Relative Sum-Rate Gains of PD-NOMA over OMA under Max-Min, Round-Robin, and Proportional Fairness

This letter presents a comparative analysis of power-domain non-orthogonal multiple access (PD-NOMA) and orthogonal multiple access (OMA) under Max-Min Fairness (MMF), Round Robin (RR), and Proportional Fair (PF) scheduling strategies, with a focus on preserving fairness across both systems. Motivated by the lack of consistent evaluation frameworks for multiuser scenarios, we develop a unified method that ensures identical user rate distributions and resource constraints. To demonstrate fairness preservation, we provide an illustrative example confirming that PD-NOMA maintains the same rate allocation profile as OMA under equivalent scheduling. A stochastic model for single-carrier downlink transmission over Rayleigh fading channels is used to simulate systems with up to 20 users. The results show that PD-NOMA consistently outperforms OMA in terms of sum-rate across all strategies, with the gain increasing with user count and median SNR, while approaching an asymptotic bound. Beyond three users, the gain per additional user becomes negligible, indicating that multiplexing two or three users is optimal for practical throughput enhancement.