Secrecy sum rate maximization in NOMA systems with wireless information and power transfer

In this paper, we study the physical layer security in the downlink Non-Orthogonal Multiple Access (NOMA) systems with the capability of simultaneous wireless information and power transfer (SWIPT). In our system, each user applies the optimum power splitting (PS), in order to coordinate the energy harvesting and information decoding at the receiver. The optimization problem is formulated as the optimal power allocation (PA) and PS ratio selection, to maximize the secrecy sum rate (SSR) of the NOMA system under the constraints on the individual user's minimum rate and harvested power. In order to derive the solutions to the optimization problem, we separate the PS ratio selection from the PA problem. We first solve to get the closed-form expressions for the optimal PS ratios. Based on the PS ratios obtained, the PA is converted to a convex optimization problem, from which we obtain the optimal PA. Finally, we study the performance based on simulations, our simulation results show that the systems with optimal PA and PS ratio significantly outperform the legacy systems with uniform PA and fixed PS ratio.