Near-Field Integrated Sensing and Covert Communications With Multiple Targets as Wardens

Both integrated sensing and communications (ISAC) and near-field communications have been considered as key techniques in sixth-generation (6G) mobile communication systems. Near-field ISAC systems can achieve beam focusing by leveraging spherical-wave propagation, thereby enabling joint estimation of distance and angle in sensing, as well as improved interference management in communications. Utilizing this property, we propose a covert communication framework for near-field ISAC systems aided by artificial noise (AN). In the proposed scheme, the transmitter utilizes AN to sense multiple targets and wardens, thereby achieving interference to the wardens while guaranteeing covert transmission to legitimate receivers. Considering both non-colluding and colluding detection strategies adopted by wardens, we formulate an optimization problem to minimize the Cram´er- Rao bound (CRB) subject to the transmit power, covertness, and minimum covert transmission rate constraints. To address the non-convexity of the problem, we propose an alternating optimization algorithm based on the Schur complement and semidefinite relaxation (SDR) technique. To further take into account the location estimation errors, a robust scheme is proposed based on CRB and convex hull with the imperfect channel state information (CSI). Numerical results demonstrate the effectiveness of the proposed scheme, revealing the trade-off between sensing and covert communication performance in near-field ISAC.