Strengthening Secure Transmission in Multi-User Systems via Double-RIS

Reconfigurable Intelligent Surface (RIS) is an emerging communication technology designed to tackle the increasing costs and energy consumption issues in wireless communication. This paper explores a scenario in which a base station and users establish secure communication utilizing a doubleRIS system, while contending with potential eavesdroppers in proximity to the users. The base station employs a strategy that integrates beamforming techniques along with phase adjustments at both RISs to ensure secure transmission, considering the indirect communication paths created by reflections between the two RISs. We employ an alternating optimization framework to formulate and iteratively enhance a three-variable problem. The conversion of the fractional problem into an equation form via the Charnes-Cooper transformation accomplishes this. Simulation results validate our theoretical findings, showing that the protective capabilities of the dual-RIS system exceed those of a single-RIS system, thereby offering enhanced defenses against eavesdropping attempts.