  Integrating unmanned aerial vehicles (UAVs) into the cellular network as new
aerial users is a promising solution to meet their ever-increasing
communication demands in a plethora of applications. Due to the high UAV
altitude, the channels between UAVs and the ground base stations (GBSs) are
dominated by the strong line-of-sight (LoS) links, thus severe interference may
be generated to/from the GBSs in the uplink/downlink, which renders the
interference management with coexisting terrestrial and aerial users a more
challenging problem to solve. In this paper, we study the uplink communication
from a multi-antenna UAV to a set of GBSs in its signal coverage region. Among
these GBSs, we denote available GBSs as the ones that do not serve any
terrestrial users at the assigned resource block (RB) of the UAV, and occupied
GBSs as the rest that are serving their respectively associated terrestrial
users in the same RB. We propose a new cooperative interference cancellation
strategy for the multi-beam UAV uplink communication, which aims to eliminate
the co-channel interference at each of the occupied GBSs and in the meanwhile
maximize the sum-rate to the available GBSs. Specifically, the multi-antenna
UAV sends multiple data streams to selected available GBSs, which in turn
forward their decoded data streams to their backhaul-connected occupied GBSs
for interference cancellation. To draw useful insights, the maximum
degrees-of-freedom (DoF) achievable by the multi-beam UAV communication for
sum-rate maximization in the high signal-to-noise ratio (SNR) regime is first
characterized, subject to the stringent constraint that all the occupied GBSs
do not suffer from any interference in the UAV's uplink transmission. Then,
based on the DoF-optimal design, the achievable sum-rate at finite SNR is
maximized, subject to given maximum allowable interference power constraints at
each occupied GBS.
