On the Limits of Covert Backscatter Communication Over Undecodable Ambient Signals

This paper investigates the limit covert rate of the backscatter communication systems based on undecodable ambient radio frequency (RF) signals over additive white Gaussian noise channel. In the considered network, source node Alice expects to transmit information to receiver Bob without being detected by warden Willie, by backscattering undecodable ambient RF signals. In this case, it is challenging for Bob to recover the information from Alice, since the undecodable signals result in both unknown additive and multiplicative interference. A subtle communication strategy is proposed to alleviate the influences of the interference, by which we prove that no matter Willie can or cannot decode the information from the RF source and then eliminate its interference, the limit covert rate is subject to the well-known square root law (i.e.,
$\mathcal {O}(\sqrt {n})$
bits per
$n$
channel uses). This interesting result means that even if the RF source is a cooperative node of Willie (e.g., a jammer deployed by Willie to restrict Alice from active transmission), Alice can still covertly and reliably transmit information to Bob by backscattering the ambient RF signals (e.g., the malicious jamming signals).