CRB-Based Sensing and Robust Beamforming for Secure ISAC in 6G

Integrated sensing and communication (ISAC) is a core enabler for 6G networks, yet ensuring physical layer security (PLS) remains challenging under mobility and imperfect channel knowledge. This paper proposes a unified framework that integrates a CRB-based mobility-aware sensing trigger with robust beamforming via semidefinite programming (SDP). The CRB-velocity policy adaptively invokes sensing based on estimation accuracy and eavesdropper mobility, thereby reducing redundant activations in low-mobility regimes while intensifying sensing when mobility threatens secrecy. Robust beamforming with artificial noise is then employed to guarantee worst-case secrecy under channel state information (CSI) uncertainty. Theoretical analysis establishes a quadratic CRB scaling law for mobility-aware thresholds and proves the rank-one tightness of the robust SDP relaxation. Extensive simulations confirm that the proposed framework reduces sensing overhead by up to 80% relative to fixed and periodic baselines, while maintaining secrecy rates above 2 bps/Hz even under large CSI errors. Tradeoff analysis further shows that the proposed method lies on the Pareto frontier between secrecy and sensing efficiency, and a multiuser extension demonstrates scalability to practical deployments. These results highlight that integrating mobility-aware sensing with robust transmission design provides an effective and practical pathway toward secure ISAC in future 6G networks.