Digital Twins and Non-terrestrial Networks: Applications from NASA Spacecraft to the Ukraine Conflict

With the space industry experiencing disruption not seen since the original space race of the 1960s, engineers face an unmanageable volume of technical and market information. As companies and governments invest in space-based capabilities, new technologies and use cases rapidly emerge.

The reduction of launch costs, advancement of reusable rockets, and meteoric development of lower earth orbit (LEO) satellite constellations drive today’s satellite industry revolution. Established industry primes and start-ups compete for market share as demand for bandwidth surges.

With so much development happening all at once, it’s difficult to keep up. From market trends to technical data, how do you stay aware of the latest information while ensuring its accuracy? Talk directly to the experts.

During Day 3 of our virtual event, Keysight World: Innovate, Dr. Todd Humphreys, researcher at the University of Texas at Austin, Dr. Allison Brooks, Research Vice President at IDC Worldwide Public Safety Practice, Dr. Mohsen Hosseinian, Wireless Systems 5G Expert from Samsung Systems LSI, and Phil Lorch, Keysight Director of Satellite and Space Mission Assurance, discussed the acceleration of non-terrestrial network (NTN) development, standardization, and services.

On the Tech Uncorked podcast, Keysight’s Richard Soden, Space Solutions Program Manager, and Chris Hawkins, Director of Sales and Marketing for Digital Twins, broke down the facts about the developing technologies of NTN, highlighting the particular utility of digital twins. On the Keysight podcasts Source De[Code] and All Things 5G, experts including Dr. Ian Rippke, Keysight Software Solutions Engineer District Manager, and Dr. Rajive Bagrodia, founder of Scalable Network Technologies, explained digital twin details.

Non-terrestrial Networks

Third Generation Partnership Project (3GPP) Release 17 enhancements enable new classes of 5G wireless devices and extend the coverage and capacity of terrestrial networks. Unlike previous generations of wireless communications technology, NTN refers to satellite constellations or high-altitude platforms (HAPs) that function as relays.

Integrating satellites and other aerial communication systems with terrestrial network infrastructure extends the range of cellular networks to remote areas. Cellular service providers continue to develop NTN technology in their effort to provide ubiquitous coverage around the world.

Dr. Humphreys used LEO non-terrestrial networks as an example to contextualize NTN architecture, shown in Figure 1, during his Keysight World: Innovate keynote. He explained that every NTN includes points of presence (POP) where the satellite connects to the terrestrial internet. The POPs connect to one or more gateways. These gateways link to satellites and function like wideband backhaul links for terrestrial cell towers. Satellites orbiting nearby may link to each other as a mesh via high-bandwidth optical inter-satellite links. Phased array antennas (PAA) with fixed locations on the ground act as user terminals.

During the Keysight World: Innovate panel discussion, Dr. Hosseinian noted that not all forms of communication happening above the Earth’s surface fall under NTN. For instance, communications among UAVs fall outside of the NTN classification.

Figure 1. Non-terrestrial network architecture

System designers must consider numerous technical and regulatory challenges when developing non-terrestrial networks. Unlike terrestrial networks where the base station remains—as the name implies—stationary, LEO satellites move at several kilometers per second. Additionally, the signal must travel a vast distance through the atmosphere to reach the end user. These factors result in the following challenges:

A combination of these factors and a greater susceptibility to interference presents difficulty for NTN design engineers. Using simulation software and digital twins helps mitigate the impact of these complexities.

Digital Twins

According to Richard and Chris, just about everyone involved in developing digital twins describes it a bit differently—but precious few manage to define a concise explanation. On the Tech Uncorked podcast, Richard went on to say that experts at Keysight think of a digital twin as, “…a digital model that keeps getting better.”

As an engineer continues to inform the model with more test data, the digital model becomes a true, digital duplicate of the physical system in question. Chris further built on this definition, emphasizing that a digital twin not only represents a physical system digitally but also models the behavior of that system.

When asked to define the difference between standard simulation and digital twins on the Source De[Code] podcast, Dr. Rippke explained, “[Simulation] was always considered an approximation...it was not a true representation of behavior…Customers want to see correlation.” He went on to say, “[Digital twins] have built-in correlation.”

