Secrecy Rate Maximization in the Presence of Stacked Intelligent Metasurface

This paper focuses on maximizing the sum secrecy rate in secure multi-user MISO communication systems that use stacked intelligent metasurfaces (SIM). SIM technology manipulates electromagnetic waves and improves secure communication by combining several metasurface layers with discrete phase-shifting capabilities. We propose a methodology for optimizing beamforming vectors at the base station and phase shifts across metasurface layers, with the goal of maximizing the sum secrecy rate while adhering to practical power constraints. The non-convex optimization problem, induced by discrete phase shifts and coupled design beamforming parameters, is dealt by using an alternating optimization (AO) method. This method employs successive convex approximation for beamforming and projected gradient ascent for phase shift adjustment, resulting in convergence to locally optimal solutions. The proposed approach is thoroughly assessed in simulated scenarios to discover how it performs under various system configurations. The findings reveal that increasing the number of metasurface layers and meta-atoms significantly increases the sum secrecy rate by improving spatial control, lowering interference, and effectively repelling eavesdropping threats. Furthermore, the AO algorithm demonstrates rapid convergence and computational efficiency, making it appropriate for practical use. The framework demonstrates strong flexibility to changes in transmit power, antenna design, and user densities while retaining stable and scalable performance. This research emphasizes the potential of SIM-assisted systems in improving security in wireless communications by enhancing spatial architecture and beamforming, which can complement existing security strategies.