Energy-Efficient Active RIS for Secure Transmission With Size and Deployment Optimization

In this paper, we propose a secure transmission scheme for active reconfigurable intelligent surface (ARIS)-aided communication systems. Different from all existing works where the size (number of elements) and location of the ARIS are fixed, we treat these parameters as variables to minimize the power consumption of the ARIS while maintaining a robust secrecy threshold constraint. Specifically, we jointly optimize the reflection coefficient, size, and location of the ARIS. Furthermore, we consider a practical scenario where only the suspicious region of the eavesdropper (Eve) is known. We make a key observation that the ARIS size must be determined before channel estimation. Since all variables are coupled, the joint optimization problem is challenging. To address this, we propose a novel two-stage framework. In the first stage, we derive a universal closed-form upper bound for the ARIS size to decouple the variables, followed by the development of a successive convex approximation (SCA) algorithm to minimize the ARIS size. In the second stage, we apply an alternating optimization (AO) algorithm to optimize the location and reflection coefficients of ARIS. Simulation results demonstrate that the ARIS requires minimal power for signal amplification and highlight the significance of optimizing the size and location of ARIS for energy-efficient secure transmission.