Improving Physical Layer Security of NOMA Networks by Using Opportunistic Scheduling

In this paper, we study how to improve physical layer security capability of multiple near users and multiple far users non-orthogonal multiple access (NOMA) networks. To this end, we propose an opportunistic user scheduling scheme, named the best-secure-near-user best-secure-far-user (BSNBSF) scheme. The BSNBSF aims to select the best near-far user pair, whose data transmission is the most robust against the interception of an eavesdropper. In order to facilitate the performance analysis of the proposed user scheduling scheme in terms of secrecy outage probability (SOP), we derive an exact closed-form expression and a tight approximate closed-form expression for the SOP of the selected near and far users, respectively. Numerical results show that the BSNBSF scheme significantly improves the secrecy outage performance NOMA networks compared to that of the random near user and random far user selection scheme. Additionally, discussions on the complicate convex characteristic of the total SOP with respect to the power allocation coefficients and the impact the number of near and/or far users are provided.