Performance Comparison of OFDMA and SC-FDMA with Clipping and Filtering

摘要

近年インターネットの普及に伴って大容量かつ高速な通信が求められている.また周波数資源は有限で あるため，限られた帯域を効率よく使う必要がある.そこで注目をあつめている方式として直交周波数分割多元接続 (OFDMA ： Orthogonal Frequency-Division Multiple Access)方法が挙げられる.しかしOFDMAは複数のサブキヤ リァを合成して波を生成するためにピーク対平均電力比（PAPR ： Peak-to-Avarage Power Ratio)が大きくなり電力 増幅器の効率が著しく低下する.そこで本稿ではPAPR特性に優れ通信方式であり，次世代の移動体通信方式の上 り回線として採用されているシングルキャリァ周波数分割多元接続（SC-FDMA: Single-Carrier Frequency-Division Multiple Access)方式に注目する.SC-FDMA方式はシングルキヤリァにおける周波数領域を異なるユーザに割り 当てることにより多元接続を行う方式であり，シングルキヤリァの特性を引き継ぐため，OFDMAと比較して低い PAPRで通信を行うことができる.本稿では，OFDMAおよびSC-FDMAにPARR低減としてクリッピングアン ドフィルタリング（CAF ： Clipping And Filtering)を適用するとともに，低密度パリティ検査（LDPC ： Low-density parity-check)符号を用いた場合の誤り率改善効果について特性比較を行う.%Recently, orthogonal frequency-division multiple access (OFDMA) has been considered as a promising candidate for next-generation mobile communications due to its robustness against multipath fading and high bandwidth efficiency. However, the OFDMA signal has a critical drawback that the peak-to-average power ratio (PAPR) is very high compared to conventional single-carrier signals. For these reasons OFDMA is not considered suitable for battery-driven mobile terminals. In this paper, we analyze the performances of the clipping and filtering (CAF) based PAPR reduction scheme for both the single-carrier frequency-division multiple access (SC-FDMA) and OFDMA systems in terms of peak power reduction capability and degradation in terms of bit error rate (BER). The clipping is performed on the baseband SC-FDMA and OFDMA signals with and without oversampling, follow by the ideal low-pass filter. First, the effect of the envelope clipping power on the PAPR of the band-limited SC-FDMA and OFDMA is studied. We then discuss the BER performances of the oversampled and clipped SC-FDMA and OFDMA signals over the additive white Gaussian noise (AWGN) channels. It is shown that the SNR penalty due to clipping can be considerably alleviated by using channel coding, which is justified by the simulation results using the low-density parity-check (LDPC) code.