  In 5G and beyond networks, Unmanned Aerial Vehicles (UAV) are an attractive
solution to enhance the secrecy of a wireless systems by exploiting their
predominant LOS links and spacial manoeuvrability to introduce a friendly
jamming. In this work, we investigate the impact of two cooperative UAV-based
jammers on the secrecy performance of a ground wireless wiretap channel by
considering secrecy-area related metrics, the jamming coverage and jamming
efficiency. Moreover, we propose a hybrid metric, the so-called Weighted
Secrecy Coverage (WSC) that can be used as a metric for gaining insights on the
optimal deployments of the UAV jammers to provide the best exploration of
jamming signals. For evaluating these metrics, we derive a closed-form
position-based metric, the secrecy improvement, and propose an analogous
computationally simpler metric. Our simulations show that a balanced power
allocation between the two UAVs leads to the best performances, as well as a
symmetrical positioning behind the line of sight between the legitimate
transmitter and receiver. Moreover, there exist an optimal UAV height for the
jammers. Finally, we propose a sub-optimal and simpler problem for the
maximisation of the WSC.
