Joint relay assignment and energy-efficiency maximization in energy-harvesting downlink/uplink clustered nonorthogonal multiple-access networks

In this article, the problem of joint relay assignment and energy-efficiency maximization (J-RA-EE-MAX) in energy-harvesting downlink (DL) and uplink (UL) clustered nonorthogonal multiple-access (NOMA) networks is considered. Specifically, the aim is to perform relay assignment to user clusters in the DL and UL directions, while simultaneously maximizing energy efficiency (EE) over each relay via multiobjective optimization, and satisfying users' quality-of-service (QoS) constraints. However, problem J-RA-EE-MAX happens to be nonconvex in each link direction, and thus is computationally prohibitive. Alternatively, a low-complexity solution procedure is devised to solve problem J-RA-EE-MAX in each link direction by: (i) optimally solving the energy-efficiency maximizing power allocation (EE-MAX-PA) for each (user cluster, relay) combination to construct the relays' preference profile, and (ii) performing relay assignment via Gale's top trading cycles (TTC) matching mechanism. In particular, the optimal solution of the EE-MAX-PA problem is obtained by transforming it into a concave maximization problem, while the TTC mechanism is executed in linear time-complexity so as to obtain a stable relay assignment. Simulation results are presented to validate the proposed solution procedure, which is shown to yield comparable user cluster sum-rate and relay EE to the J-RA-EE-MAX scheme, and superior to other schemes in both link directions, however, at lower computational complexity, while satisfying users' QoS constraints. Finally, this work sheds light on the importance of decoupling relay assignment in the DL/UL directions, which significantly improves user cluster sum-rate and relay EE in comparison to coupled relay assignment schemes, and thus serves the requirements of 5G networks and beyond.