DF Relay Assisted Covert Communications: Analysis and Optimization

Covert communication is crucial to preventing the warder's detection of wireless transmission. In general, small transmit power is required to satisfy the covertness requirement, which limits the system performance between the transceivers. To overcome the limitation, a multi-antenna relay is utilized and the coordinated direct and relay transmission (CDRT) is proposed in this paper. Since both the direct link and the relay link impact the system performance, an optimization problem is formulated to maximize the covert rate, where the beamforming vectors at the source and the relay are coupled. To address this problem, a semidefinite relaxation based line search method is proposed to obtain the globally optimal beamforming vectors, which balances the direct and the relay link to satisfy the covertness requirements and achieve the maximum covert rate simultaneously. Numerical results demonstrate the superiority and robustness of the proposed CDRT scheme compared with the systems without the direct or the relay link, especially when the warder-related links are highly correlated with the direct or the relay link. Besides, the CDRT scheme also outperforms the cooperative jammer assisted covert system even when the transmission power is high since the covertness requirements can be satisfied by joint beamforming in the CDRT scheme.