Digital Twin Value Proposition

Beyond defining the digital twin concept, Richard and Chris also highlighted what makes digital twins so valuable. They explained that just building a model only provides engineers with a static representation of a physical asset. “…as things go out into the real world…how the system interacts with the universe might be different than you’d imagined,” Richard explained. Digital twins enable engineers to maintain dynamic models—they require a data feedback loop continuously informing the model of real-world system behavior. Traditionally, sensors capture this data from the physical twin in real time, but measurement data recorded from physical testing also helps engineers update the model.

This data-based feedback loop elevates digital twins beyond standard simulation. For the product engineers who manipulate and analyze the model, this level of fidelity provides the unparalleled accuracy needed to successfully develop components and systems deployed in space.

As an example, Richard relayed how the Keysight Eggplant team used digital twins when developing software solutions for the NASA Orion spacecraft. “The team tested the software using a digital twin of a human…acting as an astronaut.” Since digital twins deliver high-fidelity representations of real-world entities, our product engineers use them to analyze the safety, performance, and functionality of their designs through virtualized simulations and emulations. Whether you’re testing a product, or testing your test system itself, the dynamic nature of a digital twin provides the most realistic results.

Dr. Rippke put it this way, “Looking at how we use these components, systems, and subsystems—we can do so much more exploration digitally…Having that real-world predictability and representation of their systems, now, [designers] are not constrained by what they can test, they’re just constrained by what they can imagine.”

Non-terrestrial Networks Need Digital Twins

Satellite networks present massive up-front investment. Service providers must implement all the infrastructure far in advance of serving customers and generating revenue. Before establishing an operational network, providers must secure a customer base and investment. To do that, they need a convincing system-level model that demonstrates the parameters and performance of their service. “If you look at these networks, they are extremely complex systems. Think of the multi-domain communications…5G terrestrial networks, satellite networks, or HAPs need to create end-to-end 5G network,” explained Dr. Bagrodia.

Additionally, errors in operational SATCOM systems risk incurring high costs for solution providers. Once a space system deploys, maintenance or troubleshooting becomes prohibitively expensive if not impossible. Studies such as NASA’s Error Cost Escalation Through the Project Life Cycle indicate that addressing errors in operational systems costs 1,000 times more than during the requirement specification phase, and 25 times more than in the research and development phase.

“There’s a whole realm of leveraging digital twins,” commented Dr. Brooks during the Keysight World panel discussion, “to model out exactly who is where…this is going to become extremely complicated—and important.” When describing 5G network digital twins on the All Things 5G podcast, Dr. Bagrodia said, “The customers can selectively dial up either fidelity or scalability as they move up the stack to look at the application or higher level end-to-end performance.”

Particularly as space becomes more congested and contested, the risk of collision and cyber-attacks remain critical concerns for system designers. “You must have high-fidelity network simulation well in advance of building hardware,” Phil added in the panel discussion. He further stated that, during early concept development, NTN designers simulate everything from traffic patterns to cyber threats to maximize the chance of deployment success. Richard expanded on this topic in the Tech Uncorked podcast stating, “Having a digital version means you can test [the system] much more—you can attack it.”

For example, Keysight EXata Network Modeling facilitates the creation of a precise network digital twin, allowing for real-time network simulation and emulation that mirrors the exact behavior of the targeted network. EXata uses a software virtual network (SVN) to digitally represent the entire network, the various protocol layers, antennas, and devices.

By providing a faithful replication of external behavior, the emulator ensures a high-fidelity representation, virtually identical to the real system. This approach presents a cost-effective means of evaluating new network technologies before the physical construction of systems.

EXata also allows network operators to assess the impact and risks of cyber threats, as shown in Figure 2. As a digital replica of the target system, EXata provides a zero-risk, cost-effective way to analyze how the system responds to cyber-attacks and evaluate different strategies safeguarding against them.

Figure 2. Modeling the December 2015 power grid attack on Ukraine using EXata

To learn more, check out our case study on the use of network digital twins for assessing cyber resilience: Ukraine Power Grid Attack

Non-terrestrial Network Emulation with Digital Twins

At Mobile World Congress Barcelona 2023, Keysight demonstrated an end-to-end test solution for NTNs, including video streaming over a live 5G NTN connection. Figure 3 shows the setup used during the demonstration. During the demonstration, the PROPSIM channel emulator functioned as the NTN digital twin, including Doppler shift, signal interference, path loss, and other distortions.

Figure 3. Digital Twin of a non-terrestrial network

The PROPSIM Channel Emulation hardware allows engineers to create realistic, real-time propagation environments. PROPSIM simulates a network of devices, as shown in Figure 4, enabling engineers to compare hardware variants and software versions during development. It also allows developers to compare devices from different vendors objectively while integrating full solutions.

