Active constellation modification techniques for OFDM PAR reduction

Abstract

OFDM is a widely adopted modulation technique for wireless communication. However, the OFDM waveform still suffers from a large peak-to-average power ratio (PAR). Larger PAR leads to higher transmit power inefficiency. In this paper, we compare two constellation modification techniques, constellation distortion (CD) and active constellation extension (ACE), to reduce the PAR of the OFDM waveform. Both techniques are similar, in the sense that the constellation symbols are modified to reduce PAR. In CD, the tradeoff is between PAR reduction and added distortion, which results in a larger bit error rate (BER). In ACE, the tradeoff is between added power and PAR reduction. A metric called normalized total power (NTP) is introduced to facilitate a comparison between these two different techniques. Using this metric, we compare the performance of the two techniques using an 802.11a OFDM waveform. We present comparisons, via simulation, in both coded and uncoded cases. The simulation results show that, for an uncoded OFDM system, ACE performs better than CD for all constellations except QPSK. However, CD is preferred over ACE when coding is present. NTP improvements using CD can be up to 4.1dB in the case of coded QPSK and up to 0.8 dB using ACE for uncoded QPSK.