  This paper considers grid formation of an unmanned aerial vehicle (UAV) swarm
for maximizing the secrecy rate in the presence of an unknown eavesdropper. In
particular, the UAV swarm performs coordinated beamforming onto the null space
of the legitimate channel to jam the eavesdropper located at an unknown
location. By nulling the channel between the legitimate receiver and the UAV
swarm, we obtain an optimal trajectory and jamming power allocation for each
UAV enabling wideband single ray beamforming to improve the secrecy rate.
Results obtained demonstrate the effectiveness of the proposed UAV-aided
jamming scheme as well as the optimal number of UAVs in the swarm necessary to
observe a saturation effect in the secrecy rate. We also show the optimal
radius of the unknown but constrained location of the eavesdropper.
