An Analysis of Uplink OFDMA Optimality

Motivated by the increasing popularity of OFDMA, this paper studies the sum-rate optimality of OFDMA as an uplink multicarrier multiple-access scheme. We address the relationship between OFDMA and the optimal multicarrier multiple- access schemes. More specifically, we are interested in answering the following three questions: (i) what are the conditions under which OFDMA is sum-rate optimal? (ii) what is the probability of OFDMA being sum-rate optimal? and (iii) in the case OFDMA is suboptimal, what is the performance gap between OFDMA and the optimal multicarrier multiple access solution? Within a generic discrete multicarrier uplink framework, we present the necessary and sufficient conditions for OFDMA optimality and derive the probabilities of these conditions for both low and high SNR regions. Our results show that the number of shared subchannels under the optimal solution should be less than the number of total users, thus the performance gap between the OFDMA scheme and the optimal solution is negligible when the number of subchannels is large.