SatCom&NewSpace&SatNet

5GNTN-S & 6G3D

Status of the NTN

NTN typesSatellite communication systems is a sustainable solution for providing connectivity to remote or rural areas unserved or under-served by terrestrial communications systems. The Satellite communications coverage is already global, composed of geostationary orbit GEO satellites, medium Earth orbit (MEO), Low Earth Orbit (LEO), and Highly Elliptical Orbiting (HEO) constellations. On top of the satellite systems, under the umbrella of non-terrestrial networks, High Altitude Platform Systems (HAPS) have been added to provide non terrestrial but local service coverage [3GPP TR38.821].GEO satellites match the rotation of the Earth as they travel, at an altitude of 35,786 km, in a position fixed relative to a point on the ground. Their height means that data transmitted to and from the satellite have a relatively high latency, with an average round-trip latency of around 500 ms. Having a large beam footprint means that only few satellites are needed to span the entire globe. Hundreds of GEO satellites are in orbit today, delivering services such as weather data, broadcast TV, and some low-speed data communicationMEO satellites are found at altitudes between LEO and GEO satellites historically been used for GPS and other navigation applications. MEO lower height means that more satellites are needed to have a global footprint, with anywhere from 5 to 30 satellites required depending on their altitudeLEO satellites are an emerging category of satellite that promises to deliver very low latency service to users, comparable to that of terrestrial networks. Operating at altitudes between 300 to 1500 km above ground, LEO satellites can have a round-trip latency as low as 20 ms. LEO satellites having significant lower beam footprint with a range between 50 to 500km.

3GPP and Non-3GPP NTNs: deployment optionsThe use of satellites within the mobile network ecosystem was historically for providing backhaul connectivity. Recent advancements in waveforms, allowed the direct UE access, NR over satellite, and led 3GPP to identify two architectures based on the satellite payload type (i.e., , transparent and regenerative).In case of a transparent payload, the satellite provides connectivity between the users and the ground gNB. The satellite payload acts as an analogue RF repeater, repeats the NR-Uu radio interface, between the NTN gateway and the satellite (feeder link) and between the satellite and the UE (service link or access link).Generally, NTN architectures generally have a single satellite network operator (SNO) in mind. Therefore, most mobile network operators (MNOs), which provide broadband wireless services in most countries by building numerous ground base stations, are using TN as their main network, and there is a need to think about an evolved open network architecture to utilize NTN as a supplement. On the other hand, in case of a regenerative payload, the signals received from Earth being regenerated on the satellite payload. In that scenario NR-Uu radio interface is on the service link while NG interface transports on the feeder link on the Satellite Radio Interface (SRI). In addition, regenerative payload, allows intersatellite link ISL communications, which is a key enabler for mobility procedures in case of a constellation of satellites. Within 3GPP Release 19, there are different scenarios currently under consideration, such as a gNB on the satellite, split gNB (DU and RU on the satellite and CU on ground), with or without ISL as well as the UPF on board.Regenerative NTN deployments have its challenges related to complexity, power consumption, and latency constraint.

Key NTN features from Release 17 to Release 19The ongoing 3GPP standardization efforts for NTN will continue into Release 19 and subsequent releases. These proposals will aim to provide new capabilities and leverage regenerative satellites for enhanced features and topologies. Mobility enhancements, enhancements for F1 interface in case of DU on-board, NG interface enhancements, gNB with UPF on-satellite for mobility and Multicast and Broadcast Services (MBS) support are among the topics discussed in recent 3GPP meetings. [3GPP RAN plenary meeting (RAN#102, December, Edinburgh)]. Figure shows the historical timeline and topics from Release 17 to current and future releases.

