  In this work, we investigate the impact of two cooperative unmanned aerial
vehicle (UAV)-based jammers on the secrecy performance of a ground wireless
network in the presence of an eavesdropper. For that purpose, we investigate
the secrecy-area related metrics, Jamming Coverage and Jamming Efficiency.
Moreover, we propose a hybrid metric, the so-called Weighted Secrecy Coverage
(WSC) and a virtual distributed multiple-input-multiple-output (MIMO)-based
zero-forcing precoding scheme to avoid the jamming effects on the legitimate
receiver. For evaluating these metrics, we derive a closed-form position-based
metric, the secrecy improvement. Our mathematical derivations and comparative
simulations show that the proposed zero-forcing scheme leads to an improvement
on the secrecy performance in terms of the WSC, and provides conditions for
improvement of Jamming Efficiency. They also show positioning trends on the
UAVs over a fixed orbit around the legitimate transmitter as well as power
allocation trends for optimal secrecy.
