Exploiting STAR-RIS for Covert Communication in ISAC Networks Under Imperfect CSI

In this paper, we investigate the use of simultaneously transmitting and reflecting reconfigurable intelligence surface (STAR-RIS) to realize covert communication in a millimeter-wave integrated sensing and communication (ISAC) network under imperfect channel state information, where a base station (called Alice) splits its multi-antenna into two groups: one for performing the single-target sensing and the other for transmitting confidential messages to a communication user (called Bob) under the surveillance of a warden (called Willie). We first explore a novel covert transmission strategy. Specifically, Alice exploits the channel uncertainties in Willie's links to hide Bob's wireless transmission. Obeying this, the minimum probability of detection error (PDE) of Willie is derived under the Saleh-Valenzuela channel fading. To gain more insight into Willie's signal detection, we extend the above work to another general Rayleigh channel fading, and the result shows that the minimum PDE is determined merely by the ratio of the power of the covert signal to that of the sensing signal, which is quite different from the one studied above. Furthermore, we propose a robust beamforming design in which the transmit beamforming at Alice and the transmission&reflection beamforming at STAR-RIS are jointly optimized to maximize the covert rate. To solve this highly non-convex problem, we use the S-procedure and general sign-definiteness to convert it into a tractable form, and then develop an effective overall iterative algorithm by invoking the successive convex approximation and penalty convex-concave procedure methods. Finally, numerical results confirm the covertness superiority of the studied network over baseline schemes.