3GPP NTN ArchitecturesFigure illustrates an example implementation of a Non-Terrestrial Network for transparent NTN payload [3GPP TS38.300].The NTN gNB communicates with the NTN UE using the NR air interface via the NTN-Gateway (NTN-GW) and the NTN payload. The NTN platform houses the NTN payload that supports radio communications. Examples of an NTN platform include a Geosynchronous Earth Orbit (GEO) satellite, Medium Earth Orbit (MEO) satellite, a Low Earth Orbit (LEO) satellite, and a High-Altitude Platform Station (HAPS). A GEO satellite-based NTN experiences very long delays (hundreds of milliseconds), while a LEO satellite-based NTN experiences variable but smaller propagation delays (e.g., tens of ms) and large and varying Doppler shifts. A HAPS resides in the stratosphere and maintains its position relative to a given point on the Erath’s surface like a GEOsatellite. The HAPS-based NTN has the least propagation delay.The link between the UE and the NTN payload is called the access link or the service link, and the link between the NTN-GW and the NTN payload is called the feeder link. The 3GPP specifications focus on the access link; the feeder link is implementation specific. In the transparent payload architecture, the gNB resides on the Earth’s surface and the NTN payload acts as a relay/repeater by performing functions such as the power amplification and (potentially) frequency translation. The 3GPP Release 17 and 18 support the transparent payload NTN architecture shown in Figure 2-2. At the time of this writing, the 3GPP is considering a regenerative payload, where certain gNB functionality resides on the NTN payload. The 3GPP has defined NR-NTN and IoT-NTN. NR-NTN utilizers high-performance NR air interface and hence is suitable for high data rate applications. IoT-NTN supports LTE-M and NB-IoT air interfaces1 and hence suitable for delay-tolerant and low data rate IoT applications.

NTN Transport NetworksLEO satellite networks are now poised to offer high-bandwidth, low-latency transport connectivity with global coverage. SNOs are building fully interconnected, high-capacity space networks designed to deliver resilient, high-performance services. This development presents a significant opportunity for Mobile Network Operators (MNOs) to collaborate with SNOs, expanding network capacity and extending coverage to previously unconnected populations.NTNs can play a critical role in providing backhaul connectivity for base stations located in remote or sparsely populated areas, where deploying traditional terrestrial infrastructure such as fiber optic cables or microwave towers is either too costly or impractical. In such scenarios, LEO satellites offer a cost-effective alternative to traditional terrestrial backhaul solutions. Additionally, in the aftermath of natural disasters or emergencies where terrestrial infrastructure may be damaged or unavailable, NTNs can provide rapid restoration of connectivity, ensuring minimal service disruption.

Role of the NTN in 6G6G is expected to seamlessly integrate multiple access types, Terrestrial Network (TN) and NTN, into a unified architecture. The vision for 6G involves NTN as a key enabler for extended coverage, security, and resilience. In addition, Positioning, Navigation and Timing (PNT) will be key challenges as well as in integrated management of unified networks and earth sensing services (Integrated Sensing and Communications - ISAC).Within 3GPP, 6G standards, on which work will commence in 2025 (Rel. 20), are expected for a potential rollout in 2030. 3GPP RAN groups have started discussions on potential architectures and other aspects, towards 6G.

Introduction

NTN encompasses satellites, high altitude platform stations (HAPS), and unmanned aerial systems (UAS)/drones operating in different orbits and ranges as depicted in figure. The non-terrestrial connectivity is provided by an NTN payload, i.e., a network node (e.g., radio relay unit) on a satellite or HAPS. Ground/terrestrial-based cellular network element components (eNB/gNB, DU/CU, etc.) send the signal via an NTN gateway interconnected by a feeder link to a satellite-based radio access network (RAN) element, to beam the cellular connectivity from space. User equipment accesses NTN cellular network services through the NTN payload via a service link. Feeder link and service link are terms associated with satellitebased communication. The feeder link is used to send instructions and data to the satellite from the ground station and receive information back. The service link is used to send and receive signals between the satellite and devices connected to it.
IMT 2020 S proposalIMT 2020-S is a reference to work in International Telecommunication Union (ITU) where requirements have been established for NTN, and new bands are being investigated These are expected to have an MSS allocation, it is unclear if they will have an ATC support but that may be a regional regulatory ruling. The spectrum allocation are expected to be finalized at the end or WRC 27 AI 1.12The ITU has been entrusted with facilitating the resource allocation, regulation, and safeguarding of the satellite component of the International Mobile Telecommunications-2020 (IMT-2020) systems. This work is guided by ITU-R Study Group 4 (Satellite Services) and Resolution ITU-R 65 to facilitate the development of proposals for the satellite component of the radio interface(s) for IMT-2020 and their subsequent evaluation. As the terrestrial component of IMT-2020 is already well-defined and established, the satellite component is expected to consider the capabilities, system architecture, and radio interface(s) of the terrestrial component to adopt a common architecture framework. The report ITU-R M.2514iv defines the vision for the satellite component of IMT-2020 for an efficient IMT service delivery with respect to application scenarios, services, system, radio, and network interface aspects. Further developments and recommendations on the satellite component of the integrated/hybrid satellite-terrestrial network systems are in progress.
3GPP work3GPP started the study on NTN for NR from Release 15 of its specification series by defining use cases and investigating the basic characteristics of NTN such as channel model and potential impacts on NR. The study phase continued in Release 16 from both system and RAN perspective. Finally, in Release 17, NTN requirements were added to the 3GPP specifications. In addition to NR NTN, this Release also supports NTN for cellular IoT (CIoT) such as NB-IoT. A brief overview of evolution of NTN in Release 17 and 18 is provided in the previous white papers from 5GAmericas.In Release 19 the following item are under development:

Downlink coverage enhancement for GSO and NGSO constellations: employing link level or

system level optimizations to improve the downlink coverage considering the limited powerof satellite and power sharing among beams.

