正交分頻多工系統中降低峰均值功率比方法之研究

Abstract

為了使傳輸速率能夠更加快速，各國的研究人員發展出一連串提升傳輸率的方式。在近幾年來，正交分頻多工系統被視為增加傳輸速率的主要核心。透過將頻帶切成許許多多正交的子載波，正交分頻多工系統可提高頻譜使用效能，並且多重路徑的效果能被以簡單的方式等化掉。總結以上的優點，正交分頻多工系統被視為近代主流的通訊系統.

然而，正交分頻多工也有許多的缺點，其中一個主要問題是其高峰均值功率比。當傳輸訊號操作在功率放大器的非線性區時，會造成訊號的失真，以及對鄰近頻帶的干擾。而在我的研究中，我們提出三種降低高峰均值功率比的方法。首先，我們提出不同於傳統SLM方法，透過使用展頻碼來傳送SLM方法的側信息，並且達到幾近相同的表現。第二，透過使用展頻碼來傳送在消波方法中被截掉的訊號，藉以改善消波方法的錯誤率表現。第三，我們提出將訊號傳送前，使用濾波器將波形整形，藉以降低峰均值功率比。從模擬的結果顯示，透過提出的這些方法，不但峰均值功率比能夠降低，錯誤率亦可改善。

In order to meet the extreme requirement of data rate, researchers around the world have developed a lot of rate-increasing methods. Recently, orthogonal frequency division multiplexing(OFDM) has been regarded as one of the core rate-increasing technologies. By dividing the entire bandwidth into narrowband subcarriers, OFDM can provide high spectral efficiency. Also, the multipath effect can be easily equalized. Summarizing the above advantages, OFDM has become a key air interface for most modern wireless communication system.

However, OFDM system suffers from several difficulties. One of the major drawbacks is the high peak to average power ratio(PAPR) at the transmitter. As signals operate on the nonlinearity of a high power amplifier(HPA), it is distorted and causes in-band distortion and out-of-band radiation. In our research, we propose three different PAPR reduction methods. First, a SLM method with CDMA side information is proposed. The difference between the conventional SLM method and the proposed one is that it provides another channel to transmit the side information, i.e., the index of phase sequence. Second, in order to improve the BER performance of traditional clipping scheme, we exploit some spreading codes to transmit the lost information caused by clipping. Third, a novel idea, in which OFDM waveform is shaped by an FIR filter or an all pass filter, is proposed. The numerical results show that by using the proposed methods, the PAPR of OFDM can be reduced, and the performance of OFDM is improved.