Sum-Rate and Energy Efficiency Optimization by Novel Relay Selection in a NOMA-Based Cooperative Network in the Presence of Interference

A non-orthogonal multiple access (NOMA) scheme as a promising strategy for future wireless communication networks is proposed in a multi-cell scenario, in which the presence of inter-cell interference from adjacent cells affects the performance of the users. Moreover, a novel relay selection criterion assigning a decode and forward (DF) relay per user is proposed to enhance the performance of cell-edge users. Interferences from adjacent cells are also considered, where the potential relays are impressed by these interferences. Two resource allocation problems are derived for sum rate and energy efficiency optimization. Due to the non-convexity of the problems, the optimization problems are solved by proposing iterative algorithms. An iterative algorithm based on the bisection method is suggested for solving the sum rate maximization problem, while an efficient combination of a three-stage bisection method and the Dinkelbach algorithm is adopted for dealing with the energy efficiency maximization problem. Simulation results confirm that the suggested method considerably outperforms the existing relay selection criterions in terms of both sum rate and energy efficiency and approaches the exhaustive search with less complexity order.