Exploiting Spectral-Energy Efficiency Tradeoff with Fairness in Non-Orthogonal Multiple Access Systems

The spectral efficiency (SE) and energy efficiency (EE) tradeoff while ensuring rate fairness among users in non-orthogonal multiple access (NOMA) systems is investigated. In order to characterize the SE-EE tradeoff with rate fairness, a multi-objective optimization (MOO) problem is first formulated, where the rate fairness is represented with the α-fair utility function. Then, the MOO problem is converted into a single-objective optimization (SOO) problem by the weighted sum method. To solve the converted non-convex SOO problem, we apply sequential convex programming, which helps to propose a general power allocation algorithm to realize the SE-EE tradeoff with rate fairness. We prove the convergence of the proposed algorithm and the convergent solution satisfies the KKT conditions. Simulation results demonstrate the proposed power allocation algorithm can achieve various levels of rate fairness, and higher fairness results in degraded performance of SE-EE tradeoff. A pivotal conclusion is reached that NOMA systems significantly outperform orthogonal multiple access systems in terms of SE-EE tradeoff with the same level of rate fairness.