Secure Relay Selection Strategy for Full-Duplex Relay System Based on Non-Orthogonal Multiple Access

This article proposes a secure two-step relay selection strategy to ensure the secure transmission of information for multiple users on the basis of improving the spectral efficiency of the system. Unlike traditional relay selection, the secure two-step relay selection strategy first ensures the secure transmission of information for weak users in the first step and selects a decoding relay set. In the second step, it maximizes the information security capacity of strong users and selects a forwarding relay from the relay set obtained in the first step. The system relies on multiple full-duplex relays to improve diversity gain while also introducing complex self-interference. This article further analyzes the probability of two events when two pairs of user information in the system are interrupted, and obtains a closed-form expression for the probability of secure interruption through probability theory and order theory. Based on the proposed secure two-step relay selection strategy, to further analyze the security performance advantages of FD-NOMA systems, a comparative analysis is conducted between HD-NOMA systems with relays working in HD mode and secure interruption performance. Simulation results verify the performance advantages of the proposed secure high spectral efficiency scheme.