Handover Analysis and Coverage Modelling in Ultra-Dense Heterogeneous Networks

Abstract

Ultra-Dense Networks (UDNs) are one of the most important trends towards next generation cellular systems. It is expected that small cell densification will offload traffic from traditional macro base stations, and thus significantly boost network capacity. Despite its promising capacity gains, UDNs can lead to frequent handovers (HOs), which in turn can cause significant network overheads and a decline in user experience. With the aim of modelling handovers in the context of ultra-dense heterogeneous networks, we first propose a low-complexity analytical framework for multi-target small cell handovers. Our proposed HO framework accurately models important context-aware parameters of user velocity, small cell density and the effect of received power filtering and HO failure. To avoid load imbalance, we derive a simple HO threshold condition that leverages multiple cell load conditions while also guaranteeing the expected throughput of small cell users. Furthermore, we also propose a novel approach to model coverage regions of overlapping small cells. Based on this model, we derive the cumulative distribution function of the sojourn time in small cells using boundary length and chord length distributions of small cell coverage regions. Our model is comprehensive enough to capture both inter-tier and intra-tier HOs in small cell networks. The derived analytical results provide guidance for optimizing handover parameters based on user velocity and small cell density to reduce network overhead and improve user experience. Finally, a downlink coverage analysis of an unmanned aerial vehicle (UAV) assisted network with clustered UEs is presented. In this model, Nakagami fading is used to capture line-of-sight channels for air-to-ground communication. Simulations show that line-of-sight channels can be well approximated with minimal computation power.