  This paper investigates the covert communications via cooperative jamming and
relay selection in a wireless relay system, where a source intends to transmit
a message to its destination with the help of a selected relay, and a warden
attempts to detect the existence of wireless transmissions from both the source
and relay, while friendly jammers send jamming signals to prevent warden from
detecting the transmission process. To this end, we first propose two relay
selection schemes, namely random relay selection (RRS) and max-min relay
selection (MMRS), as well as their corresponding cooperative jamming (CJ)
schemes for ensuring covertness in the system. We then provide theoretical
modeling for the covert rate performance under each relay selection scheme and
its CJ scheme and further explore the optimal transmit power controls of both
the source and relay for covert rate maximization. Finally, extensive
simulation/numerical results are presented to validate our theoretical models
and also to illustrate the covert rate performance of the relay system under
cooperative jamming and relay selection.
