On physical layer security of multiple-relay assisted NOMA systems

This paper considers the secrecy outage performance of a multiple-relay assisted non-orthogonal multiple access (NOMA) network over Rayleigh fading channels. Two slots are utilized to transmit signals from the base station to destination. In the first slot, the base station broadcasts the superposition signal of two users to all decode-and-forward relays by message mapping strategy. Then the selected relay transmits superposition signal to the two users via power-domain NOMA. Two relay selection (RS) schemes, i.e., optimal single relay selection (OSRS) scheme and two-step single relay selection (TSRS) scheme, are proposed. As a benchmark, we also examine the secrecy outage performance of the traditional multiple relays combining (TMRC) scheme. The closed- form expressions for the security outage probability (SOP) of the proposed schemes are derived and validated via simulations. To get more insights, we also derive the closed-form expressions for the asymptotic SOP for all the schemes with fixed and dynamic power allocations. Monte-Carlo simulations are performed to verify our analytical results.