Performance Analysis and Secure Resource Allocation for Relay-Aided MISO-NOMA Systems

In this article, we investigate the physical layer security for a relay-aided multiple-input single-output (MISO) nonorthogonal multiple access (NOMA) system, where an eavesdropper tries to intercept confidential information transmission from the source and the relay by employing selection combining and maximal ratio combining, respectively. Specifically, we propose an optimal transmit antenna selection scheme to exploit the inherent spatial diversity gain for security enhancement. The closed-form expressions for the secrecy outage probability are derived to facilitate the system performance evaluation. At a more pragmatic level, we consider multiple users in the relay-aided MISO NOMA system and thus propose a user pairing algorithm to perfect successive interference cancellation. The algorithm avoids full search over all users by exploiting two-sided matching and low-complexity greed, thereby reducing the total complexity. Furthermore, aiming to maximize the secrecy rate, we formulate an optimization problem. Hence, the power allocation schemes are developed by jointly considering power limits and rate requirements. The scheme achieves closed-form solutions of power allocation for the data rate requirements of each user. Finally, simulation results validate the accuracy of the derived analysis and the improvement significant in secrecy performance by the proposed algorithm and scheme.