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.

Den nästa generationens trådlösa kommunikationssystem förväntas möjliggöra sömlös global täckning genom integrering av Non-Terrestrial Networks (NTNs) med terrestra nät. Till skillnad från stationära terrestra Base Stations (BSs) med stabil strömförsörjning innebär NTN-plattformar såsom Rotary Wing Drones (RWDs), Fixed Wing Drones (FWDs), High-Altitude Platforms (HAPs) och Low Earth Orbit (LEO)-satelliter utmaningar kopplade tillplattformsrörlighet, energi begränsningar och arkitektoniska funktioner. Dessa faktorer påverkar direkt tjänstetid—eller motsvarande tillgänglighet—förkommunikationslänkar till användare på marken.

Denna avhandling analyserar designfaktorer och arkitektoniska avvägningar som styr tjänstetillgänglighet över olika NTN-plattformar. För Aerial Base Stations (ABSs) utvecklas ett enhetligt ramverk för att utvärderaenergiförbrukning och solenergiskörd. Resultaten visar att RWDs endast kanupprätthålla tjänster i 5–60 minuter med obetydlig nytta av solenergi, medan FWDs och HAPs kan förlänga driftstiden till flera timmar respektivedagar. En nätverksdimensioneringsstudie kvantifierar dessutom antalet ABSsoch reservbatterier som krävs för kontinuerlig täckning, vilket belyser praktiska begränsningar för energibegränsade luftburna system.

För satellitbaserade NTNs introduceras ett digitalt tvillingramverk föratt modellera end-to-end-handoverfördröjningar under realistiska 3rd Generation Partnership Project (3GPP)-kompatibla antaganden. Resultaten visaratt placering av gNB ombord minskar den kumulativa Conditional Handover (CHO)-fördröjningen med cirka 25–30% jämfört med Split 7.2x, till priset av 55–70% högre beräkningsbelastning ombord. Konstellationsdesign harstor inverkan på tillgängligheten: ökad satellittäthet bortom en viss gräns germinskad tillgänglighet på grund av fler handover. En konstellation med medelhög täthet på låg höjd uppvisar omkring 11 minuters daglig nertid, vilketökar till 13–16 minuter vid högre täthet, medan en glesare konstellation påhögre höjd endast medför 5–7 minuter. Analysen visar att det ofta citeradetillgänglighetsmålet på 99.9% för LEO-system i praktiken är orealistiskt; enmaximal tillgänglighet runt 99.2% är möjlig, och funktionssplitten kan minska detta ytterligare (t.ex. från 99% till 98.5% vid övergång från gNB ombordtill Split 7.2x).

Sammantaget ger avhandlingen ett enhetligt perspektiv på tjänstetid somen grundläggande prestandametrik för NTNs—styrd av energibegränsningari luftburna plattformar och handoverdynamik i LEO-konstellationer—och erbjuder praktiska insikter för framtida design och optimering av NTN.

The evolution toward 6G networks introduces unprecedented challenges and opportunities, particularly in the realm of serving both aerial and ground users seamlessly. In this article, we propose a holistic adaptive combined airspace and non-terrestrial network (NTN) architecture designed to address the unique requirements of the 6G era. Three principle features - joint sensing, communication, and computation (JSCC) in three dimensions (3D), cloud-native and artificial intelligence (AI) native, and the flexibility of radio access network (RAN) and core functions of the proposed architecture - are presented. Next, two application scenarios are analyzed: one catering to aerial users and the other supporting ground users, each, in particular, supporting communication links. Finally, we look into the network management and control aspects of the proposed architecture and discuss challenges and future research directions.

