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Resource-Efficiency and Fairness Tradeoff in Downlink NOMA Systems

Non-orthogonal multiple access (NOMA) is a promising and key wireless technology for the next generation of mobile communications. Considering that spectral efficiency (SE) and energy efficiency (EE) are the main metrics for designing wireless networks, many previous works have focused on the relationship between EE and SE, and provided a good insight into the resource efficiency (RE), i.e. the joint EE-SE tradeoff in NOMA networks. Moreover, fairness among the devices in terms of resources sharing should be particularly concerned because of the characteristic of NOMA. In this paper, we formulate a multi-objective optimization (MOO) problem to achieve a good trade-off between EE, SE and fairness. It is a new paradigm called the RE-fairness tradeoff. In order to obtain the feasible (Pareto-optimal) solutions, the equivalent single objective optimization (SOO) problem is explored by combining the multi-objectives through a weighted-sum objective function. Then based on the sequential convex approximation (SCA) technique, the optimal and suboptimal approaches are proposed separately. Simulation results reveal that the proposed approaches can perform the RE-fairness tradeoff flexibly compared to the only RE design, while meeting the requirements of quality-of-service (QoS) and the upper limit for the number of users in a NOMA group.