Mobility-Aware Covert Communication via RIS: Phase Optimization Under Mobile Detection Constraints

Abstract

This paper investigates covert communication in reconfigurable intelligent surface (RIS)-assisted wireless systems when an adversarial detector (Willie) patrols the environment along a circular trajectory. Unlike prior works assuming stationary eavesdroppers, we explicitly model adversary mobility and derive analytical feasibility conditions for covert transmission under average leakage constraints. Leveraging Rayleigh quotient optimization with unimodular constraints, we obtain a closed-form RIS phase design maximizing legitimate received power while satisfying covert constraints. Extensive Monte Carlo simulations validate the theoretical predictions, revealing a fundamental trade-off between Willie’s mobility radius and achievable performance at Bob. Sensitivity analyses demonstrate that RIS size and covert constraint tightness critically impact system behavior, confirming the importance of modeling adversary mobility in practical RIS-assisted covert communication systems.