  For users in hotspot region, orbital angular momentum (OAM) can realize
multifold increase of spectrum efficiency (SE), and the flying base station
(FBS) can rapidly support the real-time communication demand. However, the
hollow divergence and alignment requirement impose crucial challenges for users
to achieve air-to-ground OAM communications, where there exists the
line-of-sight path. Therefore, we propose the air-to-ground cooperative OAM
communication (ACOC) scheme, which can realize OAM communications for users
with size-limited devices. The waist radius is adjusted to guarantee the
maximum intensity at the cooperative users (CUs). We derive the closed-form
expression of the optimal FBS position, which satisfies the antenna alignment
for two cooperative user groups (CUGs). Furthermore, the selection constraint
is given to choose two CUGs composed of four CUs. Simulation results are
provided to validate the optimal FBS position and the SE superiority of the
proposed ACOC scheme.
