Sensing-Based Two-Stage Transmission Framework for Near-Field mmWave Covert Communications

Abstract

This letter investigates a two-stage near-field covert communication for wideband millimeter-wave (mmWave) systems. During the sensing stage, we propose a novel sensing scheme exploiting controllable near-field beam squint, enabled by phase shifters (PSs) and true-time delays (TTDs), to acquire the channel state information (CSI) of a passive warden Willie. The sensing accuracy is analytically characterized by the Cramér-Rao bounds (CRBs), which establish a CRB-driven uncertainty model for Willie’s channel. During the communication stage, relying on this imperfect CSI, we maximize the achievable covert capacity by jointly optimizing the sensing duration and wideband analog beamforming, subject to a Kullback-Leibler divergence-based covertness constraint. To tackle the intractable problem, we develop a robust fully-digital approximation scheme coupled with a one-dimensional search, followed by an alternating optimization algorithm to extract the practical TTD and PS configurations. Finally, simulation results validate the proposed framework, revealing significant covert capacity gains over TTD-free baselines and highlighting a fundamental sensing-communication trade-off.