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Existing work regarding the performance comparison between nonorthogonal multiple access (NOMA) and orthogonal multiple access (OMA) can be generally divided into two categories. The work in the first category aims to develop analytical results for the comparison, often with fixed system parameters. The work in the second category aims to propose efficient algorithms for optimizing these parameters, and compares NOMA with OMA by computer simulations. However, when these parameters are optimized, the theoretical superiority of NOMA over OMA is still not clear. Therefore, in this paper, the theoretical performance comparison between NOMA and conventional OMA systems is investigated, from an optimization point of view. First, sum rate maximizing problems considering user fairness in both NOMA and various OMA systems are formulated. Then, by using the method of power splitting, a closed-form expression for the optimum sum rate of NOMA systems is derived. Moreover, the fact that NOMA can always outperform any conventional OMA systems, when both are equipped with the optimum resource allocation policies, is validated with rigorous mathematical proofs. Finally, computer simulations are conducted to validate the correctness of the analytical results.

An Optimization Perspective of the Superiority of NOMA Compared to Conventional OMA