Non-terrestrial networks

Employing Non-Terrestrial Networks for IoT connectivityConnectivity technologies are rapidly evolving beyond terrestrial boundaries. Non-terrestrial networks (NTNs) have emerged as a focus area in the 3GPP standards, with ramifications for the entire ecosystem from chipset makers, equipment manufacturers, satellite operators, network providers and mobile operators. As they support both terrestrial and satellite access capabilities under a converged standard, IoT technologies are opening up an unprecedented opportunity to provide a cost-effective way of connecting places that have lacked coverage to date. The introduction of NTN capability into the 3GPP standards-based cellular IoT ecosystem is set to extend connectivity to areas not covered by terrestrial mobile networks, including some of the most remote regions of our planet, oceans and flight paths. However, adoption and implementation of any new connectivity technology faces obstacles from lack of awareness to end customer requirements, use cases and finally monetisation.
The main standards for NTN connectivity are being defined by 3GPP, the organisation responsible for defining 5G and other wireless communication standards. 3GPP Rel. 17 has added two mainspecifications: NB-IoT NTN and eMTC/LTE-M NTN for IoT use cases, and NR NTN for data and voice-oriented applications. Additional enhancements will come in subsequent Rel-18 and Rel-19specifications.
Characteristics of an IoT NTNThe IoT NTN ecosystem aims to provide a seamless NTN/TN hybrid experience for devices, ensuring they meet stringent connectivity, power and user experience criteria. However, to achieve this, the network must confront the challenges associated with NTNs and reassess the behavioural similarities of terrestrial networks. The following characteristics play a pivotal role in this endeavour:
  • Global coverage: An advantage of NTNs is their ability to offer global coverage, as satellites can cover vast areas, making it possible to establish communication links over challenging terrains.
  • Network latency: Propagation delay due to satellite distance, orbital dynamics, and signal transmission delays will impact latency, requiring the IoT NTN network to provide connectivity steering through the network interfaces.
  • Network interconnect: Providing roaming interfaces between NTNs and TNs makes it straightforward to provide seamless connectivity for UE and devices in the field. 
  • Regulatory compliance: The network must be compliant with national and international regulations, including licensing, frequencies and adherence to lawful and responsible operation.
  • Standards compliance: NTN network infrastructure compliance with industry standards enables the partner ecosystem to seamlessly advance technology and onboard well-known established processes.

Operational requirementsThe demand for continuous network connectivity has increased as IoT, consumer and other critical use cases have emerged. Various scenarios, such as a hiker needing network access during an emergency, enabling an oil pipeline to report damage, or the need to trace a container crossing oceans, have significant social impacts. Inevitably, every device in the future will support hybrid connectivity, ensuring seamless connection regardless of its geographical location. However, accessing the network, whether terrestrial or non-terrestrial, needs to be made reliable and simpler to achieve these goals.Seamless switching between terrestrial and non-terrestrial networks requires a network infrastructure that complies with a set of standards while considering the unique attributes of eachnetwork, such as latency, bandwidth and coverage. Similarly, the chipset or device ecosystem mustpossess the necessary capabilities to enable seamless navigation across these networks.Creating a reliable network infrastructure is just the first step. To ensure optimal results, we need to focus on operational aspects that are crucial in maintaining the efficiency, reliability, and effectiveness of the switching process. These operational aspects include:
  • Network monitoring,
  • Redundancy that provides uninterrupted service,
  • Capacity planning to anticipate network demands,
  • Parameterising the network configuration for efficient data routing in critical use cases,
  • Implementing strict policies with predefined rules and standards for security and regulatory guidelines,
  • Providing training for network operators to handle the complexities of seamless switching and scaling to accommodate the growing number of use cases.
Switching between terrestrial and non-terrestrial networks requires a comprehensive understanding of the technical nuances of each network, the development of capabilities that enable seamless switching between networks, and the creation of a network infrastructure that can accommodate the demands of terrestrial and non-terrestrial networks. An NTN modem, together with the device application, needs to incorporate the required technical nuances to provide a much simpler user interface and a seamless user experience.
