Two-Phase Covert Communication With a Wireless-Powered Jammer by RIS

In this work, we consider a two-phase covert communication system assisted by a wireless powered jammer via a reconfigurable intelligent surface (RIS) with discrete phase shifts. Specifically, in the first phase, the jammer harvests energy from the same transmitter of covert signals with the aid of RIS and in the second phase the jammer uses all the harvested energy to generate interference for facilitating covert transmissions. We first derive the minimum detection error probability at a warden, based on which we optimize the phase switching ratio between the two phases to maximize the effective transmission rate subject to a covertness constraint. Interestingly, our analysis indicates that the achievable communication covertness does not depend on the transmit power of Alice, which remains the same across the two phases for simplifying system implementation. Our examinations also show that a shorter duration should be allocated to energy harvesting for maximizing the covert communication performance when the capability of the RIS increases, e.g., the element number or discrete phase shift level increases.