  This paper investigates unmanned aerial vehicle (UAV)-aided jamming technique
for enabling physical layer keyless security in scenarios where the exact
eavesdropper location is unknown. We assume that the unknown eavesdropper
location is within an ellipse characterizing the coverage region of the
transmitter. By sequentially optimizing the transmit power, the flight path of
the UAV and its jamming power, we aim at maximizing the average secrecy rate
with arbitrary eavesdropper location. Simulation results demonstrate that the
optimal flight path obtains better secrecy rate performance compared to that
using direct UAV flight path encasing the transmitter and the legitimate
receiver. Most importantly, even with the unknown eavesdropper location, we
obtained a secrecy rate that is comparable to a scenario when the
eavesdropper's location is known. However, the average secrecy rate with the
unknown eavesdropper location varies depending on the proximity of the
eavesdropper to the known location of the transmitter. We also observe that due
to the UAV-aided jamming, the average secrecy rate stabilizes at some point
even though the average received envelope power of the eavesdropper increases.
This essentially demonstrates the effectiveness of the proposed scheme.
