  This paper explores the effects of three-dimensional (3D) antenna radiation
pattern and backhaul constraint on optimal 3D path planning problem of an
unmanned aerial vehicle (UAV), in interference prevalent downlink cellular
networks. We consider a cellular-connected UAV that is tasked to travel between
two locations within a fixed time and it can be used to improve the cellular
connectivity of ground users by acting as a relay. Since the antenna gain of a
cellular base station changes significantly with the UAV altitude, the UAV can
increase the signal quality in its backhaul link by changing its height over
the course of its mission. This problem is non-convex and thus, we explore the
dynamic programming technique to solve it. We show that the 3D optimal paths
can introduce significant network performance gain over the trajectories with
fixed UAV heights.
