  In this work, we explore the potential benefits of deploying unmanned aerial
vehicles (UAVs) as aerial base stations (ABSs) with sub-6GHz band and small
cells terrestrial base stations (TBSs) with millimeter wave (mmWave) band in a
hybrid heterogeneous networks (HetNets). A flexible non-orthogonal multiple
access (NOMA) based user association policy is proposed. By using the tools
from stochastic geometry, new analytical expressions for association
probability, coverage probability and spectrum efficiency are derived for
characterizing the performance of UAV-aided HetNets under the realistic
Air-to-Ground (A2G) channels and the Ground-to-Ground (G2G) channels with a LoS
ball blockage model. Finally, we provide insights on the proposed hybrid
HetNets by numerical results. We confirm that i) the proposed NOMA enabled
HetNets is capable of achieving superior performance compared with the OMA
enabled ABSs by setting power allocation factors and targeted
signal-to-interference-plus-noise ratio (SINR) threshold properly; ii) there is
a tradeoff between the association probabilities and the spectrum efficiency in
the NOMA enabled ABSs tier; iii) the coverage probability and spectrum
efficiency of the NOMA enabled ABSs tier is largely affected by the imperfect
successive interference cancellation (ipSIC) coefficient, power allocation
factors and SINR threshold; iv) compared with only sub-6GHz ABSs, mmWave
enabled TBSs are capable of enhancing the spectrum efficiency of the HetNets
when the mmWave line-of-sight (LoS) link is available.
