RIS-Assisted Covert Communication for Millimeter-Wave MIMO Channels in Railway Internet of Things

As the railway industry undergoes digital transformation driven by AI, big data, and 5G/6G, secure data transmission for IoT-oriented railway operations has become a critical challenge in high-speed railway (HSR) wireless communications. Covert communication, which conceals wireless transmissions from eavesdroppers, together with reconfigurable intelligent surfaces (RISs), MIMO, and millimeter-wave technologies, offers a promising solution for enhancing security in such dynamic environments. This paper presents a comprehensive review of RIS-assisted covert communication for mmWave MIMO channels in HSR IoT networks. We examine security threats and the limitations of existing schemes, introduce the system architecture and channel modeling approaches, and summarize Doppler shift compensation techniques for high-mobility scenarios. In particular, this review introduces a service-priority-based covert constraint design framework as an original analytical perspective, rather than a fully validated engineering solution, to improve adaptability in diverse HSR operational contexts. We further provide a critical comparative review of optimization algorithms, including convex relaxation, heuristic, and deep reinforcement learning approaches, with a focus on their applicability and complexity trade-offs, and outline future research directions. Beyond summarizing the existing literature, this review integrates new analytical perspectives and simulation-based illustrations derived from our recent publications, offering both a systematic reference and forward-looking insights for researchers in this emerging field.