As Non-terrestrial Networks (NTNs) becomes integral to future 6G systems, ensuring seamless connectivity and service continuity over Low Earth Orbit (LEO) satellite constellations is essential. This work investigates the impact of Open Radio Access Network (RAN) functional splits on handover performance in NTNs, focusing on minimizing service interruptions. We propose Effective Service Time as a novel availability metric that accounts for end-to-end Conditional Handover (CHO) delay, Radio Link Failures (RLFs), coverage gaps, and constellation-specific propagation dynamics-factors often simplified or ignored. Unlike baseline models that assume ideal, instantaneous switching with no protocol delays or topology changes, our CHO model reflects 3GPP-compliant, real-world constraints. Leveraging a digital twin-based satellite handover framework, we evaluate availability across multiple constellations, geographic regions, and Open RAN architectures (gNB onboard, Split 2, and Split 7.2x). Results reveal that increasing satellite density beyond a threshold yields diminishing returns, as denser constellations suffer more frequent handovers and higher downtime. For instance, a medium-density constellation with lower altitude achieves an average of 11 minutes of daily downtime, which rises to 13-16 minutes under a denser deployment. In contrast, a higher-altitude but sparser constellation provides only 5-7 minutes of downtime, benefiting from fewer handovers. Our analysis revealed that the claim of 99.9% availability in LEO is impractical, where we demonstrated that maximum 99.2% can be achieved with lower-altitude constellations. Moreover, functional splits impact performance: transitioning from gNB onboard to Split 7.2x can reduce availability from say about 99% to 98.5%. Finally, we construct a four-dimensional suitability map to identify optimal constellation-architecture pairings across a variety of service requirements defined by delay, modulation, reliability, and availability. Notably, stringent 50 ms delay requirements are not supported by higher-altitude constellations despite their higher availability, whereas lower-altitude constellations can satisfy them. This study provides valuable insights into NTN design, highlighting the interplay between satellite constellation, network architecture, and service-level guarantees.

Given the advancements in next-generation low Earth orbit (LEO) satellites, there is an expected shift from transparent architectures (acting as radio repeaters) to regenerative architectures (hosting a part or all of the gNodeB (gNB) onboard). Such regenerative architectures enable disaggregation and distribution of radio access network (RAN) functions between the ground and space. Open RAN is a promising approach for non-terrestrial networks and offers flexible function placement through open interfaces. The present study examines three open RAN-based regenerative architectures, namely, Split 7.2x (low-layer physical functions onboard), Split 2 (Layers 1 and 2 onboard), and a gNB onboard the satellite. Handover (HO) management becomes increasingly complex in this disaggregated RAN, particularly for LEO satellites, where the part of the gNB is constantly in motion. The choice of regenerative architecture and its dynamic topology influence the additional HO control signals required between the satellite and ground stations. Using a realistic dynamic LEO constellation model, we analyze the interplay among conditional handover (CHO) delay, computational complexity, and control signaling overhead under different network architectures. Our findings reveal that transitioning from a transparent architecture to Split 7.2x does not reduce CHO delay despite the introduction of additional onboard processing. The gNB onboard the satellite minimizes cumulative CHO delay but demands 55%-70% more computational resources than the Split 7.2x architecture. Conversely, although Split 7.2x is computationally more efficient, it increases the cumulative CHO delay by 25%-30%. Additionally, we observed that under limited onboard processing conditions, only the transparent and Split 7.2x architectures supported delay-sensitive services up to 100 ms. In contrast, under ample processing conditions, gNB was suitable for stringent 50 ms requirements, while Split 2 best supported delay-tolerant services with 200 ms requirements.

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.

High orbital velocity of Low Earth Orbit (LEO) satellites introduces frequent handovers and high Dopplers, making reliable mobility management a critical challenge. Earth-fixed beam operation has been proposed to reduce handover frequency, but this comes at the cost of increased Doppler dynamics, which may degrade link performance. Larger Subcarrier Spacing (SCS) improves Doppler tolerance but reduces spectral efficiency, creating a trade-off between availability and throughput. We propose a Joint Satellite and SCS Selection (JSSS) algorithm that dynamically selects the serving satellite and SCS to maximize successfully transmitted data per time slot while accounting for Doppler feasibility and handover delay. This paper investigates how the different placement of baseband processing functions (Split 7.2x, Split 2, and gNB) across Non-clustered (NC), 3-clustered (C3) and 5-clustered (5C) architectures-with varying cluster size and satellite density (1584- 1980)-affects mobility, availability, and throughput. Results show that Earth-fixed beams can achieve up to ~99.9% availability. With fixed CS algorithm, the optimal SCS is 240 kHz, yielding ~35 Mbps, while JSSS provides a modest additional gain of ~5%. Clustering does not yield a higher optimal throughput than NC, and even with a 25% increase in the number of satellites, C5 still fails to outperform NC.

