Optimal sum-throughput analysis for downlink cooperative SWIPT NOMA systems

In this paper, we investigate the sum-throughput of cell-center and cell-edge users in downlink scenarios of cooperative simultaneous wireless information and power transfer (SWIPT) non-orthogonal multiple access (NOMA) systems. Specifically, we consider a two-user NOMA system, in which the cell-center user acts as a power-splitting (PS)-based SWIPT relay aiming to improve the performance of the cell-edge user. In order to evaluate the performance of the considered system, we first derive a closed-form expression for the outage probability (OP) of the cell-center user and a closed-form approximate expression for the OP of the cell-edge user. Adopting the gradient decent method, we then propose an algorithm that finds the optimal value of the PS coefficient, which results in a maximum sum-throughput of the system. With the obtained optimal PS coefficient, we show that the optimal SWIPT NOMA system provides essential throughput improvement for the cell-edge user while the use of SWIPT-based relaying transmission does not jeopardize the sum-throughput of the system compared to that of non-cooperative NOMA systems.