Secrecy outage performance of NOMA relay networks using partial relay selection in the presence of multiple colluding eavesdroppers

Nowadays, security becomes one of the utmost important issues in designing mobile communication networks, especially for non-orthogonal multiple access (NOMA), in which the users share the same bandwidth and time resource. Here, we rigorously derive the closed-form formulas of secrecy outage probability (SOP) for each user as well as the overall SOP of a NOMA relay networks consisting of two users, against the wiretap from multiple colluding eavesdroppers. All channels are assumed to be Rayleigh fading. To enhance the reliability of the legitimate communication, we exploit a partial relay selection (PRS) algorithm to select the best relay for forwarding instead of using the traditional multiple-relay (MR) forwarding scheme. For practical consideration, the successive interference cancellation (SIC) at the NOMA users is assumed to be imperfect, which is different from most of the previous works. The SOP obtained in this work is also benchmarked with the one that is obtained by using MR instead of PRS and OMA instead of NOMA, respectively. The analytical results are verified by Monte Carlo simulations, which also confirm that the PRS method outperforms the conventional MR approach