Relaying via Cooperative Jamming in Covert Wireless Communications

In this work, a covert communication scheme under block-fading channels is proposed where the relay, apart from forwarding source's information to the destination, also acts as a cooperative jammer. Stating precisely, the relay amplifies source's signal when required to relay the signal to the destination, otherwise it transmits artificial noise with random power, causing errors in the received signal statistics of the adversary, Willie, who employs a radiometer for detection of covert transmissions. Stating the condition under which Willie makes detection errors, we first find the optimal detection threshold of Willie's detector and his minimum detection error probability and then derive the expected value of this error probability, establishing Willies detection performance from the perspective of the covert communication pair. Furthermore, for a given maximum transmit power at the relay, we derive a closed-form expression for the optimal transmit power of covert communication. Our numerical results illustrate the advantage cooperative jamming offers for achieving covertness, and demonstrate how the choice of transmit power used for covert transmissions is affected by Willie's error performance and required level of covertness.