A Buffer-Aided Cooperative NOMA Scheme with Improved Physical Layer Security

A novel buffer-aided cooperative non-orthogonal multiple access (C-NOMA) scheme with improved physical layer security (PLS) is proposed and its performance is analysed. Assuming that both source and relay transmissions are vulnerable to the eavesdropper, a randomize-and-forward relaying protocol is employed to degrade the eavesdropping rate. Considering a proactive jamming (PJ) scheme and targeting the maximization of the average secrecy rate (ASR), an optimization problem is formulated by jointly taking into account the buffer stability, the quality of service (QoS) guarantee and the energy constraint of the nodes. As this is a non-convex mixed-integer problem, a dynamic recursion algorithm based on Lagrange duality and variable discretization is proposed as a low complexity solution which requires a polynomial time. Performance evaluation results have shown that, as compared to other equivalent C-NOMA schemes which do not employ buffering or PJ, the proposed scheme achieves higher PLS performance. They have also shown that by increasing the power of PJ, further improvements of the ASR are possible.