Contents 1. Introduction
OFDM is a multicarrier modulation scheme that divides the incoming bitstream into parallel, lower rate substreams and transmits them over orthogonal subcarriers, so that the bandwidth of each subcarrier is much smaller than channel coherence bandwidth and hence, each subcarrier will experience relatively flat fade [1]. It is a bandwidth efficient modulation scheme and has the advantage of eliminating intersymbol interference (ISI) in frequency selective fading channels. OFDM today used for terrestrial digital video broadcasting (DVB-T), digital audio broadcasting (DAB-T), and adopted in wireless local area networks (IEEE 802.11 a, ETSI Hiperlan2) as well as wireless metropolitan area networks (IEEE 802.16d). The main drawback of OFDM is its high peak-to-average power ratio (PAPR) which causes serious degradation in performance when nonlinear power amplifier is used. This high PAPR forces the transmit power amplifier to have a large backoff in order to ensure linear amplification of the signal, which significantly reduces the efficiency of the amplifier. Furthermore, high PAPR requires high resolution for the receiver A/D converter. Since the dynamic range of the signal is much larger for high PAPR signals, a high-resolution quantizer is required to reduce quantization error, which requires more bits and places a complexity and power burden on the receiver front end. In the literature, many solutions have been proposed and suggested to reduce PAPR [2]. However, there is no single winning solution, everyone come at the cost of bit error rate, data rate, simplicity. Nevertheless, simplicity and effectiveness are important requirements. Signal clipping and -law companding have been suggested as simple and effective PAPR reduction schemes, [3]–[7]. In [3], the effectiveness of clipping as PAPR reduction scheme is evaluated. It is shown that clipping is an excellent P APR reduction scheme; however, when considering the more important measure of total degradation, it is shown that clipping is ineffective at improving performance. The distortion caused by the clipping offsets the gain in PAPR reduction. In [7], the use of -Iaw companding for PAPR reduction is explored for a link involving a nonlinear transmit power amplifier with OFDM. The results from the study show that at low backoffs, impairments from nonlinear amplification can be significant. Increasing the compression (via the companding parameter, ) reduces the nonlinear distortion and suggests that moving the operating point toward saturation might be beneficial. However, increasing the companding parameter also leads to noise amplification during expansion at the receiver, which potentially can largely negate any performance gains that would otherwise result from higher values of . This paper analyzes and compares the performance of clipping and -law companding APR reduction schemes for M-PSK/OFDM signal in AWGN channel with the presence of nonlinear power amplification. The simulation is performed in MATLAB® environment. The paper is organized as follows: Section 2 introduces the system model and is addressing PAPR and the relation between it and the number of subcarriers. Sections 3 and 4, analyze clipping and companding schemes respectively. Section 5 introduces the nonlinear power amplifier model, while section 6 discusses the simulation results. The paper is finally concluded in section 7.
