  In this paper, we study the trajectory and resource allocation design for
downlink energy-efficient secure unmanned aerial vehicle (UAV) communication
systems, where an information UAV assisted by a multi-antenna jammer UAV serves
multiple ground users in the existence of multiple ground eavesdroppers. The
resource allocation strategy and the trajectory of the information UAV, and the
jamming policy of the jammer UAV are jointly optimized for maximizing the
system energy efficiency. The joint design is formulated as a non-convex
optimization problem taking into account the quality of service (QoS)
requirement, the security constraint, and the imperfect channel state
information (CSI) of the eavesdroppers. The formulated problem is generally
intractable. As a compromise approach, the problem is divided into two
subproblems which facilitates the design of a low-complexity suboptimal
algorithm based on alternating optimization approach. Simulation results
illustrate that the proposed algorithm converges within a small number of
iterations and demonstrate some interesting insights: (1) the introduction of a
jammer UAV facilitates a highly flexible trajectory design of the information
UAV which is critical to improving the system energy efficiency; (2) by
exploiting the spatial degrees of freedom brought by the multi-antenna jammer
UAV, our proposed design can focus the artificial noise on eavesdroppers
offering a strong security mean to the system.
