Security Performance Analysis of Single-User Downlink Communication Based on Double-RIS

In this article, the base station and the user communicate through double-RIS, while there is an eavesdropper on the user side, and the base station transmits confidentially through joint beamforming and phase optimization of the two RISs. This process considers the reflected communication channel between the two RISs. For the non-convex solution problem, we use an alternating optimization scheme for step-wise optimization. First, we can solve directly the closure expression for the beam assignment scheme. In contrast, the two-phase optimizations are complex fractional optimizations, which are then solved by converting the fractional form into an equation optimization with the assistance of transformations. Implementing the simulation surface, whether we deploy a single RIS on the user side or the base station side is inferior to the double-RIS scheme.