  Multiple-input multiple-output (MIMO) techniques can help in scaling the
achievable air-to-ground (A2G) channel capacity while communicating with
drones. However, spatial multiplexing with drones suffers from rank deficient
channels due to the unobstructed line-of-sight (LoS), especially in
millimeter-wave (mmWave) frequencies that use narrow beams. One possible
solution is utilizing low-cost and low-complexity metamaterial-based
intelligent reflecting surfaces (IRS) to enrich the multipath environment,
taking into account that the drones are restricted to fly only within
well-defined drone corridors. A hurdle with this solution is placing the IRSs
optimally. In this study, we propose an approach for IRS placement with a goal
to improve the spatial multiplexing gains, and hence to maximize the average
channel capacity in a predefined drone corridor. Our results at 6 GHz, 28 GHz
and 60 GHz show that the proposed approach increases the average rates for all
frequency bands for a given drone corridor, when compared with the environment
where there are no IRSs present, and IRS-aided channels perform close to each
other at sub-6 and mmWave bands.