Challenges and evolutionTo achieve the ful digitisation of society and industry, global coverage of IoT networks is a must and satellite solutions are well positioned to solve the dead zone of terrestrial networks. But this is not enough. In the past, satellite operators have deployed IoT solutions from space, but the implementation costs and operational costs prevented mass adoption. Now the integration of 3GPP terrestrial networks and non-terrestrial networks under the same standard has dramatically reduced the cost of the user equipment and, at the same time, facilitates the seamless integration and operation between both networks. Satellite operators recognise in 3GPP Rel 17, and beyond, a big opportunity to extend NB-IoT coverage for networks compliant with GSMA roaming interfaces and 3GPP protocols.To achieve this goal, one of the most important challenges is to speed-up the UE certification process and avoid proprietary solutions that can monopolise the market and result in customer and technology lock-in. MNOs, MVNOs and IoT service providers all need to support the process.
Smooth interworking and integration of terrestrial network (TN) and non-terrestrial network (NTN) components is the next logical step on the coverage journey to provide enhanced mobile broadband (eMBB) to consumer smartphones (direct-to-smartphone) and Internet of Things (IoT) use cases. Integration with satellite networking technologies that can provide coverage in areas that TNs cannot reach would help to deliver resilient services to people and businesses currently unserved in both developed and undeveloped parts of the world, bringing significant social and economic benefits. Different satellite systems have been used for years to provide services such as TV broadcasting, navigation, communications, surveillance, weather forecasting and emergency systems. Figure illustrates the orbits of the three main satellite types – geostationary (GEO), medium-Earth orbit (MEO) and low-Earth orbit (LEO) – in comparison to a commercial aircraft and high-altitude platform system (HAPS) providing local service coverage. The first commercial rocket launches by SpaceX in the mid-2010s coincided with an ongoing paradigm shift in the space industry that soon resulted in a significant drop in the cost of launches as well as an increase in capacity. The “New Space Era” that  has emerged since then has been defined by a dramatic increase in annual private venture capital investments in large LEO constellations focusing on fixed broadband internet services for residential and business users in existing and planned satellite constellations such as Starlink, OneWeb and Amazon Kuiper. The next step in the development of mobile satellite services (MSS) focuses on the ability to communicate with standard smartphones. Three development tracks have emerged: legacy MSS, legacy Long-Term Evolution (LTE) and 5G NTN.Non-terrestrial networks (NTN) became part of the 3rd Generation Partnership Project standard in Release 17, establishing a strong foundation for direct communication between satellites, smartphones and other types of mass-market user equipment. Modern satellites typically divide their service areas into several hundred sub-areas, which they serve with individual beams (spot beams). In general, each of these areas corresponds to one cell, and can have a diameter of tens or even hundreds of kilometers. Alternatively, a beam steering mechanism can be implemented on the satellite to steer the beams toward a fixed area on the Earth for as long as possible. This concept, known as Earth-fixed beams, allows a device to remain in the same beam and cell for several minutes. While both alternatives are supported in Rel-17, a particular benefit of the Earth-fixed beam concept is that it avoids frequent handover between cells.The fundamental challenge for any satellite communication system is how to overcome the large round-trip delays and frequency shifts due to the movement of the satellite relative to Earth, also known as Doppler shifts. The 3GPP solution to this challenge is to require the UEs to compensate delay and service link Doppler shift before accessing the network. To this end, the satellite broadcasts its ephemeris corresponding to its position and velocity. The UE is required to be equipped with a Global Navigation Satellite System (GNSS) module, which it uses to determine its own position before accessing the network.From its own position and the satellite ephemeris, the UE calculates the distance to and relative velocity of the satellite, and it