  This paper investigates a new downlink nonorthogonal multiple access (NOMA)
system, where a multiantenna unmanned aerial vehicle (UAV) is powered by
wireless power transfer (WPT) and serves as the base station for multiple pairs
of ground users (GUs) running NOMA in each pair. An energy efficiency (EE)
maximization problem is formulated to jointly optimize the WPT time and the
placement for the UAV, and the allocation of the UAV's transmit power between
different NOMA user pairs and within each pair. To efficiently solve this
nonconvex problem, we decompose the problem into three subproblems using block
coordinate descent. For the subproblem of intra-pair power allocation within
each NOMA user pair, we construct a supermodular game with confirmed
convergence to a Nash equilibrium. Given the intra-pair power allocation,
successive convex approximation is applied to convexify and solve the
subproblem of WPT time allocation and inter-pair power allocation between the
user pairs. Finally, we solve the subproblem of UAV placement by using the
Lagrange multiplier method. Simulations show that our approach can
substantially outperform its alternatives that do not use NOMA and WPT
techniques or that do not optimize the UAV location.
