Washington View: Beam Me Up (And Down), Scotty - Inside GNSS - Global Navigation Satellite Systems Engineering, Policy, and Design

Washington View: Beam Me Up (And Down), Scotty

A look at the state of 5G NR NTN.

If beyond visual line of sight (BVLOS) operations are the “Holy Grail” of the UAS industry, harnessing the capability of non-terrestrial networks (NTNs), or Space-Based Adaptive Communications Node networks, appears to be the equivalent for a 5G communication system network—particularly in low earth orbit (LEO) satellite communications (SatComs) constellations.

UAS, among many other systems, can play a vital role in developing requirements, which could, in turn, help mitigate current technological limitations for BVLOS and a wide range of other use cases that depend on accurate positioning, navigation and timing (PNT). However, as in so many other areas of emerging technology, challenges in implementation and standardization remain unsolved. Here’s a rundown of the state of play for 5G LEO SatCom networks.

The Basic Plan

To appreciate the game-changing nature of 5G New Radio (NR), you must first have a basic understanding of how traditional mobile telecommunications work.

Telecoms traditionally consist of four key components, according to Ericsson, an information and communication technology (ICT) service provider. Most of us engage directly with user equipment (UE) devices such as smartphones and tablets. The Radio Access Network (RAN) wirelessly connects UEs using radio frequencies (RF). Coordination between various parts of the RAN and the connection to the internet occurs through the core network (CN). Finally, the transport network supplies the connection between the RAN and the CN.

Complex integrated hardware and software enable these functions, even in traditional comms. Baseband equipment that performs all of the signal processing functions required for wireless communications (e.g., for multiple antennas, to detect/correct transmission errors, provide security and manage resources) contains high performance electronics and cutting edge software. Radios ensure signal transmissions travel on the correct bands at the required voltage and actually convert digital information into those signals. Antennas beam out those electric signals into radio waves.

5G, which requires Multiple-Input Multiple-Output (MIMO), adds layers of complexity to this basic telecom system. It requires cross-functional integration, such as integrating radios and baseband hardware and software with antennas. 5G NR RAN (which replaces the Long Term Evolution (LTE) high speed and low latency RAN) and CN software can be deployed and managed on the same infrastructure. 

For this reason, RAN radios (baseband and antenna-integrated) and CN sites depend on software, on each other and on complex code. For maximum coverage, companies have built additional base radio stations, called gNB (Next Generation/gNodeB, which replaces the eNB or eNodeB or Evolved Node B) and deployed AI and ML to orchestrate and balance traffic. 

Why does this matter? 5G’s unified interface enables higher speeds, reduces latency and increases the availability and reliability of connections. 

Christian Ardito of StarNav preparing the company’s satellite-based PNT system for flight tests on an L-29 jet.

The Value Proposition

According to Qualcomm, 5G will fuel “massive IoT” and drive global growth. The company’s landmark 5G Economy Study found 5G could potentially enable up to $13.1 trillion worth of goods and services across a diverse group of businesses worldwide by 2035. More than 60 countries have already deployed 5G.

Now add in NTNs, which deliver 5G/NR service via space (satellite) or air (airborne platform) to the 5G mix. This multi-layered network can include SatCom networks, high altitude platform systems (HAPS), UAS and other air-to-ground networks.

According to 3GPP, the NTN 5G value proposition is clear. NTN systems can significantly bolster 5G service continuity where a single or series of combined terrestrial networks cannot, particularly for mobility assets and mission-critical communications. NTN can bridge 5G service coverage gaps where terrestrial networks do not exist or simply do not reach. This includes oceans, deserts, wildernesses and urban areas. Scalability, through NTN’s wide area coverage and ability to multicast, also tops its list of benefits. 

NTN 5G can support a wide range of use cases, including aeronautical and maritime tracking systems. Specifically, Automatic Dependent Surveillance-Broadcast (ADS-B), which is based on the capability of the aircraft to navigate to a destination using GNSS data and barometric altitude, allows for communication with air traffic control, cooperative surveillance, separation and situational awareness. It depends on aircraft navigation system data derived primarily from GNSS signals and then broadcast to aircraft and ground-deployed infrastructure. 

But this infrastructure does not exist in a number of areas, including over oceans and in the Arctic. LEO-based ADS-B receivers could contribute to the ATC relay network. This would result in low latency and secure coverage globally. In the maritime sector, the equivalent tracking system, Automatic Identification System (AIS), also benefits from space-based receivers.

The benefits of NTN 5G extend beyond transportation and more broadly for internet of things (IoT) applications, from surveillance of infrastructure to precision agriculture. 

Standards Moving Forward

The 3rd Generation Partnership Project (3GPP), a global partnership of telecommunications standard development organizations, started working on 5G NR NTN about 5 years ago. Its first study, Release 15 (Rel-15) documented in TR 38.811, targeted deployment scenarios and models that included not only LEO and HAPS, but GEO satellites as well. It addressed issues such as relevant beams, elevation angles, satellite deployment footprint, various NTN terminals and antenna arrays. 

