By Nancy Friedrich
The benefits of non-terrestrial networks (NTNs) range from connectivity for rural and isolated regions and better disaster response, to new consumer, industrial and scientific applications.
By transmitting and receiving more information via satellite for communications and data transfer, NTNs enable new capabilities and features in machine-to-machine (M2M) applications such as agriculture, transportation, environmental monitoring and asset tracking.
For cellular networks, the integration of satellites supports direct-to-device capabilities and services. As NTNs connect the earth with space across populations, industry standards set up national-to-international precedents for performance and interoperability. By uniting development and deployment efforts, these standards empower accelerated adoption and leverage NTN advantages.
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.
Not all NTN solutions and services will operate within the 3rd Generation Partnership Project (3GPP) standards (Figure 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.
Video broadcast
The Digital Video Broadcasting (DVB) specifications define digital broadcasting using DVB satellite, cable, and terrestrial broadcasting infrastructures. These specifications have been standardised, mostly by the European Telecommunications Standards Institute (ETSI) for international adoption and utilisation:
• The Digital Video Broadcasting-Satellite (DVB-S) standard defined satellite framing structure, channel coding, and modulation. Features included support for a single MPEG transport stream for standard TV.
• DVB-Satellite Second Generation (DVB-S2) was designed for standard and high-definition television (HDTV) and interactive services including internet access.
• DVB-Return Channel via Satellite (DVB-RCS) was the first to support data throughputs beyond 4G, and two-way interactive satellite communications. A second generation provided improvements, including improved bandwidth efficiency.
• DVB-Satellite Second Generation Extensions (DVB-S2X) extended the second generation for direct to home (DTH), very small aperture terminal (VSAT), and digital satellite news gathering (DSNG)]. It also expands the range of applications in emerging markets, such as mobile, interactive, and broadband use cases.
Interoperability standards
The DIFI Consortium provides an open, interoperable digital intermediate frequency/radio frequency (IF/RF) standard for communication on ground systems. Unlike in analogue IF, there is almost an infinite number of ways to encode digital IF bits into a standard IP packet.
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.
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. These NTN platforms include non-geosynchronous orbit (NGSO) and geo-synchronous orbit (GSO, including 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.
Release 17 introduced support for 5G new radio (NR) and narrowband-IoT (NB-IoT) and eMTC. 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.
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 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.
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.
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 direct-to-handset communications. Several proposals are under consideration:
Frequency C bands are likely to be added in the Ku-band for NR NTN. Satellite providers are pushing for Frequency C bands in the Ku-band for NR NTN to be added to optimise spectrum, although details like carrier aggregation still need to be resolved. Release 19 also includes additional capacity enhancements, such as enabling multiplexing of multiple UE in a single subcarrier.
As NTNs and their services increase, standards will continue to evolve to ensure their interoperability, compliance, and performance. 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.
