  In this paper, we propose a cell-free scheme for unmanned aerial vehicle
(UAV) base stations (BSs) to manage the severe intercell interference between
terrestrial users and UAV-BSs of neighboring cells. Since the cell-free scheme
requires enormous bandwidth for backhauling, we propose to use the
sub-terahertz (sub-THz) band for the backhaul links between UAV-BSs and central
processing unit (CPU). Also, because the sub-THz band requires a reliable
line-of-sight link, we propose to use a high altitude platform station (HAPS)
as a CPU. At the first time-slot of the proposed scheme, users send their
messages to UAVs at the sub-6 GHz band. The UAVs then apply match-filtering and
power allocation. At the second time-slot, at each UAV, orthogonal resource
blocks are allocated for each user at the sub-THz band, and the signals are
sent to the HAPS after analog beamforming. In the HAPS receiver, after analog
beamforming, the message of each user is decoded. We formulate an optimization
problem that maximizes the minimum signal-to-interference-plus-noise ratio of
users by finding the optimum allocated power as well as the optimum locations
of UAVs. Simulation results demonstrate the superiority of the proposed scheme
compared with aerial cellular and terrestrial cell-free baseline schemes.
