFDM signals. Exhaustive test in the 8-subcarrier case when the subcarriers are partitioned into two equal-sized subblocks of consecutive subcarriers shows that all quaternary length 8 Golay sequences are selected with linear phase variation.

Simulations with 802.11a signals applying linear phase variations show that when the PAPR reduction is 4.1dB at PAPR CCDF value 10-3, the output backoff required to satisfy the spectrum mask and EVM is reduced by approximately 0.7dB and 0.54dB, i.e. the amplifier power efficiency is raised indeed.

並列關鍵字

OFDM ； PAPR ； linear phase ； cyclic shift

參考文獻

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