Active STAR-RIS Assisted Covert Communications Against a Proactive Warden

In this correspondence, we investigate covert communications assisted by an active simultaneous transmitting and reflecting reconfigurable intelligent surface (aSTAR-RIS) in the presence of a proactive warden. The transmitter adopts non-orthogonal multiple access (NOMA) to serve both a covert user and a public user in the downlink. We consider that the proactive warden introduces additional interference channels, which affects the minimum error detection probability (DEP) at the warden. The DEP is derived and is influenced by the transmit power and interference power. Additionally, we propose an optimization framework to maximize the covert rate of the system while considering the constraints of transmitting power and covertness. The simulation results demonstrate that the proactive warden pose more challenges to covert systems compared to the passive warden. Furthermore, the proposed scheme significantly outperforms the passive STAR-RIS scheme in ensuring the covert performance of the system.