Weighted Ergodic Sum Secrecy Rate Maximization in a NOMA System With Untrusted User Terminals

Non-Orthogonal Multiple Access (NOMA) is a wireless communication technique that uses the Successive Interference Cancellation (SIC) strategy to achieve high efficiency. However, SIC can pose security risks in an untrusted NOMA environment where users can suffer from internal eavesdropping. In this research paper, we propose security solutions for downlink NOMA transmission involving two untrusted user terminals and imperfect SIC. Specifically, we investigate the ergodic secrecy rate performance in NOMA for two user terminals against mutual eavesdropping. To achieve the maximum weighted ergodic sum secrecy rate of the system while meeting the quality of service (QoS) requirements of both user terminals, we formulate an optimization problem for power allocation. Our solution approach considers a decoding order that yields the best possible outcome and employs an iterative method to obtain the optimal power allocation. Simulation results confirm the analytical claims and provide key insights on the optimal design parameters. Lastly, we numerically demonstrate that our proposed power control and decoding order design outperforms the benchmark schemes in terms of weighted ergodic sum secrecy rate performance.