  Unmanned aerial vehicles (UAVs) are foreseen to constitute promising airborne
communication devices as a benefit of their superior channel quality. But
UAV-to-ground (U2G) communications are vulnerable to eavesdropping. Hence, we
conceive a sophisticated physical layer security solution for improving the
secrecy rate of multi-antenna aided U2G systems. Explicitly, the secrecy rate
of the U2G MIMO wiretap channels is derived by using random matrix theory. The
resultant explicit expression is then applied in the joint optimization of the
MIMO transceiver and the UAV location relying on an alternating optimization
technique. Our numerical results show that the joint transceiver and location
optimization conceived facilitates secure communications even in the
challenging scenario, where the legitimate channel of confidential information
is inferior to the eavesdropping channel.
