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5G from Space: An Overview of 3GPP Non-Terrestrial Networks
Xingqin Lin, Stefan Rommer, Sebastian Euler, Emre A. Yavuz, Robert S. Karlsson
TL;DR
5G must be adapted to the complex, multi-layer requirements of non-terrestrial satellite networks. The paper surveys 3GPP’s design work across radio access, services, architecture, management, core, and terminals, finding that NR and the 5GC provide a basis for NTN with targeted enhancements for delay, mobility, and moving platforms. It also emphasizes that prototypes are needed to validate key design aspects beyond standardization.
Problem
Adapting 5G to non-terrestrial satellite networks requires coordinated design across radio access, services, system aspects, core, terminals, and management.
Method
The paper provides a comprehensive overview of 3GPP NTN work, detailing design aspects and the rationales influencing standardization across multiple working areas.
Results
3GPP found that NR and the 5GC are well prepared to support NTN with enhancements for delay, synchronization, HARQ, mobility, and management of moving non-terrestrial components.
Takeaways & Limitations
Early prototypes are needed to validate key NTN design aspects and provide feedback to standardization as normative work progresses.
Abstract
from arXiv · showhide
We provide an overview of the 3rd generation partnership project (3GPP) work on evolving the 5G wireless technology to support non-terrestrial satellite networks. Adapting 5G to support non-terrestrial networks entails a holistic design spanning across multiple areas from radio access network to services and system aspects to core and terminals. In this article, we describe the main topics of non-terrestrial networks, explain in detail the design aspects, and share various design rationales influencing standardization.
I. INTRODUCTION
3GPP is adapting 5G to support non-terrestrial networks, with satellite communications as the primary focus. This article provides a dedicated, detailed overview of the work across radio access, services, system aspects, core, and terminals.
- Article focus: The article’s objective is to explain the state of 3GPP NTN work, detailed design aspects, and the rationales influencing standardization.Its coverage spans radio access network, services and system aspects, core, and terminals.
- NTN scope: NTN is an umbrella term covering satellite communication networks, high altitude platform systems, and air-to-ground networks.Satellite platforms include LEO, MEO, and GEO satellites; HAPS include airplanes, balloons, and airships.
- 5G foundation: Satellite operators are engaging in 3GPP to integrate satellite networks into the 5G ecosystem and benefit from its economies of scale.Recent interest has especially centered on broadband delivered by large LEO satellite constellations.
- NTN scope: 3GPP NTN work has primarily focused on satellite communications, with implicit compatibility for HIBS and air-to-ground networks.Mobile-enabled low-altitude UAVs are handled in a separate 3GPP track, so the article gives them minimal treatment.
- 5G foundation: 5G NR provides a foundation for NTN adaptation through forward compatibility, low-latency support, advanced antennas, and flexible operation across low, mid, and high frequencies.3GPP is also studying NTN support for NB-IoT and LTE-M for massive IoT use cases.
II. RADIO ACCESS NETWORKS FOR NR NTN
3GPP’s Rel-15 study established reference NTN deployment scenarios and channel models for adapting NR to satellite and HAPS environments. The models account for geometry-dependent propagation effects across representative deployments and frequencies.
- Reference scenarios: The Rel-15 study selected reference NTN scenarios spanning S-band and Ka-band, GEO, LEO, and HAPS, earth-fixed and moving beams, and handheld and VSAT terminals.It also specified footprint sizes, minimum elevation angles, and antenna models.
- Channel models: The channel models support urban, suburban, and rural deployment scenarios.
- Channel models: NTN large-scale channel parameters differ from terrestrial propagation because satellite paths are nearly parallel and depend on the serving satellite’s elevation angle.Relevant parameters include line-of-sight probability, angular spread, and delay spread.
- Channel models: Path-loss modeling combines free-space path loss with clutter loss, shadow fading, atmospheric-gas absorption, and ionospheric and tropospheric scintillation losses.Clutter and shadowing values are tabulated by elevation angle and by S-band or Ka-band; some additional losses matter mainly at low elevation angles or under specific conditions.
- Channel models: The study developed generic frequency-selective and simpler two-state flat-fading models, plus clustered-delay-line and tapped-delay-line models for link-level simulations.The two-state model targets situations such as low frequencies, large elevation angles, and near-line-of-sight conditions.
