  This paper investigates a multiple unmanned aerial vehicles (UAVs) enabled
cooperative secure transmission scheme in the presence of multiple potential
eavesdroppers. Specifically, multiple source UAVs (SUAVs) send confidential
information to multiple legitimate ground users and in the meantime multiple
jamming UAVs (JUAVs) cooperatively transmit interference signals to multiple
eavesdroppers in order to improve the legitimate users' achievable secrecy
rate. By taking into account the limited energy budget of UAV, our goal is to
maximize the system secrecy energy efficiency (SEE), namely the achievable
secrecy rate per energy consumption unit, by jointly optimizing the UAV
trajectory, transmit power and user scheduling under the constraints of UAV
mobility as well as the maximum transmit power. The resulting optimization
problem is shown to be a non-convex and mixed-integer fractional optimization
problem, which is challenging to solve. We decompose the original problem into
three sub-problems, and then an efficient iterative algorithm is proposed by
leveraging the block coordinate descent and Dinkelbach method in combination
with successive convex approximation techniques. Simulation results show that
the proposed scheme outperforms the other benchmarks significantly in terms of
the system SEE.
