On the Detection Optimality and Exact BER Analysis of NOMA

Non-orthogonal multiple access (NOMA) detection using successive interference cancellation (SIC) and joint multiuser maximum likelihood (JML) has been extensively studied, although their presumed optimality for individual-user bit error rate (BER) minimization is unestablished. Moreover, existing BER analyses overlook the SIC-induced statistical changes. This letter derives optimal per-user maximum a posteriori (MAP) detectors, identifies when the near-user detector reduces to conventional SIC, and proves the equivalence of conventional SIC and JML. An exact average BER analysis accounting for SIC-induced statistical changes is presented. The results show that the proposed detector achieves modest but consistent BER gains at low-to-moderate signal-to-noise ratio (SNR).