  Communications between unmanned aerial vehicles (UAVs) play an important role
in deploying aerial networks. Although some studies reveal that drone-based
air-to-air (A2A) channels are relatively clear and thus can be modeled as
free-space propagation, such an assumption may not be applicable to drones
flying in low altitudes of built-up environments. In practice, low-altitude A2A
channel modeling becomes more challenging in urban scenarios since buildings
can obstruct the line-of-sight (LOS) path, and multipaths from buildings lead
to additional losses. Therefore, we herein focus on modeling low-altitude A2A
channels considering a generic urban deployment, where we introduce the
evidence of the small-size first Fresnel zone at the millimeter-wave (mmWave)
band to approximately derive the LOS probability. Then, the path loss under
different propagation conditions is investigated to obtain an integrated path
loss model. In addition, we incorporate the impact of imperfect beam alignment
on the path loss, where the relation between path loss fluctuation and beam
misalignment level is modeled as an exponential form. Finally, comparisons with
the 3GPP model show the effectiveness of the proposed analytical model.
Numerical simulations in different environments and heights provide practical
deployment guidance for aerial networks.
