Active Reconfigurable Intelligent Surface for Secure Energy Efficiency in Wireless Network

Reconfigurable intelligent surface (RIS) has emerged as a potential technology for the design of 6G wireless communication systems. However, the “double fading” effect in the principal reflection link limits its practical development. In this paper, we explore the use of active RIS (ARIS) to enhance a downlink multiple-input-single-output (MISO) system in the presence of a single eavesdropper (Eve) whose channel state information (CSI) is imperfect. We fomulate a secure energy efficiency (SEE) maximization problem which is addressed by jointly optimizing the BS transmit beamforming and the ARIS reflection coefficient matrix. We first decompose the SEE into a linear function exploiting a quadratic transform. The uncertainty of the channels related to the Eve is approximated by using the S-Procedure. Then, we apply the successive convex approximation (SCA) technique and semidefinite relaxation (SDR) to handle the non-convexity. An alternating optimization (AO) framework is proposed to address the coupled variables.