  Future Internet-of-Things (IoT) has high demand for energy-saving
communications, especially in remote areas and smart cities. To meet this
demand, we propose novel Unmanned Aerial Vehicle-assisted backscatter
communications, where a UAV first collects data from multiple terrestrial
backscattering tags via time division multiple access, and then flies into the
coverage region of a terrestrial base station to upload its collected data to
its associated base station. To determine the optimal UAV data collection
location, we first analyze the system average outage probability, and then
optimize the energy efficiency with the optimal backscattering location through
Golden Section method under UAV energy constraint. Our analytical and
simulation results illustrate that there is a trade-off between UAV data
collection location and the outage probability, and the optimal UAV data
collection location to achieve maximum energy efficiency needs to be closer to
the tags for lower UAV transmit power.
