Secrecy Performance Analysis in Downlink NOMA Systems with Cooperative Full-duplex Relaying

We introduce the cooperative full-duplex relaying to a downlink non-orthogonal multiple access (NOMA) system and analyze the secrecy performance of each NOMA user. In particular, we enable the near NOMA user, which is close to a base station (BS), to act as a full-duplex relay for the far NOMA user, which is far away from the BS. By taking imperfect self-interference cancellation into consideration, we analytically derive the secrecy outage probabilities of both NOMA users with integral expressions. To reduce the computational complexity of both secrecy outage probabilities, we adopt approximation techniques and obtain approximated secrecy outage probabilities with closed-form expressions. Simulation results validate the correctness of our analysis and reveal that suppressing the self-interference at the full-duplex relay can enhance the secrecy performance of NOMA users. Meanwhile, simulation results demonstrate the advantages of the cooperative full-duplex relaying in the NOMA system over the OMA system.