Uplink capacity optimization: Due to large footprint of an NTN cell, a large number of UEs

are in the coverage area of the cell, competing for the resources. Therefore, the limitedspectrum resource needs to be efficiently managed (e.g., by means of multiplexing multipleUEs using orthogonal cover codes).

Enhanced Multicast-Broadcast Services (MBS): MBS may be intended for a sub-region of

the entire cell coverage. The existing mechanisms need to be updated to indicate whichregion is the target of MBS.

Support of NTN architecture with regenerative payload: This support is necessary to enable

UE-Satellite-UE and Network-UE communications. In addition, mechanism for intra andinter-gNB mobility needs to investigated for any necessary enhancements.

Introduction of RedCap and eRedCap NTN on FR1: RedCap devices can outperform 4G

IoT technology (i.e., NB-IoT) while keeping the complexity level low. In R19, the essentialchanges required to support NTN RedCap/eRedCap is under study.

Inter-RAT mobility from E-UTRA TN to NR NTN: Mobility inside NR from TN to NTN and viceversa is defined in Release 17 and 18. Release 19 is focusing on idle-mode mobility based on cell reselection from E-UTRA TN to NR NTN.

Some of these items are also studied for IoT NTN, such as store and forward, which supports cost-effective and delay-tolerant IoT applications. In addition, due to a massive number and diverse types of NB-IoT devices in the coverage area, the uplink capacity enhancement for IoT system is being studied.

Non-Terrestrial Networks (NTNs) are advanced wireless communication systems that operate above the Earth's surface using satellites, high-altitude platform stations (HAPS), and drones. Unlike traditional terrestrial networks that rely on ground-based infrastructure, NTNs leverage spaceborne and airborne assets to provide connectivity, especially in remote and underserved areas. They play a crucial role in enhancing global communication coverage and are integral to the 5G and future 6G ecosystems, supporting a wide range of applications from internet access to critical communications. Scientists and visionaries have long envisioned creating networks of low-Earth orbit (LEO) satellites that could connect everyone on Earth. Current NTN commercialization efforts can trace their roots back to the 1990s, when two companies, Globalstar and Iridium Communications, formed and launched major satellite networks that aimed to realize that goal.
Since 2022, Smart Phone access to satellite development progress could be classified into four technical options:

#1 option consists in integrating the legacy satellite communication solutions into new 5G smart phones. Because the legacy LEO satellites have limited capability (e.g., equivalent isotropic radiated power (EIRP), antenna gain-to-noise-temperature (G/T)), and are unable to make up for the link budget loss caused by replacing bulky antennas with regular smart phone antennas, the achievable data rates will be generally low and only suitable for messaging services (e.g., emergency use).
#2 option relies on proprietary satellite onboarded Long Term Evolution (LTE) base station, i.e., eNB technology implementation to compensate for the latency, Doppler, and link budget limitations. Due to various LTE user equipment (UE) capability restrictions (e.g., 10 ms scheduling delay tolerance and maximum Doppler tolerance in high speed train scenarios), the interoperability testing (IOT) could be quite challenging in the field because legacy LTE UE implementation did not assume the eNB signal may come from LEO satellites instead of terrestrial base stations.
#3 option makes use of the Third Generation Partnership Project Internet of Things NTN (3GPP IoT NTN) technology implementation on smart phones for satellite services (e.g., messaging, small data). According to previous field experiments, new 5G satellite Narrow-Band Internet of Things (NB-IoT) technology establishing a bi-directional link with several kbps from MediaTek’s satellite-enabled standard NB-IoT devices to a commercial Geosynchronous Equatorial Orbit (GEO) satellite would be feasible, thanks to sophisticated radio technologies designed for cellular IoT. By leveraging the global coverage strength of the GEO satellite technology, the vision of always-connected smart phones was recently realized for satellite messaging systems with Bullitt Satellite Connect service.
#4 option makes use of the 3GPP New Radio (NR) NTN technology on smart phone for wideband satellite services (e.g., voice call, satellite data services). Although NR NTN can support wider signal bandwidth compared to NB-IoT NTN, the experiment observes that more advanced LEO satellite capabilities (e.g., EIRP, G/T) are essential to close the link budget gap and offer Mb/s level data rate. This option promises to be the long-term trend, enabling future 5G smart phones to directly connect with LEO satellites.

