Nancy Friedrich, Marketing Manager for RF Aerospace Defense Products, Keysight Technologies

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 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 precedents for performance and interoperability on a national to international level.

5G NTNs draw many features from 5G terrestrial networks and face many of the same challenges, adding higher reliability expectations compared to earlier satellite communication networks. Base stations, which are typically part of a terrestrial network comprising towers on the ground, are instead shifted to the air and space. The 5G core network is called the next generation core. A 5G NTN includes user equipment (UE), namely a mobile device such as a cell phone or a sensor. If needed, the UE communicates with base stations, each of which is called a gNodeB.

The introduction of 5G NTNs has disrupted the traditional 5G terrestrial network architecture. Many alternatives exist for satellites and high-altitude platform systems (HAPS) participating in the gNodeB and radio access network domains, some with multiple satellites in chains scattered across miles.

Not all NTN solutions and services will operate within the Third Generation Partnership Project (3GPP) standards. Many vendors outside 3GPP already rely on proprietary waveforms, with more in development.

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, and promoted for international adoption and utilisation:

  • DVB-satellite (DVB-S): Published in 1994, the DVB-S standard defined a satellite framing structure, channel coding and single carrier QPSK modulation with Reed-Solomon and convolutional coding.
  • DVB-satellite second generation (DVB-S2): Released in 2005, DVB-S2 offered many improvements over the first generation including adaptive code modulation; more modulation formats such as 8, 16, and 32 amplitude and phase-shift keying; and tighter roll offs to alpha = 0.2.
  • DVB-return channel via satellite (DVB-RCS): Published in 2001, this generation was the first to support data throughputs beyond 4G and two-way interactive satellite communications. DVB-RCS2 followed in 2011 with improved efficiency, advanced modulation and enhanced forward error correction.
  • DVB-satellite second generation extensions (DVB-S2X): Released in 2014, DVB-S2X enhances performance in core markets such as direct-to-home, very small aperture terminal and digital satellite news gathering, while expanding applications in emerging markets such as mobile, interactive and broadband.

Interoperability standards

The DIFI Consortium provides an open, interoperable digital intermediate frequency/radio frequency (IF/RF) standard for communication on ground systems. Analogue IF systems, with their inflexible chain of hardware, are difficult to scale and complicated to operate. They struggle to handle today’s capacity and cannot scale to meet tomorrow’s needs. According to the consortium, the satellite industry is solving these challenges by inserting IF converters as close to antennas as possible, to create a digital IF packet stream that can be transported and processed digitally. But, unlike 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 3GPP standards is the ability to access satellite networks with existing, unmodified 5G and long-term evolution (LTE) devices. 3GPP considers LTE NTN synonymous with internet of things (IoT) NTN. Narrowband IoT (NB-IoT) NTN and enhanced machine-type communication (eMTC) are subsets of IoT NTN. 3GPP originally defined NTN for 5G before prioritising IoT NTN, as it presented fewer 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. 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, 3GPP only referred to LEO and GEO, but it then opted to generalise to NGSO and GSO. These are the elements of NTN defined by 3GPP, but proprietary NTNs outside of 3GPP also include HAPS and crewless aerial vehicles. They have a separate work item in the 3GPP standards.

Release 17 introduced support for two types of NTNs: 5G new radio (NR), and 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.

Release 17’s non-terrestrial updates address the technical hurdles inherent in communication between handsets, IoT devices and satellites in enabling NTN support.

3GPP Release 18: Enhancing performance

Release 18’s enhancements related to LTE NTN focused on mobility management, throughput, power-saving and discontinuous coverage. For example, its improvements for NTN mobility included the integration of time-based and location-based measurement triggers so the UE can initiate neighbour cell measurements before it 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 included features disabling HARQ feedback to mitigate the impact of stalling on UE data rates.

Release 18’s enhancements for NR NTN included uplink coverage and NTN-TN and NTN-NTN mobility and service continuity enhancements. The release enabled the network to verify UE location as per regulatory requirements, as well as the opening of frequencies beyond 10 GHz. Ka band was enabled only for NR NTN but not for LTE NTN. 3GPP defined other new frequency bands below 3 GHz: extended L-band (for LTE NTN only) and 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 is focused on approaches to direct-to-handset communications. Although 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 5 MHz
  • Specify high-power UE (PC2, PC1.5 and PC 1) for NR-NTN and IoT-NTN in the 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 the FR1-NTN bands
  • Define Ku bands

The Ka band covers 17-30 GHz, but a new portion of spectrum is now being slated for use by 3GPP. According to the current standard scope, the downlink for the Ku band for GSO and non-GSO is 10.70-12.75 GHz for ITU Regions 1 and 3. Uplink will span either 12.75-13.25 GHz, or 13.75-14.5 GHz. With a drop in frequency from 10 GHz to 14.5 GHz, this is a totally new section of frequency between FR1 and FR2. On the downlink, this will range closer to 7 GHz. In ITU Region 2, the downlink is expected to span 10.70-12.70 GHz, while uplink will span 12.70-13.25 GHz and 13.75-14.5 GHz.

Frequency C bands are likely to be added in the Ku band for NR NTN. Satellite providers are pushing for these to optimise spectrum, although details such as carrier aggregation still need to be resolved. Release 19 will also include 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. 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 communication providers incorporate NTNs, pushing beyond terrestrial-based infrastructure, they will enable immersive experiences such as the metaverse while transforming industries, from manufacturing to 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.