Robust and Secure Active IRS-aided Covert Multiuser MIMO Communications with a Multi-antenna Energy-harvesting Eavesdropper

This paper investigates robust and secure active intelligent reflecting surface (IRS)-aided covert multi-user multiple-input multiple-output (MIMO) communications in the existence of a multi-antenna energy-harvesting eavesdropper. In particular, an active IRS is deployed to establish a favorable wireless communication environment for improving both security and covertness of the system. We aim to maximize the total system secrecy rate by jointly optimizing the transmit beamforming and artificial noise matrices at the base station (BS) and the reflection-coefficients of the active IRS. Furthermore, we formulate the design as a non-convex optimization problem, explicitly taking into account energy harvesting capabilities of the eavesdropper, information security requirements of legitimate users, and covertness constraint against adversarial detection. To address the inherent non-convexity of the optimization design problem, we propose an effective iterative suboptimal algorithm. In particular, we first transform the original optimization problem into a tractable form by employing the weighted minimum mean-square error (WMMSE), general sign-definiteness, and S-Procedure methods. Subsequently, we exploit the block coordinate descent (BCD) approach to decompose the coupled optimization matrices. Simulation results demonstrate that the proposed scheme can substantially outperform various baseline schemes adopting existing solutions. Besides, our results highlight the superiority of integrating active IRS into MIMO communication systems for secure and covert transmission.