  This paper exploits the multi-antenna technique to enhance the covert
communication performance in a relay system, where a source S conducts covert
communication with a destination D via a relay R, subjecting to the detections
of transmissions in the two hops from a single-antenna warden W. To demonstrate
the performance gain from adopting the multi-antenna technique, we first
consider the scenario when S, R and D all adopt single antenna, and apply
hypothesis testing and statistics theories to develop a theoretical framework
for the covert performance modeling in terms of detection error probability
(DEP) and covert throughput. We then consider the scenario when S, R and D all
adopt multiple antennas, and apply the hypothesis testing, statistics and
matrix theories to develop corresponding theoretical framework for performance
modeling. We further explore the optimal designs of the target rate and
transmit power for covert throughput maximization under above both scenarios,
subjecting to the constraints of covertness, reliability and transmit power. To
solve the optimization problems, we employ Karushi-Kuhn-Tucker (KKT) conditions
method in the single antenna scenario and a search algorithm in the
multi-antenna scenario. Finally, we provide extensive numerical results to
illustrate how the multi-antenna technique can enhance the covert performance
in two-hop relay systems.
