  The large-scale and ever-growing use of unmanned aerial vehicles (UAVs) in a
wide range of applications is foreseen to be a major part of beyond 5G and 6G
wireless networks in the next decade. The effective support of such massive
deployment of UAVs requires offering reliable, secure, and cost-effective
wireless connectivity. In this regard, cellular networks play essential roles
in serving UAVs acting as flying user equipments. While the cellular networks
provide promising connectivity solutions for UAVs, enabling robust UAV
operations faces several challenges. In this paper, an overview of key barriers
and design considerations of widespread commercial use of flying UAVs are
presented along with their potential solutions. In addition, we discuss how
cellular networks can support UAVs by relying on their advanced features,
network intelligence, and exploiting new tools from machine learning. Finally,
we shed light on offering wireless services to high altitudes and the
integration of non-terrestrial networks with terrestrial networks towards
limitless connectivity in 6G.
