Interleaving effects on BER fairness and PAPR in OFDMA System

Abstract

Considering an Orthogonal Frequency Division Multiple Access (OFDMA) system, in this study we analyze the role of interleavers from their capability in ensuring uniformity of Bit Error Rate (BER) performance amongst the active users. In addition, we also investigate their Peak to Average Power Ratio (PAPR) properties. From the uniformity or fairness in BER perspective, we first consider a generic system and show that for a slowly changing multipath channel, individual user's BER performance can vary from each other to a great extent, implying that the propagation channel effect is unfairly distributed on the users. Applying different types of frequency interleaving mechanisms, we demonstrate that the choice of interleaving can ensure better BER fairness on an individual user basis. In particular, by introducing the application of cyclically shifted random interleaver, we demonstrate its effectiveness in achieving BER fairness dispersion in individual users BER reduced by 89% compared to no interleaving at 15 dB Signal to Noise Ratio. We then explore the comparative performance of different interleavers considering variable number of total subcarriers, variable number of users and variable number of subcarriers per users. Finally, going back to the generic OFDMA, we investigate the PAPR distribution of different interleavers at various active user densities. Based on the results we conclude that when the total number of subcarriers per user is relatively low, i.e., a system which is heavily loaded with active users, cyclically shifted random interleavers can effectively ensure uniform performance amongst active users with reduced system complexity and manageable PAPR. In all other cases, interleaving with fixed amount of subcarrier spacing is the best solution.