This paper addresses the challenge of optimizing handover (HO) performance in non-terrestrial networks (NTNs) to enhance user equipment (UE) effective service time, defined as the active service time excluding HO delays and radio link failure (RLF) periods. Availability is defined as the normalized effective service time which is effected by different HO scenarios: Intra-satellite HO is the HO from one beam to another within the same satellite; inter-satellite HO refers to the HO from one satellite to another where satellites can be connected to the same or different GSs. We investigate the impact of open radio access network (O-RAN) functional splits (FSs) between ground station (GS) and LEO satellites on HO delay and assess how beam configurations affect RLF rates and intra- and inter-satellite HO rates. This work focuses on three O-RAN FSs -- split 7.2x (low layer 1 functions on the satellite), split 2 (layer 1 and layer 2 functions on the satellite), and gNB onboard the satellite -- and two beam configurations (19-beam and 127-beam). In a realistic dynamic LEO satellite constellation where different types of HO scenarios are simulated, we maximize effective service time by tuning the time-to-trigger (TTT) and HO margin (HOM) parameters. Our findings reveal that the gNB onboard the satellite achieves the highest availability, approximately 95.4%, while the split 7.2x exhibits the lowest availability, around 92.8% due to higher intra-satellite HO delays.

翻译：本文致力于优化非地面网络中的切换性能，以提升用户设备的有效服务时间——即扣除切换延迟与无线链路失效时段后的活跃服务时长。可用性定义为归一化的有效服务时间，其受不同切换场景影响：星内切换指同一卫星内不同波束间的切换；星间切换指不同卫星间的切换，这些卫星可连接至相同或不同的地面站。我们研究了地面站与低轨卫星间开放无线接入网功能分割对切换延迟的影响，并评估了波束配置如何影响无线链路失效率及星内与星间切换率。本研究聚焦于三种O-RAN功能分割方案——分割7.2x（低层物理层功能部署于卫星）、分割2（物理层与数据链路层功能部署于卫星）以及星载gNB方案，以及两种波束配置（19波束与127波束）。在模拟各类切换场景的真实动态低轨卫星星座中，我们通过调整触发时间与切换边界参数来最大化有效服务时间。研究结果表明，星载gNB方案可获得最高可用性（约95.4%），而分割7.2x方案因较高的星内切换延迟导致可用性最低（约92.8%）。