  Ultra-dense networks (UDNs) envision the massive deployment of heterogenous
base stations (BSs) to meet the desired traffic demands. Furthermore, UDNs are
expected to support the diverse devices e.g., personal mobile devices and
unmanned ariel vehicles. User mobility and the resulting excessive changes in
user to BS associations in such highly dense networks may however nullify the
capacity gains foreseen through BS densification. Thus there exists a need to
quantify the effect of user mobility in UDNs. In this article, we consider a
three-dimensional N-tier downlink network and determine the association
probabilities and inter/intra tier handover rates using tools from stochastic
geometry. In particular, we incorporate user and BSs' antenna heights into the
mathematical analysis and study the impact of user height on the association
and handover rate. The numerical trends show that the intra-tier handovers are
dominant for the tiers with shortest relative elevation w.r.t. the user and
this dominance is more prominent when there exists a high discrepancy among the
tiers' heights. However, biasing can be employed to balance the handover load
among the network tiers.
