Non-Terrestrial Networks (NTN) is a relatively new term in the communications field. Before this term was introduced, the more common term was Satellite networks, namely Geostationary Earth Orbits (GEO) satellites.
GEO satellites were the first iteration of NTN due to the high-cost of sending communication systems to space; these systems needed to be robust, have a long-life cycle, and cover the largest possible areas with the least number of satellites to make the business case feasible. As several technological advancements emerged in the space satellites domain (antennas, chipset size, power consumption, reusable rockets, smaller satellites), this led to significant reductions in the cost for launching satellites, particularly for both Medium and Low Earth Orbit (MEO, LEO) starting at 100,000 USD per kilo in the early 2000 and dropping to 1,400 USD per kilo. Utilizing closer orbits for satellites helped solve latency challenges of traditional GEO systems, improving the potential for the provision of broadband high-speed connectivity. In addition, more communication systems utilizing airborne vessels (aircrafts, drones, High-Altitude Platforms HAPS) were also coming into play. With such a variety of systems on the rise with similar characteristics and challenges, the term Non-Terrestrial Networks was born to help identify these elements; space-borne and airborne, compared to their terrestrial counterparts. A representation of such systems in described in Figure 1 below.
Figure 1: NTN categories and types
Current Communications from Space
Space communications started with GEO satellites as they made it possible to provide the necessary basic voice and data services to cover remote and rural areas. With more demand for higher bandwidth and lower latency, Non-Geostationary Orbit (NGSO) satellites started to emerge and help serve different use cases; mainly broadband communications, connectivity to enable Internet of Things for industrial application, provision of backhaul for terrestrial networks, and advanced earth observation applications. This is notable from the perspective of the number of launched satellites over the past decade, with the number of GEO satellites being constant over the years and the number of NGSO increasing steadily, as shown in the following Figure 2:
Figure 2: The number of launched satellites per type. Source: prepared with data from the UCS Satellite Database
It should be noted that while one GEO satellite can cover very large portions of Earth, NGSO satellites are composed of several hundreds or even thousands of satellites, required to cover the same portions, or virtually, the whole Earth. This also explains the surge in launch numbers.
Nevertheless, from the shared market point of view comparison, GEO has the higher share to-date considering GEO High-throughput satellites (HTS) are the main providers of broadband connectivity and backhaul services today (around 90%). However, this distribution is forecasted to shift within the next following years, as it is expected that 90% of the HTS capacity will be provided by NGSO systems.
While GEO satellites advanced to provide higher throughputs and NGSO satellites solved latency issues, other challenges came into play. NGSO satellites networks are facing a challenge of physical orbital slots allocations that requires thorough planning and coordination between countries. Additionally, space traffic management became more complex and still requires clear policies to help manage it, as previous policies dealing with GEO satellites were no longer effective.
The new communications domain: Airborne Communications
Although the terrestrial networks expanded significantly since the 1990s, they still face the challenges of natural obstacles and distances. In this context, airborne-aided communication systems, such as the High Altitude Platforms (HAPS), or as some might call them “towers in the skies”, can be considered an effective solution as they relocated the base station element of typical terrestrial networks to the skies; allowing them to cover larger areas of land with a single flying platform. The Kingdom of Saudi Arabia conducted the world’s first 5G test using HAPS during 2022. The trials allowed analyzing the technical challenges and functionalities of this technology.
Air-to-Ground (A2G) systems on the other hand relied on existing terrestrial infrastructure to point large antennas to the aircraft routes, allowing them to connect airplanes to the terrestrial network directly without the need to connect to traditional satellite systems; improving bandwidth and latency. Drones and Unmanned Aerial Vehicles (UAV) are also playing a part to extending the coverage of traditional terrestrial networks when needed, especially in emergencies. For example, after hurricane Ian hit Florida a mobile provider deployed a cell site over a tethered drone to provide quick coverage in certain areas where land infrastructure was lost.
While the technical capabilities and viability of these solutions has been proven, currently the market viability of the use cases is still being built. With the COVID-19 pandemic affecting commercial flights during 2020 and 2021 as well as the expansion of experimental connectivity projects, efforts are still undergoing to consolidate commercial opportunities. Nevertheless, currently the A2G and HAPS solutions are gaining momentum worldwide, with interest in their deployment being fueled by the recovery of the aviation industry, consumer demand for ubiquitous connectivity and governments interest in solving connectivity gaps on rural and urban areas. Currently, it is forecasted that the HAPS market could generate 4 billion USD in value by 2029 driven mainly by the increased interest in these solutions by governments, airlines, and traditional terrestrial telecommunication providers.
