  The purpose of this article is to bestow the reader with a timely study of
UAV cellular communications, bridging the gap between the 3GPP standardization
status quo and the more forward-looking research. Special emphasis is placed on
the downlink command and control (C&C) channel to aerial users, whose
reliability is deemed of paramount technological importance for the commercial
success of UAV cellular communications. Through a realistic side-by-side
comparison of two network deployments -- a present-day cellular infrastructure
versus a next-generation massive MIMO system -- a plurality of key facts are
cast light upon, with the three main ones summarized as follows: (i) UAV cell
selection is essentially driven by the secondary lobes of a base station's
radiation pattern, causing UAVs to associate to far-flung cells; (ii) over a 10
MHz bandwidth, and for UAV heights of up to 300 m, massive MIMO networks can
support 100 kbps C&C channels in 74% of the cases when the uplink pilots for
channel estimation are reused among base station sites, and in 96% of the cases
without pilot reuse across the network; (iii) supporting UAV C&C channels can
considerably affect the performance of ground users on account of severe pilot
contamination, unless suitable power control policies are in place.