This paper explores the evolution of Radio Access Network (RAN) architectures and their integration into Non-Terrestrial Networks (NTN) to address escalating mobile traffic demands. Focusing on Low Earth Orbit (LEO) satellites as key components of NTN, we examine the feasibility of RAN function splits (FSs) in terms of fronthaul (FH) latency, elevation angle, and bandwidth (BW) across LEO satellites and ground stations (GS), alongside evaluating performance of Conditional Handover (CHO) procedures under diverse scenarios. By assessing performance metrics such as handover duration, disconnection time, and control traffic volume, we provide insights on several aspects such as stringent constraints for Low Layer Splits (LLSs), leading to longer delays during mobility procedures and increased control traffic across the feeder link in comparison with the case when gNodeB is onboard satellite. Despite challenges, LLSs demonstrate minimal onboard satellite computational requirements, promising reduced power consumption and payload weight. These findings underscore the architectural possibilities and challenges within the telecommunications industry, paving the way for future advancements in NTN RAN design and operation.

Aerial base stations (ABSs) have emerged as a promising solution to meet the high traffic demands of future wireless networks. Nevertheless, their practical implementation requires efficient utilization of limited payload and onboard energy. Understanding the power consumption streams, such as mechanical and communication power, and their relationship to the payload is crucial for analyzing its feasibility. Specifically, we focus on rotary-wing drones (RWDs), fixed-wing drones (FWDs), and high-altitude platforms (HAPs), analyzing their energy consumption models and key performance metrics such as power consumption, energy harvested-to-consumption ratio, and service time with varying wingspans, battery capacities, and regions. Our findings indicate that FWDs have longer service times and HAPs have energy harvested-to-consumption ratios greater than one, indicating theoretically infinite service time, especially when deployed in near-equator regions or have a large wingspan. Additionally, we investigate the case study of RWD-BS deployment, assessing aerial network dimensioning aspects such as ABS coverage radius based on altitude, environment, and frequency of operation. Our findings provide valuable insights for researchers and telecom operators, facilitating effective cost planning by determining the number of ABSs and backup batteries required for uninterrupted operations.

As Non-terrestrial Networks (NTNs) becomes integral to future 6G systems, ensuring seamless connectivity and service continuity over Low Earth Orbit (LEO) satellite constellations is essential. This work investigates the impact of Open Radio Access Network (RAN) functional splits on handover performance in NTNs, focusing on minimizing service interruptions. We propose Effective Service Time as a novel availability metric that accounts for end-to-end Conditional Handover (CHO) delay, Radio Link Failures (RLFs), coverage gaps, and constellation-specific propagation dynamics-factors often simplified or ignored. Unlike baseline models that assume ideal, instantaneous switching with no protocol delays or topology changes, our CHO model reflects 3GPP-compliant, real-world constraints. Leveraging a digital twin-based satellite handover framework, we evaluate availability across multiple constellations, geographic regions, and Open RAN architectures (gNB onboard, Split 2, and Split 7.2x). Results reveal that increasing satellite density beyond a threshold yields diminishing returns, as denser constellations suffer more frequent handovers and higher downtime. For instance, a medium-density constellation with lower altitude achieves an average of 11 minutes of daily downtime, which rises to 13-16 minutes under a denser deployment. In contrast, a higher-altitude but sparser constellation provides only 5-7 minutes of downtime, benefiting from fewer handovers. Our analysis revealed that the claim of 99.9% availability in LEO is impractical, where we demonstrated that maximum 99.2% can be achieved with lower-altitude constellations. Moreover, functional splits impact performance: transitioning from gNB onboard to Split 7.2x can reduce availability from say about 99% to 98.5%. Finally, we construct a four-dimensional suitability map to identify optimal constellation-architecture pairings across a variety of service requirements defined by delay, modulation, reliability, and availability. Notably, stringent 50 ms delay requirements are not supported by higher-altitude constellations despite their higher availability, whereas lower-altitude constellations can satisfy them. This study provides valuable insights into NTN design, highlighting the interplay between satellite constellation, network architecture, and service-level guarantees.