Since 2022, 3GPP NTN NR and IoT solutions were standardized in Release 17. However, the baseline NR and NB-IoT waveforms and protocols were defined in Release 15 and Release 13/14, respectively. Hence 5G NTN is an add-on solution which leverages all the legacy NR and NB-IoT R&D effort for economics of scale advantage. It also means there is still room for enhancement if NTN requirements are considered from Day 1 when redesigning the radio interface for 6G. Three types of satellite service links are supported in 3GPP standards:

Earth-fixed provisioned by beam(s) continuously covering the same geographical areas all the time.
Quasi-Earth-fixed provisioned by beam(s) covering one geographic area for a limited period and a different geographic area during another period.
Earth-moving provisioned by beam(s) whose coverage area slides over the Earth surface.

ITU-R Work on IMT-Satellite/NTNThe Non-Terrestrial Network (NTN) will complement existing terrestrial mobile networks and enhance the next generation of mobile networks and services, striving to improve connectivity for users in unserved and underserved areas, benefiting both consumers and industries.From the very beginning of the IMT-development, a satellite component was part of the ITU-R work and a series of ITU-R Recommendations for the satellite component for the according IMT generation has been developed:

IMT-2000. Recommendations ITU-R M.818-2, M.1167, M.1182-1, M.1850-2 andM.2014-1
IMT-Advanced. Recommendation ITU-R M.2047
IMT-2020. Report ITU-R M.2514 – Vision, requirements and evaluation guidelines for the satellite radio interface(s) of IMT-2020; Ongoing work in ITU-R WP 4B regarding the according IMTprocess, incl. the evaluation of potential RIT-candidates.
IMT-2030. TBD The phase (2024-2027) will be the definition of relevant requirements and evaluation criteria for potential radio interface technologies (RIT) including NTN.

Around 2030, 6G, an IMT2030 and beyond technology, will be commercially available as the result of a global standard that will be initiated by 3GPP from 2024. The 6G ecosystem will be more diverse, in terms of e.g., traffic and device types, spectrum ranges and regimes, and networking topologies driving the 6G design. The convergence of terrestrial and satellite networks will be an integral part of 6G, building on the early adoption of open standard satellite communication systems based on 5G technology. This will lead to an expansion of the digital transformation of our society, whether for individual consumers, businesses or governments across various customer markets. It will strive to provide universal and affordable access to the Internet - anywhere and at any time. Convergence of the technology designs of terrestrial and satellite networking is key for enabling the economies of scale needed todrive satellite connectivity to the mass market of end devices. This was a key requirement behind 5G NTN, and will continue to be essential for 6G NTN.

Non-Terrestrial Networks (NTNs) has become an umbrella term for any network that involves non-terrestrial flying objects. The NTN family includes satellite communication networks, high altitude platform systems (HAPS), and air-to-ground networks.

Satellite communication networks utilize spaceborne platforms which include low Earth orbiting (LEO) satellites, medium Earth orbiting (MEO) satellites, and geosynchronous Earth orbiting (GEO) satellites. Over the past several years, the world has witnessed resurging interest in the broadband provisioned by LEO NTNs with large satellite constellations (Starlink, Kuiper, OneWeb). To benefit from the economies of scale of the 5G ecosystem, the satellite industry has engaged in the 3GPP process to integrate satellite networks into the 5G ecosystem.
HAPS are airborne platforms which can include airplanes, balloons, and airships. In the 3GPP NTN work, the focus is on high altitude platform stations as International Mobile Telecommunications base stations (HIBS). A HIBS system provides mobile service in the same frequency bands used by terrestrial mobile networks.
Air-to-ground networks aim to provide in-flight connectivity for airplanes by utilizing ground stations which play a similar role as base stations (BSs) in terrestrial mobile networks. But the antennas of the ground stations in an air-to-ground network are up-tilted towards the sky, and the inter-site distances of the ground stations are much larger than that of terrestrial mobile networks.

The focus of 3GPP NTN work has been on satellite communications networks, with implicit compatibility to support HIBS systems and air-to-ground networks. It is worth noticing that 3GPP has also been working on mobile enabled low-altitude unmanned aerial vehicles (UAVs, aka. drones), which can be considered as part of the NTN family in a wide sense. However, this line of work has been carried out in a separate track in 3GPP?! 5G New Radio (NR) based NTN has been the main focus in 3GPP.