Figure 4. Keysight PROPSIM channel emulator simulates a network of devices and interlinking subsystems

Non-terrestrial Network Use Cases

The primary benefit of NTN is extended coverage. Richard explained during the Tech Uncorked podcast that, while many NTN use cases exist, they vary in scope. Some focus on direct-to-device services while others center on handling backhaul traffic. NTN provides an opportunity for network service providers to offer premium services in an otherwise untapped market of customers for whom fiber or terrestrial microwave connections remain impractical.

For remote and underserved regions, including offshore oil platforms, high-speed rail, and in-flight aircraft, applications like broadband LEO communications and fixed cell connectivity enable users access to 5G services. The most ambitious NTN proposals call for direct-to-handset links with global coverage and low latency (beyond 2 Mb / second). “I want better Wi-Fi on my flight, okay?” Phil joked during the panel discussion. “In-flight connectivity is a big market. Being able to do that directly to your phone…is a promise the industry’s going to fulfill for us.”

NTN developers also plan to enhance machine-to-machine (M2M) communication for industrial applications such as environmental monitoring and asset tracking, with ubiquitous and reliable connectivity to the Internet. Wide-area IoT will leverage satellite links to augment terrestrial networks, enabling IoT services over a wide area for relatively high-latency applications including energy networks, transportation, and agriculture. In addition to expanding network coverage, mobile network operators (MNOs) also plan to use non-terrestrial networks to fill gaps at the edge of existing terrestrial coverage areas.

Beyond industrial and private sector applications, this technology offers tremendous benefits to public service, rescue, and emergency communications. “As someone who’s had a family member stranded overnight on a ski hill…this is exactly the scenario. It is a life and death opportunity,” implored Dr. Brooks.

In the event of natural disasters, network outages, or regional conflicts NTN provides backup connectivity to ensure continued service for mission-critical, time-critical communications. As such, any data-dependent and data-intensive use case stands to benefit from NTN infrastructure. “I like the eBook [Keysight] put out on this whole area…If people are looking for some sort of ground truth on what’s happening out there,” noted Dr. Brooks. To download our space and satellite report, visit: Defying Gravity

Dr. Brooks went on to describe how the war on Ukraine showcases open-source intelligence capacity when leveraging satellite imagery. “The most jarring visual that we all saw early on in the war was…a 65-kilometer-long caravan…perched outside of Kiev,” She additionally noted, “…this is what they’re calling the first open-source war.” Dr. Brooks further described how developments in commercial satellite and radar technologies serve to support detection and surveillance efforts pertaining to the conflict. Though the private sector space race accelerated only recently, Dr. Brooks argued that the satellite technology advancements enabling NTN development already, “…fundamentally, completely, re-calibrated land war.”

Building on Dr. Brooks’ comments regarding NTN utility, Dr. Hosseinian stated, “It’s happening now.” He went on to discuss standardization efforts currently happening around NTN technology. He explained how NTN standard development started with Rel 15 3GPP and continues to focus on how to integrate non-terrestrial networks with existing terrestrial network infrastructure.

Phil Lorch added that Sateliot, a small company based in Spain, recently launched the first narrowband internet of things (NB-IoT), Rel-17 based satellite. The company intends to use this satellite to support narrowband messaging from cell phones.

Originally envisioned as a technology to connect low-cost IoT devices with limited power consumption and throughput, developers now plan to integrate NB-IoT with NTN, as shown in Figure 5. NB-IoT will support ubiquitous coverage to mobile handsets for texting, voice communication, and multicast firmware updates.

Figure 5. NB-IoT developments enable massive device connectivity for low-power, low-throughput, high-density applications

“Release 18, which is being worked on now in the 3GPP organization, will be the next phase…on the path to 5G-Advanced,” Phil stated.

For the latest on NTN, visit: Enabling Next-Generation Non-Terrestrial Networks

Non-terrestrial Network Future Development

In his Keysight World: Innovate keynote, guest speaker Dr. Humphreys reviewed LEO NTN case studies from industry heavyweights like Starlink and T-Mobile, Apple and GlobalStar, and AST Space Mobile. In the context of these case studies, Dr. Humphreys highlights the key market dynamics for LEO broadband, visualized in Figure 6.