B. Release-16 Study Item on NR NTN
The Rel-16 study identified the minimum architecture, protocol, and physical-layer features needed for NR NTN support. It concluded that existing NR functionality provides a good basis, while NTN-specific enhancements address delay, Doppler, and mobility.
- Study scope: Rel-16 followed the Rel-15 scenario and channel-model study by investigating solutions for adapting NR to NTN.
- Study scope: The study targeted a minimum feature set covering architecture, higher-layer protocols, and physical-layer aspects for NR support of satellite communications.
- Architecture and protocols: The study found no showstoppers among the explored NTN-based NG-RAN architecture options.It examined the split of a 5G base station into central and distributed units and the impacts of long propagation delays on user-plane protocols.
- Architecture and protocols: Mobility procedures require NTN-specific system information, earth-fixed tracking areas, cell-selection assistance, and enhanced handover procedures for moving satellites.These measures address frequent tracking-area updates and handovers, particularly for LEO satellites.
- Physical layer: Handheld UEs can be served by LEO and GEO in S-band, while high-gain terminals such as VSATs can use LEO and GEO in both S-band and Ka-band.The evaluation covered link-level and system-level performance in the two nominal frequency bands.
- Physical layer: Rel-15 and Rel-16 NR functionalities form a good basis for NTN, but enhancements are needed for timing, uplink time and frequency synchronization, and HARQ.
C. Release-17 Work Item on NR NTN
Rel-17 turns the Rel-16 findings into NR NTN work targeting LEO and GEO systems, with implicit support for HAPS and air-to-ground networks. The work adapts architecture, protocols, synchronization, retransmission, timers, and mobility procedures to long delays and moving coverage.
- Work-item scope: Rel-17 specifies enhancements for LEO and GEO NTN while implicitly targeting HAPS and air-to-ground networks.The work covers physical layer, protocols, architecture, radio resource management, RF requirements, and frequency bands.
- Architecture: The target architecture uses a transparent payload, earth-fixed tracking areas, and FDD systems with GNSS-capable UEs.A ground gNB connects through an NTN gateway and feeder link to the satellite or HAPS payload, which serves UEs over the service link.
- Synchronization: NTN uplink timing must accommodate propagation times much longer than a transmission slot.GNSS-based UEs estimate satellite relative speed and RTT for Doppler pre-compensation and combine UE-satellite RTT with a common timing advance.
- HARQ: 32 HARQ processes are used to mitigate transmission stalling caused by long RTTs when stop-and-wait feedback is pending.Rel-16 supports up to 16 processes, which can all await feedback under long-delay conditions.
- HARQ: When HARQ feedback is disabled, the UE can save energy, but robustness relies on lower target block-error rates and higher RLC retransmission and status-reporting activity.
- Mobility: MAC and RLC timers are extended, while satellite reselection, conditional handover, and measurement triggering incorporate UE location and satellite coverage timing.
A. Services and Requirements
3GPP identified satellite access and backhaul use cases for 5G and translated them into service requirements. A Rel-18 study further addresses the regulatory challenges of satellite and high-altitude coverage beyond national borders.
- Satellite access use cases include broadcasting, guaranteed IoT coverage, and mission-critical connectivity during disasters.
- Satellite backhaul use cases include fixed links for remote base stations and links for moving base stations on trains.
- The use-case analysis produced requirements covering NTN RAN satellite access, satellite backhaul, and non-3GPP satellite radio technology.
- The Rel-18 study addresses extra-territorial satellite and high-altitude coverage across multiple countries and international waters.
B. Architecture
3GPP studied how satellite access and backhaul affect the 5G system architecture while seeking to reuse terrestrial 5G solutions. The resulting design preserves core mobility and QoS frameworks with targeted enhancements for geographic movement, regulation, and satellite delay.
- Architecture: SA2 investigated satellite access and backhaul impacts on 5G architecture, aiming to reuse the existing 5GC and terrestrial solutions.
- Architecture: Earth-fixed tracking areas and geographically mappable cell IDs let the 5GC and service layer represent UE location despite moving radio cells.
- Architecture: Multicountry satellite coverage may require the AMF to verify that a UE is in a country it is permitted to serve.
