Non-terrestrial networks
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.
Here's the consolidation on NR NTN and IoT NTN package in Rel-19: NR-NTN (Phase 3):
- 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
- 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.
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.
- 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.
- 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:
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.