Covert Backscatter Communication With Multitags

This work proposes a covert backscatter communication (BackCom) system with multiple backscatter transponders (Tags) using random excitation power. In the proposed framework, multiple Tags perform covert transmissions to deliver backscattered information to a covert receiver, while attempting to evade detection by a warden, who monitors potential communication activities. This motivation is driven by the aim of significantly improving the covert rate through the use of multiple Tags, while ensuring the preservation of covertness. To minimize collisions among Tags while maintaining detection resistance, each Tag adopts a Gold sequence to increase the self-correlation properties and substantially spread the spectrum of reflected signals. Then, we propose successive interference cancellation (SIC) technology for Bob to resolve the collisions. To achieve an optimal tradeoff between covertness and reliability, we formulate an excitation power covert randomness strategy that systematically designs key system parameters through careful consideration of Willie's detection error probability (DEP) and Bob's connection outage probability (COP). Moreover, we investigate the covert performance with the imperfect channel state information of the warden's link [Willie's channel state information (WCSI)] between Tags and Alice. Extensive numerical results demonstrate that increasing the number of Tags and their reflection coefficients enhances covert transmission efficiency at the expense of reduced covertness. In addition, the proposed scheme significantly outperforms existing state-of-the-art approaches in terms of both covertness and reliability metrics.