Reconfigurable Intelligent Surfaces-Assisted Secure Wireless Communication in Wiretap Systems With Multiple Base Stations

In this paper, we consider a multi-RIS-assisted multi-BS and multi-user wiretap system with malicious eavesdroppers in hotspots. To suppress the behavior of eavesdroppers and achieve secure communication with high secrecy rate (SR) for as many legitimate users as possible, the secure BSs-users association is essential for multi-BS. Since the introduction of RISs extends BSs signal coverage, providing more access possibilities for each BS to associate with potential users, the mutual impact of passive beamforming (PBF) at RISs and secure BSs-users association should be considered together. Therefore, we formulate a sum secrecy rate (SSR) maximization problem by jointly optimizing transmit beamforming (TBF) at BSs, PBF at RISs, and secure BSs-users association. To tackle such non-convex problem, we decompose it into three subproblems and solved them through alternating optimization. The TBF subproblem is efficiently solved by performing successive convex approximation and semi-definite relaxation methods. And the PBF subproblem is tackled utilizing the quadratic transform approach. Meanwhile, we handle the secure BSs-users association subproblem with auction theory. Simulation results demonstrate the proposed algorithm can achieve higher system SSR and SR for each user compared to baselines, which further emphasizes the effectiveness of jointly considering the PBF at RISs and secure user association in enhancing system security in the multi-BS wiretap system.