Coupled Phase-Amplitude RIS for Secure SCMA in Cognitive Satellite-Terrestrial Networks: An MADRL Optimization Framework

The cognitive satellite-terrestrial network (CSTN) has emerged as a transformative architecture for enabling ubiquitous global connectivity, yet its broadcast nature and heterogeneous service demands pose critical security challenges against increasingly sophisticated wiretap threats. This paper proposes a novel reconfigurable intelligent surface (RIS)-assisted secure sparse code multiple access (SCMA) framework in CSTN, which aims to maximize the achievable secrecy rate by jointly optimizing the transmit beamforming, RIS reflection matrix, and SCMA codebook configuration while satisfying power constraints at the satellite and base station and meeting the quality-of-service demands of legitimate users. Specifically, a realistic RIS model incorporating coupled phase-amplitude constraints is considered for practical deployment scenarios. To solve the above non-convex optimization problem in dynamic environments, an intelligent decision-making mechanism based on a modified multi-agent two-delay deep deterministic (MMTD3) algorithm is developed to effectively decouple continuous beam control and discrete codebook selection, and provide a new paradigm for AI-driven cross-domain security optimization in CSTN. Simulation experiments demonstrate that the proposed framework outperforms existing benchmarks in key metrics including convergence, reward values, and secrecy rate, highlighting the framework’s potential for supporting ubiquitous security and massive heterogeneous service demands in CSTN.