  Thanks to their quick placement and high flexibility, unmanned aerial
vehicles (UAVs) can be very useful in the current and future wireless
communication systems. With a growing number of smart devices and
infrastructure-free communication networks, it is necessary to legitimately
monitor these networks to prevent crimes. In this paper, a novel framework is
proposed to exploit the flexibility of the UAV for legitimate monitoring via
joint trajectory design and energy management. The system includes a suspicious
transmission link with a terrestrial transmitter and a terrestrial receiver,
and a UAV to monitor the suspicious link. The UAV can adjust its positions and
send jamming signal to the suspicious receiver to ensure successful
eavesdropping. Based on this model, we first develop an approach to minimize
the overall jamming energy consumption of the UAV. Building on a judicious
(re-)formulation, an alternating optimization approach is developed to compute
a locally optimal solution in polynomial time. Furthermore, we model and
include the propulsion power to minimize the overall energy consumption of the
UAV. Leveraging the successive convex approximation method, an effective
iterative approach is developed to find a feasible solution fulfilling the
Karush-Kuhn-Tucker (KKT) conditions. Extensive numerical results are provided
to verify the merits of the proposed schemes.
