Secrecy Performance of a RIS-Assisted Wireless Network: A Comprehensive Analysis Under Outdated CSI

Reconfigurable intelligent surfaces have manifested notable merits in enhancing networks' security from a physical layer perspective by leveraging smart genuine signal reflection using its reflective elements (REs) towards legitimate users. However, outdated channel state information (CSI) poses a major challenge to RIS's physical layer security gains. This work explores the impact of varying degrees of outdated CSI on the secrecy performance of RIS-assisted wireless networks. More specifically, the system in consideration consists of a source communicating with a legitimate receiver via an RIS in the presence of a potential eavesdropper. To this end, the secrecy outage probability (SOP) metric is derived under the presence of outdated CSI (aging) at the RIS controller, considering four scenarios based on the availability of direct links between the transmitter and both receivers (i.e., the legitimate one and the eavesdropper). This provides a comprehensive evaluation of the system's resilience to CSI aging. Additionally, numerical evaluations are provided to determine the minimum number of RIS REs required to achieve a target SOP. The obtained results demonstrate that the adopted system can achieve an acceptable SOP level even with 10% outdated CSI (i.e., ρ = 0.9 ). This resilience indicates that the system can reduce channel estimation frequency to reconfigure the RIS, thus decreasing overall system overhead and complexity and improving energy efficiency. Moreover, increasing the number of REs can mitigate CSI aging effects where an SOP level of 3 × 10^-5 can be achieved with 140 REs at ρ = 0.9 in the absence of a transmitter-receiver direct link. Lastly, Monte Carlo simulations validate the analytical findings.