Energy-Efficiency Maximization in Downlink Clustered NOMA Networks with Energy-Harvesting Relays

In this paper, the problem of joint relay assignment and energy-efficiency maximization (J-RA-EE-MAX) in energy-harvesting downlink clustered non-orthogonal multiple-access (NOMA) networks is considered. Specifically, the aim is to perform relay assignment to user clusters, while simultaneously maximizing energy-efficiency over each relay via multi-objective optimization, and satisfying users' quality-of-service (QoS) constraints. However, problem J-RA-EE-MAX happens to be nonconvex (i.e. computationally-prohibitive). Alternatively, a low-complexity solution procedure is devised to solve problem J-RA-EE-MA by: (1) optimally solving the energy-efficiency maximizing power allocation (EE-MAX-PA) for each (user cluster, relay) combination to construct the relays' preference profile, and (2) performing relay assignment via Gale's top trading cycles (TTC) matching mechanism. Simulation results are presented to validate the proposed solution procedure, which is shown to yield comparable energy-efficiency value per relay to the J-RA-EE-MAX scheme, while satisfying users' QoS constraints.