Robust Beamforming for Covert Communications Aided by Reconfigurable Dual-Functional Surface

In this paper, we propose a robust beamforming scheme for the simultaneous transmitting and reflecting intel-ligent surface (STAR-RIS)-aided covert communication system with imperfect channel state information. Specifically, STAR-RIS is exploited to facilitate the covert transmission when the legitimate receiver and the warden are located in the different sides of the surface. To maximize the covert rate, we formulate an optimization problem to jointly design the active and passive beamforming, which is non-convex due to the CSI error-incurred semi-infinite constraints and the coupled optimization variables. To solve the problem, we employ the techniques of matrix lifting, S-procedure, and successive convex approximation, and de-velop an alternating optimization framework. Numerical results show that the proposed STAR-RIS-aided robust beamforming scheme significantly improves the covert rate, compared to its transmitting-only counterpart and the existing baseline.