In this paper, we study wireless powered communication networks (WPCNs) with base stations (BSs) mounted on unmanned aerial vehicles (UAVs). For implementation simplicity and uncoordinated access, the energy harvesting (EH) users (EHUs) employ slotted ALOHA to transmit information at a fixed rate to a common UAV-mounted BS. The UAV flying along a circular trajectory facilitates a line of sight (LoS) between the BS and each EHU, and also mitigates the adverse double near-far effect as it periodically comes close to the EHUs. Due to the periodicity of the LoS channels, the channel can be tracked without performing energy-costly channel estimation, and the channel attenuation can be compensated to avoid channel outages. For such a WPCN, we optimize the UAV radius, the channel access probability, and the rate of each EHU to enable proportionally fair resource allocation. Our solution reveals that, when a certain UAV altitude is exceeded, proportional fairness is attained when the UAV is in the hovering mode.
- slotted ALOHA
- unmanned aerial vehicles
- Wireless powered communication networks