Energy-Efficient Covert Communications for Bistatic Backscatter Systems

In this paper, we consider covert communications for bistatic backscatter systems, where a tag transmits information passively to a reader by reflecting incident carrier signals with artificial noise (AN) generated from a dedicated carrier emitter (CE) under the supervision of a warden. By exploiting the channel uncertainty introduced by CE, we analytically derive the warden's minimum sum of error probabilities to evaluate communication covertness. We find that communication covertness can be improved by reducing the tag's reflection coefficient, but is not affected by the CE's transmit power. Therefore, in order to achieve energy-efficient design, we optimize the tag's reflection coefficient to minimize the CE's transmit power under the constraints of communication covertness and reliability. Our analysis shows that the minimum CE's transmit power can be further reduced with a minor loss of the achievable covert rate.