Improved PAPR Reduction in OFDM Systems

Abstract

Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier modulation scheme, which is widely used in various wireless communication systems and standards due to its high data rate, high spectral efficiency and robustness to multipath fading channel. However, peak-to-average ratio (PAPR) reduction and intercarrier interference (ICI) cancellation are two major challenges in implementing an OFDM system. High PAPR results from large envelope fluctuations in OFDM signal which requires a highly linear power amplifier (PA). Power amplifiers with large linear range are bulky, costly and difficult to manufacture. Therefore, various PAPR reduction techniques have been proposed in literature to reduce the PAPR of OFDM signal. In OFDM system the subcarriers are narrowband and require tight frequency synchronization between the transmitting and receiving ends. A small carrier frequency offset (CFO) between transmitter and receiver local oscillators can disturb the orthogonality among the subcarriers and cause intercarrier interference (ICI). This thesis focuses mainly on PAPR reduction of OFDM systems by using companding, partial transmit sequence (PTS) and selected mapping schemes (SLM). We have proposed a quadrilateral companding transform (QCT) to reduce the PAPR of OFDM signal. It transforms the original distribution function of OFDM signal magnitude into a quadrilateral distribution function. The QCT has better design flexibility and more degrees of freedom than the other schemes, therefore a favorable trade-off between PAPR reduction and BER performance can be achieved. QCT can effectively reduce the effect of companding distortion such that BER performance of the OFDM system gets improved in comparison to existing companding schemes for a given PAPR.