  Reconfigurable intelligent surfaces (RISs) bring various benefits to the
current and upcoming wireless networks, including enhanced spectrum and energy
efficiency, soft handover, transmission reliability, and even localization
accuracy. These remarkable improvements result from the reconfigurability,
programmability, and adaptation capabilities of RISs for fine-tuning radio
propagation environments, which can be realized in a cost- and energy-efficient
manner. In this paper, we focus on the upgrade of the existing fifth-generation
(5G) cellular network with the introduction of an RIS owning a full-dimensional
uniform planar array structure for unleashing advanced three-dimensional
connectivity. The deployed RIS is exploited for serving unmanned aerial
vehicles (UAVs) flying in the sky with ultra-high data rate, a challenging task
to be achieved with conventional base stations (BSs) that are designed mainly
to serve ground users. By taking into account the line-of-sight probability for
the RIS-UAV and BS-UAV links, we formulate the average achievable rate, analyze
the effect of environmental parameters, and make insightful performance
comparisons. Simulation results show that the deployment of RISs can bring
impressive gains and significantly outperform conventional RIS-free 5G
networks.
