Non-Terrestrial Network Advantages, Challenges, and Applications

What is a non-terrestrial network (NTN)?

A non-terrestrial network (NTN) is a communications network incorporating airborne or spaceborne assets as well as assets on the ground. These hybrid networks apply satellite communication (SATCOM) technology to extend existing cellular communications. A non-terrestrial network or NTN usually refers to networks connecting the cellular world and satellite links and how to attain direct access to satellite networks for end users.

Today’s new space race primarily focuses on NTNs as governments and corporations seek to advance communications, surveillance, sensing, and monitoring capabilities. The NTN is the newest SATCOM evolution and a key supporter of space commercialization. The NTN has considerable inherent complexity, as the non-terrestrial network uses the NTN satellite for backhaul traffic, transforming it from a mere relay into a crucial component.

What makes up a non-terrestrial network (NTN)?

An NTN is a network that includes nodes not physically located on Earth. Although we think primarily of satellites in NTNs, other components can consist of low-altitude platforms (LAP) or drones, high-altitude pseudo satellites (HAPS), drones, or balloons across the various orbits, or a combination of these elements. Most development focuses on satellite-to-handset capabilities for cellular phones and other user devices.

Every non-terrestrial network (NTN) has several points of presence where the satellite network connects to the terrestrial internet. Fiber-optic links connect land stations together, while laser-optical links are used between satellites. From satellite gateways on the ground, wideband links connect cellular networks to satellite constellations with massive 20-plus gigabits per second connections, referred to as feeder links. These points of presence connect to one or more gateways. The graphic illustrates these links between the gateways and the satellites as the wideband backhaul links for terrestrial cell towers.

Do non-terrestrial networks (NTNs) only operate in low Earth orbits (LEO)?

Non-terrestrial networks (NTNs) operate across the geostationary or geosynchronous equatorial orbit (GEO), medium Earth orbit (MEO), and low Earth orbit (LEO). The concentration of 5G NTN’s primary applications is in GEO and LEO. Today’s explosive growth in LEO satellites provides the foundation for most NTN use cases across the commercial, government, and military industries. Low Earth orbit satellites also provide the advantage of lower latency due to the shorter distance to Earth, allowing them to support real-time NTN applications.

Each orbit creates different challenges in communication networks. With LEO, the satellite operates at a nearer distance but moves more quickly. Due to the proximity, you can have low-latency communications from satellite to ground. In contrast, traditional GEOs provide long duration fixed connections with much longer delay in the signal path between satellites in that orbit and ground stations. This process exponentially increases latency depending on the number of times the signal must travel between points. For example, if the satellite must travel around the globe, noticeable latency will occur.

Latency also arises in store and forward scenarios. Here, a satellite receives a signal, then transmits that signal later when it gains visibility to the target ground station. This occurrence is often called discontinuous transmission.

What are the advantages of NTNs?

The primary benefit of NTNs is extended coverage. Remote and underserved regions such as rural areas, islands, and isolated communities can benefit from the technology. NTNs can also provide service to ships at sea and aircraft in flight. NTNs enable network service providers to operate in an otherwise untapped market and offer premium services beyond the capabilities of traditional terrestrial networks. NTNs satisfy the ever-increasing demand for data, transmitting and receiving more information through their satellites for communications and data transfer. Machine-to-machine (M2M) applications, including agriculture, transportation, environmental monitoring, and asset tracking, can tap into NTNs for ubiquitous and reliable connectivity to the Internet.

Non-terrestrial networks also add a layer of resilience and redundancy to the existing 5G network. In the event of natural disasters, regional conflicts, or network outages, NTNs can provide backup connectivity to ensure continued service for mission-critical communications. The advantage of a distributed LEO satellite constellation is that it spreads risks and costs across hundreds or thousands of satellites.

By boosting non-terrestrial communications, NTNs provide numerous benefits including:

• Ubiquitous coverage.
• Critical emergency support improvements.
• Diagnostic enhancement functionality for farming via sensing capabilities.
• Accurate monitoring of earth and climate variables.
• Effective spread of risk and cost expenses across satellites.

What are the challenges facing NTNs?

A number of challenges face NTNs and their applications, and additional obstacles will arise as these networks evolve.

The space environment: Space is the foremost challenge for NTNs. Once deployed, equipment is inaccessible. Furthermore, systems must operate in an extremely harsh environment with extreme temperatures and radiation. For successful performance, systems also need to provide consistent power generation and storage. Building mesh networks in space exacerbates these complexities by multiplying the chances for problems.

Size, weight, power, and cost: Another concern is the physical limits of placing high-frequency RF and computing resources in the sky. Size, weight, power, and cost (SWaP-C) become issues when moving away from the GEO 20 tonners into more compact LEO satellites and HAPS platforms, and payloads must transform accordingly.

Connecting in constant motion: Non-terrestrial networks put some things, or perhaps everything in the network, in constant motion. NTN satellite and HAPS movements factor into connection setup, signal quality, and handovers. In a 5G NTN, gNodeB instances and parts of the radio access network (RAN) flying aloft add to the movement of any user equipment (UE) at the surface.

