Secure Optimization Method for Cell-Free MIMO STINs

Satellite-Terrestrial Integrated Networks (STINs) are key enablers for achieving all-time, all-weather connectivity, yet they face critical challenges in spectrum management and interference mitigation. To address these issues, this paper proposes a downlink high-secrecy-rate Cell-Free massive MIMO (CFmMIMO) transmission scheme for STINs under active interference. An optimal beamforming design problem is formulated to maximize the system Secrecy Rate (SR) while satisfying the Quality of Service (QoS) requirements of both satellite and terrestrial users, explicitly targeting the cell-free security challenges in this integrated framework. A non-Semidefinite Relaxation (non-SDR) algorithm based on Successive Convex Approximation (SCA) is developed to achieve low computational complexity while maintaining near-optimal performance. Moreover, the security performance of the proposed CF-STIN system is analyzed under cross-layer eavesdropping scenarios, providing valuable insights into its anti-eavesdropping robustness in complex network environments. The structure of this paper is shown as follows: the system model and problem formulation are presented first, followed by the proposed non-SDR optimization algorithm. Simulation evaluation are then discussed to validate the effectiveness of the proposed method, and the main findings are summarized in the conclusion.