MC-CDMA系統傳輸端非線性的預先補償

Predistortion Compensation for Transmitter Nonlinearity in MC-CDMA systems

劉家馨(Chia-Hsin Liu)

指導教授 ： 李穎

元智大學/工程學院/電機工程學系/碩士(2001年)

摘要

多載波分碼多工(Multi-Carrier Code-Division-Multiple-Access，簡稱MC-CDMA)是傳統的直接序列展頻分碼多工(DS-CDMA)與正交分頻多工(OFDM)的結合體。它具有高速資料傳輸、抗多路徑延遲擴展、頻譜利用率高的優點，是下一代無線通訊系統中熱門的傳輸調變選擇。由於MC-CDMA幾乎使用OFDM的結構，因此MC-CDMA訊號有很高的傳輸功率峰值對平均值比(Peak-to-Average power Ratio；簡稱PAR)，所以易受無線通訊系統傳輸端的功率放大器(Power Amplifier，簡稱PA)非線性扭曲失真的影響，而嚴重降低訊號傳輸的品質。本研究將以基頻數位預先補償器的方法，補償MC-CDMA系統中功率放大器之非線性現象。使用的補償器包含原用於OFDM系統的並列可適性預先補償線性器(Parallel Adaptive Predistortion Linearizer；PAP-LZR)、MC-CDMA系統的實數值定點遞迴(Real-valued Fixed Point Iteration)補償器、應用於OFDM系統的FLUT方法，並對FLUT加入內插法，以提升FLUT Predistorter之補償效果。由電腦模擬結果比較上述幾種方法收斂速度、訓練時間、補償性能、頻域上outband IM壓抑的程度的差異。本研究並提出基於頻域上功率分佈的性能指標，可提供Predistorter設計的參考。

關鍵字

多載波分碼多工 ； 直接序列展頻分碼多工 ； 正交分頻多工 ； 功率放大器 ； 預先補償器 ； 快速查表法

並列摘要

Multi-Carrier Code-Division-Multiple-Access technique is the combination of conventional DS-CDMA with OFDM. The advantages of MC-CDMA are high data rate transmission, robustness against multipath delay spread and high spectral efficiency. MC-CDMA is a promising candidate for future wireless communication systems. However, MC-CDMA is significantly more sensitive to nonlinear distortions caused by power amplifier(PA) due to its high peak to average power ratio(PAR) due to OFDM. In this thesis, we use baseband digital adaptive signal predistorter to compensate for distortions introduced by nonlinear PA. The predistorters we tested include Parallel Adaptive Predistortion Linearizer previously used for OFDM signals, Real-valued Fixed Point Iteration Predistorter used for MC-CDMA signals and Look Up Table based Predistorter(FLUT) previously used for OFDM signals. To improve the performance, we propose to use interpolation in FLUT. Simulations are conducted to compare the convergence rate, training time, performance in terms of total degradation and the spectral distribution of signal powers. We propose to use the sum of the in-band signal power and the difference of the in-band and out-band signal power as a performance measure to be maximized. Compare to total degradation, this measure can more directly reflect the suppression of outband intermodulation.

並列關鍵字

MC-CDMA ； DS-CDMA ； OFDM ； Power Amplifier ； Predistorter ； FLUT

參考文獻

[1]K. Fazel and S.Kaiser, “Analysis of non-linear distortions on MC-CDMA, ” IEEE Communications, ICC 98. Conference Record,pp1028-1034, 1998.

[2]Jaehyun Jeon, Yoan Shin, Sungbin Im “A data predistortion

technique for the compensation of nonlinear distortion in MC-CDMA”, IEEE Workshop , pp. 174-177, 1999.

[4]Won Gi Jeon, Kyung Hi Chang, and Yong Soo Cho, “An adaptive data predistorter for compensation of nonlinear distortion in OFDM Systems”, IEEE Transactions on Communications, vol.45 , NO. 10 pp. 1167—1171, OCTOBER 1997.

[5]Geog Geun, Jeong Goo Kim, and Eon Kyeong Joo, “A novel subblock partition scheme for partial transmit sequence OFDM”, IEEE Transactions on broadcasting, vol.45, NO. 3 pp. 333 -338 , September 1999.