以訊號扭曲法改善正交分頻多工訊號之非線性容忍度

Nonlinear Tolerance Improvement for OFDM Signals with Signal Distortion Techniques

游益儀(Yih-Yi Yu)

指導教授 ： 李穎

元智大學/工程學院/電機工程學系/碩士(2003年)

摘要

正交分頻多工技術具有高頻寬效率，且能在多路徑傳輸環境中提供高可靠度的通訊。其一主要問題是，訊號的峰均比高（PAPR），因此對傳輸端非線性容忍度低，造成功率效率不佳。本論文探討以訊號扭曲改善此問題的方法，包括使用裁減相關方法降低功率峰均比及預先矯正。本文使用滿足系統規格對訊號品質要求下的最小功率反減（OBO），及滿足系統要求且達到最小整體衰減的最小功率反減為性能指標。非線性容忍度的改善可視為能降低滿足訊號品質要求所需的最小功率反減值。以802.11a訊號模擬使用不同方法並改變裁減臨限的結果顯示：以訊號扭曲降低峰均比使帶內帶外品質均變差，無法改善非線性容忍度。濾波處理(filtering)、波峰修整（peak windowing）及（peak cancellation）等可抑制帶外干擾，但也損及帶內訊號品質。整體說來，訊號扭曲法降PAPR雖然可有效降低PAPR但不能有效改善非線性容忍度。而預先矯正則可改善放大器飽和點以下的非線性影響，提升訊號的帶內帶外品質。

關鍵字

正交分頻多工訊號 ； 功率峰均比 ； 非線性容忍度 ； 訊號扭曲法 ； 預先矯正 ； 波峰裁減 ； 波峰修整 ； 波峰消減

並列摘要

Orthogonal Frequency Division Multiplexing (OFDM) signal has high spectrum efficiency and high communication reliability under multipath transmission environment. One major difficulty is its large peak-to-average power ratio (PAPR), which reduces its nonlinear tolerance and the efficiency of the power amplifier. This thesis investigates signal distortion techniques that aim to improve the nonlinear tolerance for OFDM signals, including PAPR reduction through clipping-related methods and predistortion. Simulations with 802.11a signals are performed for performance comparison. Nonlinear tolerance improvement is viewed as the reduction of the minimum output backoff (OBO) required to meet the 802.11a transmit spectrum mask, error vector magnitude (EVM), and the OBO that achieves the minimum total degradation (MTD). Simulations show that reducing PAPR with signal distortion schemes induce the penalties of additional inband intermodulation and outband interference. Suppressing outband interference with low pass filter or peak windowing leads to more inband distortion and increased bit error rate. We conclude that clipping-related PAPR reduction methods are not effective for nonlinear tolerance improvement since the required OBO cannot be reduced even though PAPR can be easily reduced. On the other hand, predistortion does reduce the required OBO and thus is effective in improving the nonlinear tolerance of OFDM signals.

並列關鍵字

OFDM ； PAPR ； Nonlinear Tolerance ； Signal Distortion ； Predistortion ； Peak Clipping ； Peak Windowing ； Peak Cancellation

參考文獻

[Cim00] L. J. Jr. Cimini, N.R. Sollenberger, “Peak-to-average power ratio reduction of an OFDM signal using partial transmit sequences,” IEEE Communications Letters , Vol. 4 , pp. 86 —88, March, 2000.

[Dav97] J.A. Davis, J. Jedwab, “Peak-to-mean power control and error correction for OFDM transmission using Golay sequences and Reed-Muller codes,” Electronics Letters, vol.33 4, pp. 267—268, 13 Feb. 1997.

[Din01] R. Dinis, A. Gusmão, “Signal Processing Schemes for Power/Bandwidth Efficient OFDM Transmission with Conventional Or LINC Transmitter Structures,” IEEE Communications Conference 2001, vol.4, pp. 1021-1027, 2001.

[Din99] R. Dinis and A. Gusmão, “On the Performance Evaluation of OFDM Transmission Using Clipping Techniques,” IEEE VTC’99 (Fall), Amsterdam, Sept. 1999.

[Hor01] T. Horiuchi, Weizu Yang, T. Ohtsuki, I. Sasase, “Performance evaluation of OFDM with the compensation technique of the nonlinear distortion using partial transmit sequence and predistortion,” IEEE 54th Vehicular Technology Conference, Vol. 2, pp. 655 —659, Fall, 2001.