Covert Ambient Backscatter Communication against Randomly Distributed Wardens

The features of minimalist hardware and passive transmission bring ambient backscatter communication (AmBC) high spectral and energy efficiency, but also in turn make AmBC face severe security problems. To guard AmBC from wardens, existing studies widely rely on covert communication, but consider only one warden and require the warden's position known to legitimate users. In this paper, we propose a covert AmBC scheme against randomly distributed wardens, which maximizes the covert rate subject to the covertness constraint by utilizing a full-duplex receiver to transmit varying-power interference signals. First, we give a comprehensive performance metric analysis in terms of the covertness, reliability, and overall covert rate by considering the joint decoding performance of the cooperative receiver in the AmBC system. Then, we derive a strong covertness constraint that one of the wardens can be located at the optimal detection location and always uses the optimal detection thresholds. Under this worst situation, we optimize the maximum artificial noise (AN) power and transmission rate to maximize the covert rate. Numerical results illustrate the effectiveness of the proposed scheme, which achieves a positive covert rate for any randomly varying range of jamming power and transmission rate that satisfies the covertness constraint, even in the presence of multiple wardens.