  Aerial base stations (BSs) attached to unmanned aerial vehicles (UAVs)
constitute a new paradigm for next-generation cellular communications. However,
the flight range and communication capacity of aerial BSs are usually limited
due to the UAVs' size, weight, and power (SWAP) constraints. To address this
challenge, in this paper, we consider dynamic cooperative transmission among
multiple aerial BSs for power-efficient aerial communications. Thereby, a
central controller intelligently selects the aerial BSs navigating in the air
for cooperation. Consequently, the large virtual array of moving antennas
formed by the cooperating aerial BSs can be exploited for low-power information
transmission and navigation, taking into account the channel conditions, energy
availability, and user demands. Considering both the fronthauling and the data
transmission links, we jointly optimize the trajectories, cooperation
decisions, and transmit beamformers of the aerial BSs for minimization of the
weighted sum of the power consumptions required by all BSs. Since obtaining the
global optimal solution of the formulated problem is difficult, we propose a
low-complexity iterative algorithm that can efficiently find a
Karush-Kuhn-Tucker (KKT) solution to the problem. Simulation results show that,
compared with several baseline schemes, dynamic multi-UAV cooperation can
significantly reduce the communication and navigation powers of the UAVs to
overcome the SWAP limitations, while requiring only a small increase of the
transmit power over the fronthauling links.
