Secure NOMA for Maximizing Ergodic Secrecy Fairness in the Presence of Untrusted Users

Abstract

Designing a secure non-orthogonal multiple access (NOMA) system has recently sparked a lot of interest among researchers due to its competency over bandwidth utilization. This paper addresses the critical security issue that arises from the adoption of successive interference cancellation based decoding approach in a two-user untrusted NOMA scenario. With the objective of maximizing secrecy fairness between users, a joint optimization problem of power allocation and decoding order is investigated to maximize the minimum ergodic secrecy rate between users. The optimized solutions are obtained by proposing a low computational complexity iterative method. In addition, closed-form optimal solutions based on a tight approximation are presented. Finally, numerical simulations validate the key nontrivial analytical claims and demonstrate that substantial performance gain is achieved over existing benchmark schemes.