On covert transmission of distributed antenna systems against heterogeneous adversaries

This paper investigates the covert transmission from multiple ground nodes to a full-duplex unmanned aerial vehicle (UAV) under the surveillance of three types of wardens. The transmit nodes constitute a distributed antenna system to form sharp and flexible beams. The UAV receiver generates jamming signals while receiving to confuse the wardens. The wardens are classified as ground wardens, near UAV wardens, and far UAV wardens based on their positions; different channel models and covert communication strategies apply to different warden types. Analytical expressions are derived to evaluate the wardens’ detection performance. Furthermore, the benefits of increasing the number of collaborative transmit nodes are revealed via asymptotic analyses. The collaborative transmit nodes’ transmit powers, phases, and the receiver’s jamming power are jointly optimized to minimize the receiver’s outage probability while satisfying the covertness constraints for all the wardens. The formulated optimization problem is subtly decomposed by using the monotonicity properties of the Marcum Q function and is tackled by alternating optimization, with one subproblem having a closed-form optimal solution and the other solved with the semidefinite relaxation method. Numerical results show significant performance gains of the proposed algorithm over the existing strategy.