  In this paper, we consider a UAV-assisted small-cell having heterogeneous
users with different data rates and coverage demands. Specifically, we propose
a novel utility-aware resource-allocation protocol to maximize the utility of
UAV by allowing it to simultaneously serve the highest possible number of
heterogeneous users with available energy resources. In this regard, first, we
derive a closed-form expression for the rate-coverage probability of a user
considering Rician fading to incorporate the strong line of sight (LoS)
component in UAV communication. Next, since this UAV utility maximization
problem is non-convex and combinatorial, to obtain the global optimal resource
allocation policy we propose an iterative feasibility checking method for fixed
integers ranging from lower to upper bound on the number of users that can be
served by UAV. To further reduce the complexity, we formulate an equivalent
problem aimed at minimizing per user energy consumption, where tight analytical
relaxation on rate-coverage probability constraint is used along with
semi-closed expressions for joint-optimal power and time allocation. Lastly,
via detailed numerical investigation, we validate our analytical claims,
present insights on the impact of key system parameters, and demonstrate that
60% more users can be served using the proposed scheme, as compared to relevant
benchmarks.
