Covert RIS-Aided THz Communication With Pointing Errors and Imperfect CSI: Joint Beamforming, Phase Shift, and Power Splitting Optimization

Abstract

Covert communication has sparked widespread attention since it seeks to secure potentially undetected transmissions by employing a variety of confusion sources. Despite the highly directed nature of Terahertz (THz) beams, receiver vibration causes beam direction misalignment, lowering communication speeds, and reducing communication covertness. In this research, we present and analyze a resilient reconfigurable intelligent surface (RIS)-aided covert transmission scheme for THz transmissions, in which a multi-antenna node transmits to both a covert user and a public user in the presence of an unfriendly warden, under both perfect and imperfect channel state information assumptions. The proposed approach is based on superimposing the two legitimate users’ signals, confusing the warden node’s detection. Closed-form expressions for the optimal detection threshold at the warden, detection error probability (DEP), and minimum DEP (MDEP) are provided. Simulation findings corroborate the analytical results, and insightful discussions on the monotonicity of the main metrics and their impact on the system’s covertness are included. Then, using an alternating optimization (AO)-based framework, an optimal design of the power allocated to each user’s signal, their respective beamforming vectors, and the RIS phase shifts is carried out to maximize the system’s achievable communication rate subject to power constraints and an MDEP threshold at the warden. The convexity of all intermediate subproblems has been thoroughly examined, and the covertness advantage of the proposed approach has been proven.