Multiple RISs for Enhancing the Secrecy Performance of NOMA Systems over Realistic Nakagami-m Fading Channels

In this paper, we utilize multiple reconfigurable intelligent surfaces (RISs) to improve the secrecy performance of non-orthogonal multiple access (NOMA) systems. In particular, multi-RIS is exploited to aid two legitimate NOMA users under the existence of colluding eavesdroppers. We successfully derive the mathematical expressions of the secrecy outage probabilities (SOPs) and ergodic secrecy capacities (ESCs) of the proposed systems under realistic Nakagami- m channels recommended for the fifth and beyond generation (5G & B5G) wireless networks. Numerical results confirm the enormous benefits of the proposed systems compared to the classical NOMA systems with multiple eavesdroppers and without RISs. More specifically, compared to the classical NOMA systems, the SOPs of the proposed systems are considerably lower, while the ESCs are dramatically higher. In addition, the transmit power to obtain the lowest SOPs of the proposed systems is lower than that to get the lowest SOPs of the classical NOMA systems. On the other hand, the effects of many crucial parameters, such as the carrier frequencies of Wi-Fi systems, the fading order, and the distances between transmitter-receiver, are thoroughly examined to obtain the behaviors of the proposed systems. Finally, Monte-Carlo simulations are provided to validate the derived expressions.