NON LINEAR DISTORTION AND COMPENSATION TECHNIQUE IN WIMAX BROADBAND

Abstract

WiMAX (Worldwide Interoperability for Microwave Access) is a wireless communications standard designed to provide 30 to 40 Mbps data rates. The physical layer of WiMAX is based on Orthogonal Frequency Division Multiplexing (OFDM) technique. OFDM is a Frequency Division Multiplexing (FDM) technique used as a digital multi-carrier modulation method. The major advantages of OFDM are robustness in multipath fading and high spectral utilization efficiency. However, it has some serious problem like highly sensitive to timing and frequency offset, and more specifically Peak-to-Average Power Ratio (PAPR) problem. Since OFDM is constructive superposition of the subcarriers, it has significant numbers of large peak resulting in high PAPR causing large fluctuation in input signal which requires the use of highly linear amplifier. Due to the amplifier imperfection the peaks are distorted non-linearly which generates inter modulation product causing both in-band distortion and Out-of-Band (OOB) radiation. The design of a compensation technique to improve the linearity of the power amplifier is performed in this thesis work. The adaptive pre-distortion method to compensate the non-linearity of power amplifier includes the LSE estimation of non-linearity and pre-distortion techniques. The analysis of Phase Realignment (PR) and Modified Phase Realignment (MPR) techniques for PAPR reduction for the performance improvement of the system showed that the combination of predistortion and PR/MPR technique has superior capability in mitigation of the nonlinear distortion as compared to the implementation of individual techniques.