  This work employs an unmanned aerial vehicle (UAV) as a jammer to aid a
covert communication from a transmitter Alice to a receiver Bob, where the UAV
transmits artificial noise (AN) with random power to deliberately create
interference to a warden Willie. In the considered system, the UAV's trajectory
is critical to the covert communication performance, since the AN transmitted
by the UAV also generates interference to Bob. To maximize the system
performance, we formulate an optimization problem to jointly design the UAV's
trajectory and Alice's transmit power. The formulated optimization problem is
non-convex and is normally solved by a conventional iterative (CI) method,
which requires multiple approximations based on Taylor expansions and an
initialization on the UAV's trajectory. In order to eliminate these
requirements, this work, for the first time, develops a geometric (GM) method
to solve the optimization problem. By analyzing the covertness constraint, the
GM method decouples the joint optimization into optimizing the UAV's trajectory
and Alice's transmit power separately. Our examination shows that the GM method
can significantly outperform the CI method in terms of achieving a higher
average covert rate and the complexity of the GM method is lower than that of
the CI method.