Choice of payload: The choice between transparent or regenerative payloads can completely change how the network organizes and the resulting signal routing. With LEO satellites in motion, all timing relationships are dynamic. At stake is the quality of service (QoS) user experience, primarily due to variable delays and complex handovers that can result in dropped connections.

Latency: The delay in signal transmission stems from the signal being sent between ground and satellite(s). Due to latency, an NTN does not currently support use cases demanding 3GPP Ultra Reliable Low Latency Communications (URLLC), such as telesurgery, which needs 1 ms with 99.99% reliability.

Security: While a distributed LEO satellite constellation spreads costs and risks across satellites, the hardware is vulnerable as it passes over unfriendly territories. National security needs exert demands for cyber protections and novel operations to protect infrastructure deployed in space. For example, the U.S. Space Force is tasked with this mission for all branches of military and government.

What is 5G NTN?

The term NTN usually includes fifth-generation (5G) cellular as an aspect of the network. 5G NTNs draw many features from 5G terrestrial networks and face many of the same challenges, adding higher reliability expectations for 5G NTN service compared to earlier SATCOM networks. Base stations, which are normally terrestrial networks consisting of towers on the ground, are moving from land to air and space. The 5G core network is referred to as the next generation core (NGC). A 5G NTN comprises 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.

This configuration is a typical 5G NTN setup. However, many variations exist. For example, not all NTN applications require the gNodeB. You might have a core network which is the internet directly connected to a gateway on a proprietary system. Another alternative approach is to use NTN for edge computing, putting the network edge in the satellite.

Various approaches to a 5G NTN architecture exist. For example, an aerial asset or satellite can operate as a bent pipe between UE and gNodeB. That device will receive signals on Frequency 1 and transmit them on Frequency 2 to facilitate non-terrestrial network communications over a wide geographical range. In this model, note that UEs need sufficient power and sensitivity to transmit and receive from the satellite bent pipe. The gNodeB can be ground-based as long as it can communicate to the NTN satellite bent pipe.

An alternative architecture has the gNodeB on the airborne or spaceborne asset itself. In this case, UE communicate to that aerial asset. The core network also is connected to that aerial or spaceborne asset. Additional examples introduce relay nodes to interface with either the standard UE with satellite bent pipe or with the UEs to an aerial or spaceborne gNodeB.

The introduction of 5G NTNs disrupts the traditional 5G terrestrial network architecture and opens a paradigm shift in connectivity. Many alternatives exist for satellites and HAPS participating in gNodeB and RAN domains, some with multiple satellites in the chain scattered across miles of sky. 5G NTNs draw many features from 5G terrestrial networks and face many of the same challenges, adding higher reliability expectations for 5G NTN service compared to earlier SATCOM networks.

Are NTNs and satellite communications the same thing?

Non-terrestrial networks herald the next wave of satellite communications (SATCOM), making SATCOM a part of cellular networks. Satellite communication technology covers difficult-to-reach areas with no infrastructure or isolated platforms to support cellular network deployments. The use of SATCOM also provides extra reliability for machine-to-machine (M2M) / IoT and connectivity for moving platforms, such as airplanes, trains, and cars.

To satisfy new performance demands for SATCOM, the satellite industry strives to reach higher throughput, wider bandwidths, and higher operating frequencies. Increasingly, networks also rely on optical photonics links. Non-terrestrial networks also need to deliver enough power to receivers despite free space, weather, clouds, and other ionospheric conditions. Given the increasing complexity of NTNs and SATCOM, you must test satellite communication systems with realistic environmental models, adding buffering for delayed signals and simulating sliding delay for creating realistic satellite kinematics. Keysight supports the development, manufacturing, deployment, and maintenance of non-terrestrial networks across the entire ground and air / space workflow, including the testing of 5G NR NTN satellite links.

How does Keysight support NTN use cases?

Keysight supports the development and ongoing performance verification for NTN use cases such as the following:

• Coverage of unserved areas
• Service for aircrafts, ships, trains, buses, and more
• Man-to-machine and IoT
• Relaxed latency requirements
• Service availability
• 5G network scalability

Keysight delivers end-to-end NTN testing environments that can virtualize network access and even the complete satellite constellation. Replace the NTN-capable radio network with Keysight UXM 5G, a fully capable NTN network emulation device. Re-create satellite links with the Keysight PROPSIM channel emulators, accompanied by the Keysight advanced VXG microwave signal generator and Keysight UXA signal analyzer. Gain full control over the system under test and complete visibility into the NTN nodes and links. You can easily add hardware to extend the frequency range to cover all main NTN bands, such as X-band and K-band.

With flexible Channel Studio scenario creation, you also can test real satellite radio hardware and the solution at both the node and network level. Troubleshoot performance problems using the Keysight WaveJudge wireless analyzer and Keysight PathWave Vector Signal Analysis (VSA) running on the UXA signal analyzers. To analyze IQ streams beyond WaveJudge, you can use the PROPSIM internal IQ capture and streaming functionality.

You also can generate realistic testing conditions in the lab combining the VXG microwave signal generator with PROPSIM and UeSIM realistic representation of user terminal behavior. Find out more about how Keysight supports the ongoing development of NTNs from ground to air across your entire workflow, helping to connect and secure future communications.