  The projected number of Internet of Things (IoT) sensors makes battery
maintenance a challenging task. Although battery-less IoT is technologically
viable, the sensors should be somehow energized, either locally or remotely.
Unmanned aerial vehicles (UAVs) can respond to this quest via wireless power
transfer (WPT). However, to achieve energy neutrality across the IoT networks
and thus mitigate the maintenance issues, the UAVs providing energy and
connectivity to IoT sensors must be supplied by recharging stations having
multi-source energy harvesting (EH) capability. Yet, as these sensors rely
solely on UAV-transferred power, the absence of UAVs causes sensor outages and
hence loss of coverage when they visit recharging stations for battery
replenishment. Hence, besides the UAV parameters (e.g., battery size and
velocity), recharging duration and station density must be carefully determined
to avoid these outages. To address that, this paper uses stochastic geometry to
derive the coverage probability of UAV-powered sensors. Our analysis sheds
light on the fundamental trade-offs and design guidelines for energy-neutral
IoT networks with recharging stations in regard to the regulatory organization
limitations, practical rectenna and UAV models, and the minimum power
requirements of sensors.
