  In this article, we consider a cellular network deployment where UAV-to-UAV
(U2U) transmit-receive pairs coexist with the uplink (UL) of cellular ground
users (GUEs). Our analysis focuses on comparing two spectrum sharing
mechanisms: i) overlay, where the available time-frequency resources are split
into orthogonal portions for U2U and GUE communications, and ii) underlay,
where the same resources may be accessed by both link types, resulting in
mutual interference. We evaluate the coverage probability and rate of all links
and their interplay to identify the best spectrum sharing mechanism. Among
other things, our results demonstrate that, in scenarios with a large number of
UAV pairs, adopting overlay spectrum sharing seems the most suitable approach
for maintaining a minimum guaranteed rate for UAVs and a high GUE UL
performance. We also find that increasing the density of U2U links degrades
their rates in the overlay-where UAVs only receive interference from other
UAVs-, but not significantly so in the underlay -- where the effect of
GUE-generated interference is dominant.
