Covert Communications in Active Reconfigurable Intelligent Surface Based Symbiotic Radio Systems

We study a covert communications problem in an active reconfigurable intelligent surface (ARIS) based symbiotic radio system. In the considered system, the ARIS helps to improve the communications performance between a pair of primary transceivers, and in the meantime, it covertly communicates with a secondary receiver via reflection modulation. To achieve this, a reflection element-grouping scheme is adopted at the side of the ARIS. Subject to a quality-of-service constraint on the primary link and a covertness constraint on the secondary link, the reflection coefficients of the ARIS are designed to maximize the signal-to-noise ratio of the secondary receiver. For such a non-convex problem, an efficient iterative method is proposed to find a sub-optimal solution. Numerical experiments are conducted to evaluate the achievable covert communications performance.