SWIPT-Aided Secure Beamforming Design for Downlink Cooperative NOMA Systems

In this paper, we consider a downlink non-orthogonal multiple access system, in which base station emits secret messages to two legitimate users in the presence of multiple eavesdroppers, and all the eavesdroppers collude to form joint receive beamforming, in an attempt to enhance their interceptions. In light of this, a cooperative simultaneous wireless information and power transfer non-orthogonal multiple access secure transmission protocol is proposed, where the legitimate user with a stronger channel condition acts as an energy harvesting relay to help that with a weaker channel condition. Following this, a secrecy sum rate maximization problem is formulated, under the constraints of transmit power and quality of service requirements. By taking both the single-antenna and multiple-antenna scenarios into account, two different optimization schemes, i.e., the power control scheme, and the null-space beamforming design scheme, are presented to maximize the secrecy sum rate. Numerical results verify the superiority of our proposed schemes over the benchmarks.