As high-performance user terminals reduce in size and cost, along with the reduction in launch cost enabling more frequency satellite deployments, the broadband service available becomes more desirable for customers. Attracting customers funnels more funding into development, allowing for more terminal installations and satellite launches.

Figure 6. Key market dynamics for LEO broadband result in a positive feedback loop

The Starlink constellation now has over 3600 satellites in operation and plans for many thousands more. Dr. Humphreys’ research lab studies Starlink signals on behalf of the Army Futures Command to potentially use Starlink signals as a backup for GPS.

Using a process called blind signal identification, Dr. Humphreys’ team determined all parameters necessary to resolve every symbol and subcarrier of the Starlink signal. To learn more about this research, read the resulting research paper: Signal Structure of the Starlink Ku-Band Downlink

From his research, Dr. Humphreys learned some interesting details about Starlink Broadband Service. They offer a fully vertically integrated system designed outside of any standards process—using orthogonal frequency-division modulated (OFDM) signals unlike those used in 4G or 5G.

T-Mobile and SpaceX

T-Mobile and SpaceX partnered to offer messaging, voice, and low-rate data service within the United States via direct satellite-to-handset link. As Starlink satellites will compensate for Doppler and timing offset on downlink and uplink, this approach requires no cell phone hardware changes.

While regulatory uncertainty around the spectrum T-Mobile plans to use remains, the approach will comply with existing 4G / 5G 3GPP standards. Dr. Humphreys predicts that the Federal Communications Commission (FCC) will almost certainly authorize T-Mobile’s use of spectrum for the effort. As for scaling globally, Dr. Humphrey’s notes that replicating T-Mobile’s unique spectrum assets worldwide presents significant difficulty.

Apple and Globalstar

Apple and GlobalStar partnered to offer a global emergency SOS service via a direct satellite-to-handset link. Though the current phone antennas lack beamforming capabilities, they manage to reach LEO. While the solution required no hardware updates for Globalstar, Apple developed a new satcom modem for handsets.

The system uses the existing Mobile Satellite Service (MSS) spectrum owned by Globalstar. Unlike Starlink’s approach, this leaves no regulatory uncertainty. However, despite leveraging a flexible bent pipe architecture, Globalstar must maintain backward compatibility. This restricts the system to 2G signals, at least in the near term.

3GPP Release 17 specifies the operator's use of 5G New Radio (NR) for NTN using the bent pipe architecture. In this configuration, the encoded signal from the 5G base station (gNB) remains unaltered in the feeder link and service link. However, bent pipe architecture does permit up-conversion and down-conversion of the signals.

Engineers refer to systems that use unaltered signals, as shown in Figure 7, as transparent. Transparent systems simplify the satellite design and benefit from mature NR technologies. In contrast, if a satellite must demodulate and modulate the signal, then engineers refer to the system as non-transparent.

Figure 7. Transparent versus non-transparent NTN

AST SpaceMobile

AST SpaceMobile plans to offer broadband satellite-to-handset service. AST SpaceMobile will partner with various MNOs to provide service to unmodified cell phones. To do this, they intend to use a 10-meter diameter phased array antenna, shown in Figure 8, on each satellite. These PAAs will enable narrow, high-gain beams down to handsets on the ground.

Figure 8. 10-meter diameter phased array antenna

Like the T-Mobile and SpaceX solution, FCC regulations remain uncertain. Unless AST Space Mobiles partners provide massive blocks of spectrum allocated exclusively for satellite communication, the system will face tremendous terrestrial interference in populated areas. While, again, Dr. Humphreys predicts that the FCC will authorize the needed spectrum, he also suspects that the large array will present significant technical risk.

Across all case studies, Dr. Humphreys highlighted that none required new standards for NTN from 3GPP. However, he posits that addressing technical issues like Doppler shift and timing compensation will require new standards in the long term.

Your Next Innovation Starts Here

“Even though we’re talking about the same thing—a 5G non-terrestrial network—the use cases are absolutely unique,” commented Chris when discussing the scope of NTN. “It’s amazing that we can be talking about one thing, and everyone is looking at it from a different position.” As advancements in NTN technology progress, the list of use cases continues to expand. With this rapid pace of development, staying informed with the latest information feels impossible.

Navigate the ever-evolving communication technology landscape with the help of industry experts. The development and deployment of non-terrestrial networks will impact your markets, your business, and the technological challenges your team will face. Demystify digital twins with the Tech Uncorked podcast, and gain crucial insights into NTN by checking out Keysight World: Innovate on-demand.

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