3GPP NTN standard is a part of the 5G connectivity infrastructure. Airborne or spaceborne 5G base stations can be launched and connected to terrestrial ground stations in a commercially feasible way. These high-altitude network segments maximize the inherent value of 5G networks by solving coverage problems and difficult use cases that ground-based infrastructure alone cannot address.

Release 15: Integration of satellite in 5G TS 22.261
Release 16: Non-Terrestrial Networks were addressed in 2 study items (SI) in TR 22.822 and TS 22.261
Release 17: NTN integration began, paving the way for seamless 5G coverage. R17 (ASN.1 frozen in June 2022) was the first release with normative requirements for NTN in 3GPP specifications.
Release 18: Enhanced features and capabilities, focusing on improved satellite-terrestrial convergence. 3GPP has two submissions for the satellite component of IMT-2020 to ITU-R WP4B: NR NTN (NR satellite access), and IoT NTN (NB-IoT/eMTC satellite access).
Release 19 (ongoing): Lays the foundation for natively integrated NTN frameworks in 6G. Results will be documented in TR 22.822, TR 23.700-29, TR 28.874, TR 23.700-01.

3GPP defines NTN as networks, or segments of networks, using an airborne or spaceborne vehicle for transmission :

Spaceborne vehicles: Satellites (including Low Earth Orbiting (LEO) satellites, Medium Earth Orbiting (MEO) satellites, Geostationary Earth Orbiting (GEO) satellites as well as Highly Elliptical Orbiting (HEO) satellites).
Airborne vehicles: High Altitude Platforms (HAPs) encompassing Unmanned Aircraft Systems (UAS) including Lighter than Air UAS (LTA), Heavier than Air UAS (HTA), all operating in altitudes typically between 8 and 50 km, quasi-stationary.

3GPP initially defined several primary NTN use cases as part of its efforts, including:

Multi-connectivity: The user equipment (UE) simultaneously attaches to terrestrial and satellite links for multi-connectivity. This setup allows routing time-sensitive low-latency traffic through the terrestrial links, while satellite links manage less mission-critical traffic.
Fixed cell connectivity: This use case enables users in remote geographical areas and other places like ships at sea or offshore oil platforms to access 5G services.
Mobile cell connectivity: The UE accesses available terrestrial networks and automatically switches to satellite links in remote areas, providing seamless 5G coverage to aircraft and high-speed rail.

The mobile communications industry has also outlined three other use cases for NTNs, including:

Increasing network resiliency by providing a backup in case of network outages and bolstering network availability by aggregating multiple network connections in parallel to prevent complete network connection outages.
Enabling mobile network operators to stitch together unconnected areas of 5G network coverage.
Delivering satellite-based television or multimedia services using 3GPP’s 5G multicast-broadcast services (MBS) specification.

Transparent payload (Rel-17/18)

● Base station (gNB) on the ground● Transparent (“bent pipe”) satellite payload: Radio interface terminated on the ground● All network interfaces terminate on the ground● Rel-17: S-band & L-band (2 GHz & 1.6 GHz) handheld devices● Rel-18: Ka-band (20/30 GHz) high-gain devices with directive antenna

Regenerative payload (Rel-19)

● Base station (gNB) on the satellite● Leverages existing inter-gNB interface (Xn)– Direct inter-satellite mobility– Dual Connectivity– Resource coordination– Load management …● Core network entities on the ground or on the satellite● Prerequisite for store & forward operation – Core Network functions deployed on board● Prerequisite for UE-satellite-UE communication● With Rel-19, 3GPP operating bands are defined in practically all satellite spectrum below 3 GHz (close to 100 MHz in total)! Rel-19: Ku-band (12/14 GHz)● Rel-19 discussing DL coverage enhancements: beam hopping, link-level repetitions● Latency performance: ~3x increase in latency 34 ms (TN) vs. 92 ms (NTN LEO@600 km)
PRO: improved performance, better resiliency, inter-satellite links, unlocks new servicesCONTRA: more complex satellite, more power-hungry, more expensive

Trends and scenarios for NTN towards 6G

● From proprietary NTN technologies to global standards, thanks to 3GPP involvement● Direct connectivity to user terminals and IoT devices● NTN and TN: from “interworking” to “integration” to “unified”?– Interworking: independently-designed elements– Integration: adding a satellite component to 5G– Unified: satellite and terrestrial components work together to address the same common requirements, with unified design principles in an optimum balance

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