Modeling and analysis of OFDM with adaptive clipping technique for PAPR reduction

Abstract

Orthogonal Frequency Division Multiplexing (OFDM) systems are better than single-carrier systems in multipath fading channel environment. OFDM systems are being adapted in many wire-line and wireless high data rate transmission systems of digital video broadcasting (DVB), IEEE 802.11, IEEE 802.16, HIPERLAN Type II, Digital Subscriber Line (DSL), and Home networking etc. There is also strong interest to use OFDM systems in 4G wireless systems. OFDM has recently received increased attention due to its capability of supporting high data rate communication in frequency selective fading environments which cause Inter symbol Interference(ISI). In order to take advantage of the diversity provided by the multi-path fading, appropriate frequency interleaving and coding is necessary. Therefore, coding becomes an inseparable part in most OFDM applications and a considerable amount of research has focused on optimum encoder and decoder design for information transmission through OFDM over fading environments. The OFDM systems use multiple orthogonal subcarriers. Transmission data is loaded on each subcarrier and transmitted after summation. When all subcarriers have same phase than instantaneous power of transmitted signal is very high. The peak power of OFDM scheme is higher than average power. This phenomenon is called PAPR problem. This is one of the main disadvantages of the OFDM system. Power amplifier characteristics are linear until some input value, so for higher peak powers the amplifier characteristic may be nonlinear. If peak powers are not handled in linear part, OFDM signals will get distorted. A definition of PAPR is log-scale of peak power over average power, and PAPR problem appears in all multicarrier systems. Traditionally several techniques are used for reducing PAPR instead of catering for higher peak powers in amplifiers. First, clipping technique is the most famous and simple technique. But it has BER(Bit Error Rate) performance degradation. Second, peak power avoidance precoding technique is used. It has some coding gain but it decrease data rate or increase bandwidth. Third, scrambling technique is used. With the scrambling technique, probability of peak power occurrence goes low, but hardware architecture is more complex. In this thesis, a joint solution is proposed with RS coding, OFDM, and PAPR clipping. We implemented the hybrid method which consists of RS coding and adaptive clipping technique over an additive white Gaussian noise (AWGN) channel. Reed Solomon RS (255, 239) coding can correct 8 symbol errors from 239 symbols data. This capability can effectively compensate for the performance degradation resulted by setting PAPR threshold to 5 in case of 256 QAM, and RS (63, 47) and threshold of 4 incase of 64 QAM. Binary data are grouped into ‘x’ bits and encoded by RS (255, 239) encoder and then modulated by 256 QAM. For 64 QAM, RS (63, 47) is used. In a typical OFDM system consists of N = 52 subcarriers and 64 point IFFT is used. The adaptive clipping technique is used with clipping ratio of 5 for 256 QAM, and clipping ratio of 4 for 64 QAM is used. The symbols are transmitted through AWGN channel. The receiver structure has reciprocal to the transmitter architecture. The implemented hybrid technique based on RS coding and adaptive clipping technique method to compensate the performance degradation caused by clipping. From the simulation results, by using hybrid technique the clipping distortion can be removed when CR = 5 and SNR = 26.5 dB for 256 QAM, and CR = 4 and SNR = 20.5 dB for 64 QAM. The simulation results show that the hybrid method is an effective technique to mitigate the clipping distortions.