determines the required pre-compensation values and applies a large frequency shift and timing advance. Thisenables the gNB to operate at its nominal frequency and with uplink (UL) and downlink (DL) timing aligned, as in a TN.The long propagation delays necessitate further changes. Scheduling timing relationships, which are designed to cater for round-trip times (RTTs) below 1ms in a TN, have been redesigned to cope with the longer delays as well.Mobility is another area in which NTNs differ significantly from TNs, most obviously in LEO constellations, where even stationary UEs will experience frequent handovers because of the orbital movement of the satellites. In TNs, UEs experience a clear difference in measured signal strength depending on the distance between the UE and the base station, whereas in NTNs all UEs have approximately the same distance to the satellite, with only a small difference in signal strength between cell center and cell edge.Rel-17 also includes adaptations to NB-IoT and LTE-M that will enable them to support NTNs. This 3GPP track is known as IoT NTN. The work item (WI) was started very late in Rel-17 with minimal scope, focusing on essential functionalities. The general approach in IoT NTN is to follow the NR NTN work as closely as possible and adapt its solutions. For example, the basic solution for precompensation of delay and Doppler shift is the same, requiring IoT NTN UEs to have GNSS support. The NR NTN enhancements to scheduling timing relationships have also been adopted for IoT NTN. No mobility enhancements (such as CHO) have been considered, however.Similar to terrestrial 5G, NTN aims to provide services beyond MBB. The ITU-R (International Telecommunication Union – Radiocommunication Sector) outlines performance requirements intended to facilitate ubiquitous and resilient coverage for MBB, massive machine-type communications (mMTC) and high reliability communications (HRC). The report elaborates on the key performance requirements for each use case in the context of a LEO 600km constellation operating over a 30MHz carrier. Notable requirements are peak data rates of 70Mbps (DL) and 2Mbps (UL), corresponding to spectral efficiencies of 3bps/Hz (DL) and 1.5bps/Hz (UL). This can be translated to DL and UL area traffic capacity of 8kbps/km2 and 1.5kbps/km2 for this particular constellation. The ITU-R vision for mMTC includes support for up to 500 devices per square kilometer, while the HRC use case is associated with a reliability of 99.9 percent.A 3GPP-compliant NTN solution makes it possible to deliver a single network that comprises both terrestrial and non-terrestrial components, incorporating the world’s largest ICT ecosystem. As satellite systems will not have the same capacity as terrestrial systems, they should be viewed as complementary rather than competing systems.
3GPP started the work on NTN standard with the study item in 2017.
  • In Release 15, the study focused on channel model, deployment scenario, and potential key impact areas for NR NTN.
  • In Release 16, the study focused on necessary features enabling NR NTN, and identifying the use cases & service requirements.
  • In Release 17, specified both 5G NR NTN & 4G IoT NTN specifications based on the previous studies, focusing on supporting & satellites using 5G, transparent payload architecture, spectrum frequency below 6GHz, UE with GNSS capabilities, and addressing identified challenges due to propagation delays, doppler, moving cell. Multi-connectivity and URLLC over satellite are not considered in Rel. 17.
  • In Release 18 specifications, focusing on the NTN coverage enhancement, NTN deployment above 10 GHz (especially Ka-band), mobility & service continuity between NTN-TN & NTN-NTN, etc.
  • Release 19 is the 2nd release of 5G-Advanced in 3GPP. Release 19 discussion started from 2023 (RAN #100) and is expected to freeze in 2025. RAN1/2/3 work package in Release 19 was decided and approved in RAN#102 meeting in December 2023. 

Here's the consolidation on NR NTN and IoT NTN package in Rel-19: NR-NTN (Phase 3):
  • DL Coverage Enhancement
  • UL Performance (Capacity/Throughput) Optimization
  • MBS (Broadcast) for NR-NTN 
  • Support Regenerative Payload with full gNB
  • Address RedCap UE within FR1 NTN
IoT-NTN (Phase 3): 
  • Support Store & Forward Satellite Operation based on Regenerative Payload
  • UL Capacity Enhancement
  • Further Mobility Enhancement
More recently, 3GPP set the standards and targets for NTNs within 5G and future 6G networks for direct-to-handset communications and IoT devices. 