- Architecture: Existing 5G QoS can be reused with enhancements because GEO satellite links may create delays beyond current standardized QoS classes.
- Architecture: New RAT type values allow core and service-layer functions to distinguish LEO, MEO, GEO, and terrestrial NR access for policy and charging.
- Architecture: The study concluded that the 5GC is well prepared for NR NTN access and satellite backhaul with small enhancements.
C. Telecom Management
3GPP studied management, orchestration, and charging for networks integrating satellite components, largely reusing existing 5G concepts. The main extensions address moving onboard gNB components, long delays, self-organization, monitoring, and terrestrial–non-terrestrial load balancing.
- Telecom Management: SA5’s study covers business roles, service and network management, orchestration, NTN RAN and non-3GPP satellite access, and backhaul.
- Telecom Management: Management impacts arise when gNB components are onboard moving LEO/MEO satellites and when long delays affect monitoring and performance indicators.
- Telecom Management: 5G self-organizing network concepts need enhancement for mobile non-terrestrial gNBs, while some HARQ-based performance measurements may be unavailable under long delays.
- Telecom Management: Load-balancing monitoring must expand from different radio technologies to balancing between terrestrial and non-terrestrial RAN.
IV. PROTOCOL AND NETWORK SELECTION ASPECTS FOR NR NTN
3GPP’s core-and-terminals work addresses how NR NTN changes UE–core protocols and network selection. Key issues are PLMN selection across international or multicountry coverage and protocol timing under long satellite delays.
- IV. PROTOCOL AND NETWORK SELECTION ASPECTS FOR NR NTN: The CT working groups define core network protocols and UE–core network protocols, with NR NTN work newly beginning in these groups.
- IV. PROTOCOL AND NETWORK SELECTION ASPECTS FOR NR NTN: CT1 handles UE–5GC protocols and network selection, making PLMN selection a central NR NTN issue.
- IV. PROTOCOL AND NETWORK SELECTION ASPECTS FOR NR NTN: NR NTN requires PLMN-selection procedures for international areas and global operators using non-country-specific MCCs, subject to national regulatory requirements.
- IV. PROTOCOL AND NETWORK SELECTION ASPECTS FOR NR NTN: CT3 and CT4 protocols between core-network functions have limited satellite impact, but may signal satellite access types and long-delay backhaul to policy and charging functions.
V. IOT NTN
3GPP is studying how LTE-M and NB-IoT can support satellite NTN scenarios, under defined assumptions about spectrum, orbits, payloads, power, and network support. The study addresses radio procedures, timing, mobility, and related system aspects, with GNSS-based compensation central to uplink operation.
- Study scope and objectives: The study targets LTE-M and NB-IoT operation below 6 GHz with LEO or GEO constellations, transparent payloads, and 20 dBm or 23 dBm device power.
- Study scope and objectives: Its objectives include identifying applicable scenarios and recommending changes for random access, HARQ, timers, mobility, system information, and tracking areas.
- Study scope and objectives: Both evolved packet core and 5GC networks are assumed to support the considered LTE-M and NB-IoT NTN scenarios.
- Timing and protocol adaptations: LTE-M and NB-IoT devices are assumed to use GNSS to estimate and pre-compensate timing and frequency offsets for uplink transmission.The study also considers GNSS power consumption, long RTT effects, and whether disabling HARQ feedback could provide benefits or drawbacks.
- Mobility: Idle-mode mobility uses legacy cell selection and reselection as the baseline, while connected-mode mobility is supported only for LTE-M.Potential conditional-handover enhancements are considered for moving- and fixed-cell scenarios; NB-IoT uses legacy radio-link-failure mobility as its baseline.
VI. CONCLUSIONS
5G provides a flexible foundation for NTNs, but satellite communications require holistic design across multiple 3GPP working groups and releases. The article surveys this standardization work while emphasizing that prototypes are needed to validate key design aspects and provide feedback.
- Conclusions: Satellite NTN design requires a holistic approach because the systems are complex and involve work from RAN through SA to CT across multiple releases.
- Conclusions: The article presents a comprehensive overview of 3GPP NTN work, covering core topics, design aspects, and rationales influencing standardization.
- Conclusions: Early prototypes are important for validating key NTN design aspects and providing prompt feedback to normative standardization.The first release of normative NTN standardization work was expected to be completed in 2022.