Handover and Coverage Analysis in 3-D Mobile UAV Cellular Networks

With their superior maneuverability and flexible deployment options, unmanned aerial vehicles (UAVs) present a viable solution to augment cellular network capabilities by serving as mobile base stations (BSs). Yet, the 3-D, dynamic nature of UAV deployments, along with changing aerial conditions, often results in frequent handovers, adversely affecting communication quality. Moreover, the impacts of transitions between Line-of-Sight (LoS) and Non-LoS (NLoS) links on handover and coverage probabilities in mobile UAV networks have not been thoroughly investigated. This article provides an in-depth analysis of how multitier deployment altitudes and variations in LoS link probabilities influence handover and coverage probabilities. Utilizing stochastic geometry, we consider two association strategies: 1) distance-based and 2) strongest average received signal strength (RSS)-based. For both strategies, we provide semi-closed-form expressions for handover probability and subsequently derive network coverage probabilities. Through numerical simulations, we not only unearth an optimal configuration of UAV density and deployment altitude that maximizes coverage probability for ground users but also uncover that the relative benefits of the RSS-based association strategy wane as UAV density escalates compared to the distance-based association strategy. Furthermore, our results underscore the potential for enhancing coverage performance by adopting a strategy of deploying UAVs at various altitudes, contrasting with the traditional approach of uniform altitude deployment.