  The line-of-sight (LoS) air-to-ground channel brings both opportunities and
challenges in cellular-connected unmanned aerial vehicle (UAV) communications.
On one hand, the LoS channels make more cellular base stations (BSs) visible to
a UAV as compared to the ground users, which leads to a higher macro-diversity
gain for UAV-BS communications. On the other hand, they also render the UAV to
impose/suffer more severe uplink/downlink interference to/from the BSs, thus
requiring more sophisticated inter-cell interference coordination (ICIC)
techniques with more BSs involved. In this paper, we consider the uplink
transmission from a UAV to cellular BSs, under spectrum sharing with the
existing ground users. To investigate the optimal ICIC design and air-ground
performance trade-off, we maximize the weighted sum-rate of the UAV and
existing ground users by jointly optimizing the UAV's uplink cell associations
and power allocations over multiple resource blocks. However, this problem is
non-convex and difficult to be solved optimally. We first propose a centralized
ICIC design to obtain a locally optimal solution based on the successive convex
approximation (SCA) method. As the centralized ICIC requires global information
of the network and substantial information exchange among an excessively large
number of BSs, we further propose a decentralized ICIC scheme of significantly
lower complexity and signaling overhead for implementation, by dividing the
cellular BSs into small-size clusters and exploiting the LoS macro-diversity
for exchanging information between the UAV and cluster-head BSs only. Numerical
results show that the proposed centralized and decentralized ICIC schemes both
achieve a near-optimal performance, and draw important design insights based on
practical system setups.
