The impact of inter-site distance and Time-to-Trigger on Handover performance in LTE-A HetNets

Future cellular networks as envisaged by mobile operators, are expected to consist of macro cells overlaid with small nodes in dense architectures. These multi-tier deployments pose challenges to Mobility Management procedures like Handover, since traditional algorithms fail to keep up with heterogeneity, thus causing degradations in Handover performance. Finding ways to reduce unnecessary Handovers, mainly for fast moving users, and boost offloading of the lower speed ones to small cells, are subjects undergoing intense study. Most studies propose solutions based on the appropriate selection of the key parameters involved in the Handover procedure, such as the Hysteresis Margin and Time-to-Trigger. However, they do not take into account the impact that inter-site distance has on the Handover performance. In this framework, we study the dependency of the Handover procedure on the inter-site distance between a small cell and the overlaid macro cell in a two-tier deployment. Moreover, a Handover performance analysis in terms of Handover, Radio Link Failure, Handover Failure and Ping-Pong probabilities is carried out and evaluated through extensive simulations. Finally, the impact of TTT on the Handover performance is presented, while it is concluded that the appropriate TTT value should be selected according to inter-site distance, user profile (i.e speed) and overall mobility in the network automatically and in line with the concept of Self-Organized Networks (SON).