A main challenge facing most of the airborne systems is how to enable them to be a more permanent element of the network. HAPS and Low Altitude Platforms (LAPS) are currently being used as temporary solutions to extend coverage as they consume significant amounts of power and fuel. Research and development in clean and renewable fuel aims to address these challenges and allow a more sustainable deployment. Likewise, access to spectrum frequencies will also be a challenge for HAPS and LAPS, as well as ensuring the coexistence with mobile terrestrial solutions, which will be one of the agenda items during the next World Radio Conference in 2023 (WRC-23). Furthermore, standardization and parameters are also relevant to ensure a smooth massification of the services as market grows. Another relevant challenge comes from the need to ensure partnerships between the participants of the value chain, in order to allow for the services to be commercially viable and available in a shorter time.
Global Policy Challenges
As with any radical emerging technology introduction, governments need to implement the suitable policies to help protect and regulate such sectors. NTN enablement requires policies on both the physical media (space and airspace) and communication services (mainly spectrum licensing). The International Telecommunications Union (ITU) currently works with administrations to help regulate and coordinate satellite orbits and spectrum use, but coordinating NGSO is still a developing process that will take time and effort.
Airborne systems are still a relatively Greenfield when it comes to policy, as the International Civil Aviation Organization (ICAO) is still developing its policies on regulating the airspace for HAPS/LAPS Governments work cooperatively to ensure spectrum is harmonized between their airspaces. Developments are still ongoing in this regards and will require global cooperation, as well as industry participation to contribute to a collaborative process.
What is Next
NGSO constellations will continue to grow along side with private funding being directed to the sector known as “new space”. Currently, there are almost 200 entities dedicated to space-focused funding sources for startups and, overall, global investment in space ventures keeps growing, going from 5,3 billion EUR in 2019 to 12,3 billion EUR in 2021.
Regarding other airborne solutions, developing standards for NTN is key to ensuring adoption and sustainability. Release 17 of the 3GPP supports NTN elements and is already part of some existing chipsets and handsets. 6G research is already adopting NTN as a core element to providing the networks of tomorrow, with research tackling some main challenges such as spectrum sharing, antenna beamforming and synchronization between different network layers and elements. Likewise, additional partnership with terrestrial providers, as well aircraft and aerospace manufacturers could be expected, to consolidate use cases and commercial viability of the proposed solutions. Furthermore, 6G developments are also likely to rely on NTN solutions to achieve the ultrafast and ubiquitous connectivity, by ensuring seamless coverage in every point on any given area. In this sense, NGSO and HAPS solutions will act as complementary alternatives to more traditional solutions to provide coverage in rural and urban areas, considering their particularities and capabilities to provide services at both local or regional (the case of HAPS) and global (NGSO) scales.
Going forward, once international technical standards and spectrum allocations are decided, governments will face the challenge of adapting their domestic regulatory frameworks to enable a competitive environment that removes regulatory burdens to allow future innovation on the NTN field.
The International Telecommunication Union (ITU) and the Communication and Information Technology Commission (CITC) of Saudi Arabia are hosting a 3-day International Forum on Connecting the World from the Skies, in collaboration with the Saudi Space Commission (SSC). This hybrid forum will take place from 8 – 10 November 2022, in Riyadh and Online, and will discuss extensively airborne and spaceborne networks, from space, science, technology and policy perspectives.
 The Recent Large Reduction in Space Launch Cost.
 NGSO Constellations Continue to Gain Momentum, Satellite Connectivity & Video Market Expected to Double Over Next Decade.
 High Throughput Satellites are en*-tering a new era of accelerated and drastic transformation.
 World-First Trial of 5G HAPS Technology Takes Place in Saudi Arabia in the Red Sea Project.
 Verizon Responds to Hurricane Ian.
 For instance, in 2021 Google’s Loon project was cancelled after nine years of investments and research.
 Air-to-ground inflight connectivity sees momentum in multiple markets and High Altitude Platform Systems Towers in the Skies.
 HAPS market to generate $4 billion by 2029.
 High Altitude Platform Systems Towers in the Skies.
 Space Startup Funding Sources.
 Space Venture Europe 2021, Entrepreneurship, and Investment in the European Space Sector.
 Joint HAP Access and LEO Satellite Backhaul in 6G: Matching Game based Approaches.
 ITU HAPS – High-altitude platform systems.