DF-Relay-Assisted Multiantenna D2D Covert Communication in the Presence of Joint Detection

This article investigates the covert performance of the finite block length device-to-device (D2D) transmission between the multiantenna transceivers assisted by a decode-and-forward (DF) relay node, where the adversary in covert communication, the warden, also employs a multiantenna maximum likelihood detector which observes the covert signals from the source and the relay for joint detection. The relay periodically transmits reference signals to improve the throughput rate over the fading channels while the source and destination estimate the channel coefficients for beamforming. The nonideal beamforming performance with limited reference signals is analyzed, the covert performances measured by the expected relative entropies of the warden’s joint detection are derived, and the optimal power allocation strategy to achieve the highest covert performance under the required throughput rate and outage probabilities is formulated in this article. The theoretical analysis and the numerical results illustrate that the communication side will achieve higher covertness if the relay and the source adopt uncorrelated codes, and the increase in the warden’s antenna number would rapidly improve its detection performance. The numerical results also depict that enhancing the source-to-relay covert transmission brings more improvements for the covertness than the relay-to-destination link, showing the asymmetry between the different links of the DF-relay-assisted covert communication.