  Backscatter communication which enables wireless-powered backscatter devices
(BDs) to transmit information by reflecting incident signals, is an energy- and
cost-efficient communication technology for Internet-of-Things. This paper
considers an unmanned aerial vehicle (UAV)-assisted backscatter communication
network (UBCN) consisting of multiple BDs and carrier emitters (CEs) on the
ground as well as a UAV. A communicate-while-fly scheme is first designed, in
which the BDs illuminated by their associated CEs transmit information to the
flying UAV in a time-division-multiple-access manner. Considering the critical
issue of the UAV's limited on-board energy and the CEs' transmission energy, we
maximize the energy efficiency (EE) of the UBCN by jointly optimizing the UAV's
trajectory, the BDs' scheduling, and the CEs' transmission power, subject to
the BDs' throughput constraints and harvested energy constraints, as well as
other practical constraints. Furthermore, we propose an iterative algorithm
based on the block coordinated decent method to solve the formulated
mixed-integer non-convex problem, in each iteration of which the variables are
alternatively optimized by leveraging the cutting-plane technique, the
Dinkelbach's method and the successive convex approximation technique. Also,
the convergence and complexity of the proposed algorithm are analyzed. Finally,
simulation results show that the proposed communicate-while-fly scheme achieves
significant EE gains compared with the benchmark hover-and-fly scheme. Useful
insights on the optimal trajectory design and resource allocation are also
obtained.
