Cooperative Double-Faced Active Reconfigurable Intelligent Surfaces Aided Secure Wireless Communication

This paper proposes a novel cooperative double-faced active reconfigurable intelligent surfaces (Co-DFA-RIS) architecture to ensure secure transmission. Two DFA-RISs work cooperatively through single reflection and inter-surface signal reflection to securely transmit information in the presence of multiple eavesdroppers. Maximizing the sum secrecy rate (SR) as the optimization objective, an optimization problem is formulated to jointly design the AP beamforming and the DFA-RIS configuration. Due to the highly coupled constraints, particularly the presence of power splitting coefficients and the inter-surface signal reflection, the optimization problem is highly non-convex. By leveraging a majorization–minimization (MM) and penalty dual decomposition (PDD) based methods, an alternating optimization (AO) algorithm is developed to update all variables via convex optimization techniques. Furthermore, a closed-form expression of the secrecy capacity (SC) is derived to reveal the impact of various involved parameters on security performance. Lastly, numerical results indicate that the proposed architecture outperforms other state-of-the-art RIS architectures in enhancing the security of wireless communication.