  We consider a cellular network deployment where UAV-to-UAV (U2U)
transmit-receive pairs share the same spectrum with the uplink (UL) of cellular
ground users (GUEs). For this setup, we focus on analyzing and comparing the
performance of two spectrum sharing mechanisms: (i) underlay, where the same
time-frequency resources may be accessed by both UAVs and GUEs, resulting in
mutual interference, and (ii)overlay, where the available resources are divided
into orthogonal portions for U2U and GUE communications. We evaluate the
coverage probability and rate of both link types and their interplay to
identify the best spectrum sharing strategy. We do so through an analytical
framework that embraces realistic height-dependent channel models, antenna
patterns, and practical power control mechanisms. For the underlay, we find
that although the presence of U2U direct communications may worsen the uplink
performance of GUEs, such effect is limited as base stations receive the
power-constrained UAV signals through their antenna sidelobes. In spite of
this, our results lead us to conclude that in urban scenarios with a large
number of UAV pairs, adopting an overlay spectrum sharing seems the most
suitable approach for maintaining a minimum guaranteed rate for UAVs and a high
GUE UL performance.
