  Unmanned aerial vehicle (UAV)-assisted communication is a promising
technology in future wireless communication networks. UAVs can not only help
offload data traffic from ground base stations (GBSs), but also improve the
quality of service of cell-edge users (CEUs). In this paper, we consider the
enhancement of cell-edge communications through a mobile relay, i.e., UAV, in
multi-cell networks. During each transmission period, GBSs first send data to
the UAV, and then the UAV forwards its received data to CEUs according to a
certain association strategy. In order to maximize the sum rate of all CEUs, we
jointly optimize the UAV mobility management, including trajectory, velocity,
and acceleration, and association strategy of CEUs to the UAV, subject to
minimum rate requirements of CEUs, mobility constraints of the UAV and causal
buffer constraints in practice. To address the mixed-integer nonconvex problem,
we transform it into two convex subproblems by applying tight bounds and
relaxations. An iterative algorithm was proposed to solve the two subproblems
in an alternating manner. Numerical results show that the proposed algorithm
achieves higher rates of CEUs as compared with existing benchmark schemes.
