SIM-assisted Secure Mobile Communications via Enhanced Proximal Policy Optimization Algorithm

TL;DR

This paper introduces an SIM-assisted secure communication system for 6G networks, utilizing an advanced reinforcement learning algorithm to optimize secrecy rates amid mobility and hardware challenges.

Contribution

It develops a novel PPO-BOP algorithm with LSTM and feedback mechanisms to enhance secure communication in SIM-assisted systems under practical uncertainties.

Findings

PPO-BOP outperforms benchmark algorithms in secrecy rate.

The proposed method effectively handles channel uncertainty and mobility.

Simulation results validate the algorithm's superior performance.

Abstract

With the development of sixth-generation (6G) wireless communication networks, the security challenges are becoming increasingly prominent, especially for mobile users (MUs). As a promising solution, physical layer security (PLS) technology leverages the inherent characteristics of wireless channels to provide security assurance. Particularly, stacked intelligent metasurface (SIM) directly manipulates electromagnetic waves through their multilayer structures, offering significant potential for enhancing PLS performance in an energy efficient manner. Thus, in this work, we investigate an SIM-assisted secure communication system for MUs under the threat of an eavesdropper, addressing practical challenges such as channel uncertainty in mobile environments, multiple MU interference, and residual hardware impairments. Consequently, we formulate a joint power and phase shift optimization problem (JPPSOP), aiming at maximizing the achievable secrecy rate (ASR) of all MUs. Given the non-convexity and dynamic nature of this optimization problem, we propose an enhanced proximal policy optimization algorithm with a bidirectional long short-term memory mechanism, an off-policy data utilization mechanism, and a policy feedback mechanism (PPO-BOP). Through these mechanisms, the proposed algorithm can effectively capture short-term channel fading and long-term MU mobility, improve sample utilization efficiency, and enhance exploration capabilities. Extensive simulation results demonstrate that PPO-BOP significantly outperforms benchmark strategies and other deep reinforcement learning algorithms in terms of ASR.10.1109/TWC.2026.3658332