  • The 3GPP published Release 17 in 2022, making it the first release to account for ground-based terrestrial networks and non-terrestrial network platforms in the 5G specifications or any previous 3GPP cellular specifications. As defined in Release 17, these NTN platforms include multiple types of satellites, high-altitude platform stations (HAPS), and crewless aerial vehicles. Release 17 introduced support for two types of non-terrestrial networks — 5G NR and narrowband-IoT (NB-IoT). 5G NR NTN supports satellite network access to handsets in the Frequency Range 1 (FR1) band for use cases such as voice and data transmission in geographic areas not served by terrestrial networks. NB-IoT NTN supports access to IoT devices directly from satellites for agriculture, transportation, and other applications. Release 17 enhancements address the technical hurdles inherent in communication between handsets, IoT devices, and satellites to enable NTN support. These challenges include propagation delay, Doppler shift, and the difficulties associated with communication between moving terminals (user equipment) and base station platforms such as satellites, HAPS, and crewless aerial vehicles. Release 17 makes several NTN-related enhancements to the 5G protocols to accommodate the longer distance between the user equipment and the satellites. These enhancements include changes to hybrid automatic repeat request (HARQ) and random channel access (RACH) procedures to allow for increases in signal propagation delay.
  • Release 18, due for completion in 2024, includes promising new NTN capabilities, coverage enhancements, performance enhancements, and support for new frequency bands. Figure 3 illustrates the timeline for this release. Some Release 18 enhancements focus on extending LTE support of NTN, while others primarily focus on enhancing 5G NR NTN capabilities for IoT. Some of the upcoming enhancements are mobility management and power-saving enhancements for discontinuous coverage.
• Improving NTN mobility by modifying support for neighbor cell measurements before the UE loses coverage due to radio link failure — and adding support for signaling neighbor cell ephemeris data for enhanced Machine Type Communication (eMTC) and NB-IoT.• Advancing overall NTN throughput performance — including disabling HARQ feedback to mitigate the impact of HARQ stalling on UE data rates and identifying global navigation satellite system (GNSS) operation improvements to reduce UE power consumption and create a new position fix for UE pre-compensation during long connection times.• Optimizing of GNSS for power efficiency for long-term connections. • Supporting new scenarios covering deployments in frequency bandsabove 10 GHz, such as the introduction of extended L-band and frequency division duplexing (FDD) LTE band operation for IoT NTN.
  • The 3GPP is currently defining Release 19, and finalization will occur in late 2025. Although 3GPP plans to limit the scope of the overall enhancements in Release 19, it will include some additional NTN enhancements. Several proposals are under consideration for Release 19, including a specification for a regenerative architecture for NTNs that includes distributed unit processing on board the satellite supporting inter-satellite links. The following are recommendations from the 3GPP:
• Enable indoor NTN access with uplink and downlink coverage enhancements.• Support increased capacity for uplink access with uplink capacity and throughput   enhancements. • Support for 5G reduced capability (RedCap) devices, including NTN assistance for 5G multicast broadcast services (MBS).• Reduce NTN’s dependence on Global Navigation Satellite System (GNSS) with enhanced GNSS operation that includes UE precompensation for uplink time and frequency synchronization in case of GNSS availability decline.• Support for NTN discontinuous coverage for IoT NTN.
Accessing NTNs with unmodified cell phonesNon-terrestrial networks provide access to unmodified cell phones by employing distortion in transmission to mitigate the impact of satellite Doppler on the signals. NTNs also use techniques to compensate for the high-velocity LEO satellites, which travel at approximately 17,000 mph.Network planning for NTNs requires adding both cellular networks and satellite cells. In unmodified 5G cell phones, networks and base stations compensate for timing and frequency errors. The goal is to create conditions for satellite cells that are similar to those of terrestrial cells. Directive antennas and beamforming split the service area of a satellite into small cells, making the network manageable. As a result, cell edges have more issues with frequency and timing errors than the middle of the cell.Integrating cellular networks and NTN devicesTo realize the mobile communications industry’s vision of integrating cellular networks with NTNs over the next several years, device makers and mobile network operators must test NTN wireless links with actual base stations and real devices.Significant challenges include simulating life-like conditions for satellite end-to-end links and the ability to connect NTN nodes and terminals before system deployment. NTN development projects need the ability to test and connect network entities and terminals in the prototyping phase and before and during deployments to avoid costly delays.
Vision, requirements and evaluation guidel​​ines for satellite radio interface(s) ​of IMT-2020Report ITU-R M.​2514​The purpose of this Report is to build a vision, the requirements and evaluation guidelines for the satellite component of IMT-2020 including use cases, application scenarios, capabilities, system,  radio interface, and the considered specific features, particularly with respect to service and technical requirements.