  The line-of-sight dominant air-ground channels have posed critical
interference issues in cellular-connected unmanned aerial vehicle (UAV)
communications. In this paper, we propose a new base station (BS) cooperative
beamforming (CB) technique for the cellular downlink to mitigate the strong
interference caused by the co-channel terrestrial transmissions to the UAV.
Besides the conventional CB by cooperatively transmitting the UAV's message,
the serving BSs of the UAVs exploit a novel interference transmission scheme to
effectively suppress the terrestrial interference to the UAV. Specifically, the
co-channel terrestrial users' messages are shared with the UAV's serving BSs
and transmitted via CB to cancel their resultant interference at the UAV. To
balance between the CB gains for UAV signal enhancement and interference
cancellation, we aim to maximize the UAV's receive
signal-to-interference-plus-noise ratio by jointly optimizing the power
allocations at all of its serving BSs for transmitting the UAV's and co-channel
terrestrial users' messages. First, we derive the closed-form optimal solution
to this problem in a special case and draw useful insights. Then, we propose an
algorithm to solve the problem optimally in the general case. As the optimal
solution requires centralized implementation with exorbitant message/channel
information exchanges among the BSs, we further propose a distributed algorithm
that is amenable to practical implementation, based on a new divide-and-conquer
approach, whereby each co-channel BS divides its perceived interference to the
UAV into multiple portions, each to be canceled by a different serving BS of
the UAV with its best effort. Numerical results show that the proposed
centralized and distributed CB schemes with interference transmission and
cancellation (ITC) can both significantly outperform the conventional CB
without applying ITC.
