Secrecy outage performance of MIMO-NOMA relay systems with MRT/MRC schemes

Non-orthogonal multiple access (NOMA) systems have indicated that they can provide higher spectral efficiency and secrecy capability compared with orthogonal multiple access (OMA) conventional systems. Therefore, the NOMA technique is expected to replace the OMA systems. Moreover, multiple-input multiple-output (MIMO) communications can achieve a high-level security thank to high channel gain between users. However, the presence of eavesdroppers is a problem for future wireless systems. Consequently, we investigate the secrecy outage probability (SOP) of the MIMO - NOMA relay system under the presence of eavesdroppers with multiple antennas over Rayleigh fading channels. In order to improve legitimate link's capacity, the maximum ratio transmission (MRT) and maximum ratio combination (MRC) schemes are applied at the transmitter and receivers, respectively. To evaluate the secrecy performance, the closed-form expressions of the SOP of each user, the overall SOP of the considered MIMO-NOMA relay system are derived, and then an impact of channel estimation error and location of eavesdroppers on the SOP of the system is explored. Moreover, Monte-Carlo simulations are utilized to verify the correctness of all derived mathematical expressions. Numerical results show that owning to utilizing multiple antennas, the considered MIMO-NOMA relay system in MRT and MRC schemes has superior secrecy performance than transmit antenna selection and selection combining schemes. Furthermore, the location of relay and power allocation coefficient of destination users are optimized in the sense of minimum SOP. © 2024 Elsevier B.V.