Joint power allocation and collaborative beamforming for physical layer security in underlay CR NOMA relay systems

Abstract In this paper, we investigate the problem of joint power allocation and collaborative beamforming for physical layer security in underlay cognitive radio non-orthogonal multiple access relay systems. We consider two relaying schemes, i.e., amplify-and-forward and decode-and-forward, with the goal of maximizing the secrecy rate for the central user subjective to the transmit power constraint at the source node, individual transmit power constraints at relays, rate requirements at the cognitive users and the interference power constraint at the primary receiver. The optimization problems are nonconvex and hard to solve. We propose an efficient iterative algorithm by the constrained concave–convex procedure to solve the nonconvex optimization problems. For a comparison, we provide another method, namely alternating optimization (AO) algorithm. In addition, simulations are provided to show the superiority of our proposed methods with comparison to the AO algorithm and traditional orthogonal multiple access schemes.