By Nancy Friedrich, Product Manager for RF Aerospace Defence Products, Keysight Technologies
Non-terrestrial networks (NTNs) are transforming global communication, particularly in remote areas. Beyond enhancing disaster response, NTNs enable machine-to-machine (M2M) applications like farming, transportation and environmental monitoring. Cellular networks benefit from satellite integration, providing direct-to-device services to previously unreachable locations. To ensure these advancements are both effective and scalable, robust global standards are critical. These frameworks not only establish benchmarks for performance and interoperability but also unite efforts across nations and industries, accelerating the adoption of NTNs and their transformative potential.
Fifth-generation cellular (5G) NTNs draw many features from 5G terrestrial networks and face many of the same challenges, adding higher reliability expectations compared to earlier SatCom networks. Base stations, which are normally a terrestrial network comprising towers on the ground, will be moving to the air and space. The 5G core network is called the next generation core (NGC). A 5G NTN comprises user equipment (UE), which consists of a mobile device like a cell phone or sensor.
If needed, the UE communicates with base stations, each called a gNodeB. The introduction of 5G NTNs disrupts the traditional 5G terrestrial network architecture. Many alternatives exist for satellites and high altitude platform systems (HAPS) participating in gNodeB and radio access network (RAN) domains, some with multiple satellites in the chain scattered across miles. 5G NTNs draw many features and face many of the same challenges as their terrestrial network counterparts, adding higher reliability expectations compared to earlier SatCom networks.
Not all NTN solutions and services will operate within the 3rd Generation Partnership Project (3GPP) standards (Fig. 1). Many vendors outside 3GPP already rely on proprietary waveforms, with more in development. For instance, DVB-S2X offers alternatives to wideband data transfer via NTNs.
Cellular standards
One of the primary reasons to include NTN in Third Generation Partnership Project (3GPP) standards is the ability to access satellite networks with existing, unmodified 5G and Long Term Evolution (LTE) devices. The 3GPP considers LTE NTN synonymous with IoT NTN. Both narrowband IoT (NB-IoT) NTN and enhanced machine-type communication (eMTC) are subsets of IoT NTN. The 3GPP originally defined NTN for 5G before prioritising IoT NTN, as it presented less challenges. The resulting timeline put the arrival of 4G NTN in parallel with 5G, as it was a late addition to the 4G 3GPP standard.
Release 17 introduced 5G NR NTN and NB IoT NTN, enabling satellite network access for remote handsets and IoT devices in applications like agriculture and transportation. Release 17 non-terrestrial updates 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 satellite communications.
3GPP Release 17: the first wave
Release 17 was the first release to account for ground-based terrestrial networks and NTN platforms in 5G or any previous 3GPP cellular specifications. As defined in the release, these NTN platforms include multiple types of satellites, specifically non-geosynchronous orbit (NGSO) and geo-synchronous orbit (GSO, which includes geo-stationary orbit, GEO). Originally, the 3GPP referred to LEO and GEO, but it then opted to generalise to NGSO and GSO. As shown in Figure 2, these are the elements of NTN defined by the 3GPP, but proprietary NTNs outside of 3GPP also include HAPS and crewless aerial vehicles. They have a separate work item in the 3GPP.
3GPP Release 18: enhancing performance
Release 18 enhancements related to LTE NTN focus on mobility management, throughput, power-saving and discontinuous coverage enhancements. For example, improving NTN mobility includes the integration of time-based and location-based measurement triggers so the UE can initiate neighbour cell measurements before the UE loses coverage due to radio link failure. This required the addition of the signalling neighbour cell ephemeris data for eMTC and NB-IoT. To advance overall NTN throughput performance, Release 18 LTE NTN includes features disabling HARQ feedback to mitigate the impact of stalling on UE data rates.
Release 18 enhancements for NR NTN include uplink coverage and NTN-TN and NTN-NTN mobility and service continuity enhancements. The release enables the network to verify UE location as per regulatory requirements and the opening of frequencies beyond 10 GHz. (Kaband is being enabled only for NR NTN but not enabled for LTE NTN.) The 3GPP defined other new frequency bands below 3 GHz: extended L-band (for LTE NTN only) or a combination of bands L and S (for both LTE and NR NTN).
3GPP Release 19: increasing capacity
The 3GPP is currently defining Release 19, with finalisation slated for late 2025. Much of the satellite industry’s attention focuses on approaches to direct-to-handset communications, as depicted by some illustrations in Figure 3. Although the 3GPP has limited the number of overall enhancements in this release, several proposals are under consideration:
• Specify the TE-emulated channel model with varying Doppler and delay shifts for NR-NTN and IoT-NTN in the FR1-NTN bands and the corresponding LTE bands for NGSO satellites
• Specify NR NTN requirements for channel bandwidths below 5MHz
• Specify high-power user equipment or UE (PC2, PC1.5, and PC 1) for NR-NTN and IoT NTN in FR1-NTN and LTE NTN bands for the single uplink carrier scenario
• Support regenerative payloads for both IoT NTN and NR-NTN
• Support Rel-17 RedCap and Rel-18 eRedCap UEs with NR NTN operating in FR1-NTN bands
• Define Ku bands
Release 19 expands capacity with new spectrum allocations in the Ka- and Ku-bands, enhancing bandwidth efficiency and performance for satellite communications. Release 19 also includes additional capacity enhancements, such as enabling multiplexing of multiple UE in a single subcarrier.
With this latest release, the 3GPP hopes to reduce NTN’s dependence on GNSS. Enhanced GNSS operation includes UE pre-compensation for uplink time and frequency synchronisation in case of GNSS availability decline. Additional support targets NTN discontinuous coverage for IoT NTN, making data gathering via these networks more resilient. The 3GPP will continue to guide the progression of NR and IoT NTN, with the Technical Specification Group Service and System Aspects Studies for 3GPP Release 20 scheduled to be completed by June 2025.
As NTNs and their services increase, standards will continue to evolve to ensure their interoperability, compliance, and performance. Though the use cases and features for those future standards are not set, 5G (and eventually 6G) NTN is clearly on the path of communications transformation.
As communications providers incorporate NTNs, pushing beyond terrestrial-based infrastructure, they will enable immersive experiences like the metaverse while transforming industries, spanning manufacturing through climate monitoring to healthcare. Expect the pathway to this technology future to be clearly laid out by standards bodies providing use cases and capabilities that will transform lives worldwide.
This article originally appeared in the April 25 magazine issue of IoT Insider.
