Partial relay selection for secure cooperative NOMA networks

In this paper, we address the physical layer security (PLS) of multi-relay non-orthogonal multiple access (NOMA) schemes with partial relay selection (PRS) using decode-and-forward (DF) and amplify-and-forward (AF) protocols. We propose a partial relay scheme aiming to select the best relay according to the highest signal-to-noise ratio (SNR) of the first link. We derive new exact and asymptotic expressions for strictly positive secrecy capacity (SPSC) and secrecy outage probability (SOP) considering Rayleigh fading channels. Numerical results demonstrate that AF and DF provide a similar system performance. They prove that partial relay selection improves SPSC and reduces SOP when the relays are near the legitimate receiver. Numerical results also reveal that the performance boosts for a high number of relays even for the high SNR of the eavesdropper channel. The secure NOMA system with PRS is capable of providing better secrecy performance than the secure NOMA system with random relay selection. Asymptotic results show that the use of multiple relays does not affect the system performance.