Covert Communications in Multi-Antenna Two-Way Relay Systems

In this paper, we study covert communication in a multi-antenna two-way relay system, where two source nodes exchange their messages through a relay. Then, the relay tries to disseminate covert messages without being detected by an unauthorized node, the warden. We first derive a closed-form expression of the detection error probability (DEP) and identify the best detection strategy from the perspective of the warden. Then, we aim to maximize the covert rate by optimizing the transmit power and the beamforming matrix at the relay with the worst-case DEP into consideration. We propose a joint optimization algorithm based on the semi-definite relaxation principle. Two additional algorithms with maximum ratio combining and zero-forcing techniques are also presented as suboptimal solutions. Numerical results verify the accuracy of our analytic derivations, and we confirm that the proposed scheme outperforms other methods.