Enhancing Security at the Physical Layer in Cognitive Radio Networks via RIS Assistance

In this paper, we investigate the secrecy performance of a cognitive radio network (CRN) in the presence of an eavesdropper (EAV) with the help of Reconfigurable Intelligent Surface (RIS). The secondary users (SUs) are subject to three constraints which include peak transmit power level and interference limitation with respect to the primary user (PU) as well as secrecy outage constraints due to the EAV. Secrecy outage is achieved when the EAV cannot decode the targeted signal, but communications in the secondary network are still possible (non-zero capacity exists). The expressions of the secrecy outage probability and the probability of non-zero secrecy capacity are derived to evaluate the secrecy performance. Monte Carlo simulations are provided to examine the accuracy of the derived approximation expressions. Based on this, power allocation policies for the SUs are derived, satisfying all the constraints while maximizing the secrecy performance as well as the quality of service performance of the secondary network.