Non-Terrestrial Network Architecture and Design : Functional Splits, Handover Performance, and Service Availability

The next generation of wireless communication systems envisions seamless global coverage through the integration of Non-Terrestrial Networks (NTNs) with terrestrial infrastructures. Unlike terrestrial Base Stations (BSs), NTN platforms such as Rotary Wing Drones (RWDs), Fixed Wing Drones (FWDs), High-Altitude Platforms (HAPs), and Low Earth Orbit (LEO) satellites introduce challenges due to platform mobility, power limitations, and architectural constraints. These factors directly affect the service time—or equivalently, the availability—of communication links. This thesis analyzes the design factors and architectural trade-offs that govern service availability across diverse NTN platforms. For Aerial Base Stations (ABSs), a unified framework is developed to evaluate power consumption and energy harvesting. The results show that RWDs sustain service for only 5–60 minutes with negligible solar harvesting benefits, whereas FWDs and HAPs extend operation to several hours and days, respectively. A network dimensioning study quantifies the number of ABSs and backup batteries required for continuous coverage, highlighting deployment constraints of energy-limited aerial systems. For satellite-based NTNs, a digital twin framework is introduced to model end-to-end handover delays under realistic 3rd Generation Partnership Project (3GPP)-compliant assumptions. The results show that placing the gNB on-board reduces cumulative Conditional Handover (CHO) delay by approximately 25–30% relative to Split 7.2x, at the expense of 55–70% higher on-board computation. Constellation design strongly impacts availability: increasing satellite density beyond a threshold yields diminishing availability due to more frequent handovers. A medium-density, low-altitude constellation exhibits 11 minutes of daily downtime, increasing to 13–16 minutes when densified, whereas a sparser, higher-altitude constellation achieves only 5–7 minutes. The commonly cited 99.9% availability target for LEO is shown to be impractical; a maximum of approximately 99.2% is achievable, with functional split choices further reducing availability (e.g., from 99% to 98.5% when moving from gNB onboard to Split 7.2x). Overall, this thesis provides a unified perspective on service time as a fundamental performance metric across NTN platforms—whether constrained by energy limitations in aerial systems or by handover dynamics in LEO satellite constellations—offering practical insights to guide the design and optimization of NTN.