  Supporting reliable and seamless mobility for aerial users, such as unmanned
aerial vehicles (UAVs), is a key challenge for the next-generation cellular
systems. To tackle this challenge, we propose a joint beamforming, user
association, and UAV-height control framework for cellular-connected multi-UAV
networks with multiple antenna base stations (BSs). With the aim of maximizing
the minimum achievable rate for UAVs subject to co-existed terrestrial user
equipment's rate constraints, we devise a hierarchical bi-layer iterative
algorithm to optimize BSs' beamforming vectors, UAV association matrix, and the
height of UAVs jointly. With the aid of projection gradient method in inner
layer iteration and geometric program modelling plus convex-concave procedure
in outer layer iteration, our proposed algorithm is proved to converge to a
local optimum. Taking mobility characteristics of UAVs into account, we also
exploit our proposed algorithm for imperfect channel estimation scenario.
Numerical results show that our proposed algorithm can achieve improved UAVs'
minimum achievable rate compared with that of the conventional nearest
association of UAVs for both perfect and imperfect channel estimation
scenarios. Moreover, we also examine the trade-off between the UAVs' minimum
achievable rate and the frequency for updating optimization variables with
single moving UAV.