The follow on study, Rel-16, focused on minimum viable architecture, higher layer protocols, and physical layer aspects (TR 38.821). This study concluded that the group’s NR work provided a solid basis to support NTN. It identified additional areas of study including: timing relationships, uplink time and frequency synchronization, and hybrid automatic repeat request (HARQ), a combination of high-rate forward error correction (FEC) and automatic repeat request (ARQ), essential for reliable data transmissions.

Earlier this year, ratified Rel-17 focused on 5G system enhancements. Among other things, Release 17 involves physical layer aspects, protocols, architecture and radio resource management. This study assumes all UEs have GNSS capabilities. 3GPP “froze” (meaning no further functions can be added to the specification) the Protocols for this study in March 2022 and the Protocol Code (OpenAPI) in June. According to 3GPP, the “Release 17 Description; Summary of Rel-17 Work Items” (TR21.917) remains in production. 

In the meantime, 3GPP launched the Rel-18 study. It focuses on 5G Advanced, addressing extraterritorial coverage of satellites and high altitude systems.

Progress continues to move forward on the coding side of the house, courtesy of the OpenAirInterface Software Alliance (OSA). This is significant because many of these systems rely on complex code stacks. The OSA, a French non-profit organization established in 2014 and funded by corporate sponsors, is the home of OpenAirInterface (OAI). OAI, an open software endeavor, has gathered a community of developers from around the world who work together to build wireless cellular RAN and CN technologies. 

The OSA OAI 5G Project Group seeks to develop and deliver a 3GPP compatible 5G gNB RAN software stack under the OAI Public License V1.1. In October, the group provided an OAI codebase status update and development roadmap.

Simultaneously, the organization’s related OAI 5G-LEO extension for 5G satellite links aims to use the OAI as a tool to assist in 5G NTN R&D. This 5G-LEO Project has four main objectives, according to the European Space Agency (ESA):

1. Select a 5G-LEO baseline scenario for 3GPP NR-NTN system deployments to implement and verify with the extended OAI library.

2. Identify fundamental codebase gaps and changes to extend OAI to the 5G-LEO baseline. 

3. Implement required OAI code adaptations for the different layers of the 3GPP protocol stack to support 5G-LEO within Rel-17 and potentially in Rel-18.

4. Set up an end-to-end 5G-LEO demonstrator in the lab for experimental validation of the OAI extension for the 5G-LEO baseline scenario.

This two-phase project, started in December 2021, is in its second phase. It’s focused on implementation, software compliance and demonstration.

Challenges and Stratospheric Possibilities

R&D continues to tackle other challenges that must be mitigated for successful LEO-based 5G NTN. Propagation delays and large Doppler shifts caused by moving cells rank high among them.

Propagation delays result in latency. Depending on the satellite’s altitude, long distances between satellite constellations, ground stations, and user terminals cause time delays in radio wave transmissions. While delays from LEO satellites are much shorter than higher altitude GEO satellites, the constellation and ground station deployments must be larger to cover wider areas. This can increase costs. Groups are exploring workarounds such as sat-to-sat relays, or mesh networks for CN functions, to mitigate these issues.

On the military side, the same mitigation measures used to solve common latency issues in GEO MIL/SATCOM applications can be applied to a LEO MIL/SATCOM architecture, said Jason “JD” Danieli, CEO of Colorado-based Giuseppe Space Enterprises. 

“Modeling, simulation and analysis, as we commonly refer to as MS&A, is an extremely important first step prior to deploying new tech. There are so many variables to consider and account for. A common SW tool, such as MATLAB, is just one of many tools we consider when solutioning,” Danieli said. “We attempt to address issues prior to deployment such as interoperability, performance and resiliency.” 

Research also remains ongoing to address Doppler effects in the LEO orbit. This phenomenon describes the increases (or decreases) in the frequency of sound, light or other waves as the source and observer move toward (or away from) each other. Waves emitted by a source traveling toward an observer get compressed. The LEO satellites and airborne platforms for 5G NTN move very fast while the user terminal remains either stationary or moves slowly. This results in large Doppler shifts experienced by the receiver, leading to communication degradations between transmitters and receivers. This is why 3GPP assumes NTN devices will be equipped with a GNSS chipset to determine position and calculate the needed frequency adjustments. 

But GNSS has its own challenges in terms of vulnerability. Efforts to make PNT more resilient continue to churn…slowly. 

On the bright side, LEO satellites offer several attributes that are attractive to supplement GNSS for positioning and timing. This includes an abundance of signals, favorable geometric configurations, and diverse signal frequencies. 

“With 5G NTN entering the stage, we will have even more satellite signals to consider on top of the 3,000+ LEO satellites whose signals have already shown potential to supplement GNSS,” said StarNav CEO Joshua Morales, who has spent nine years building PNT systems that use cellular and LEO satellite signals as a backup to GPS.

New partnerships have cropped up to tackle these challenges and take advantage of the benefits that NTN has for the future of 5G. Last summer, for example, Ericsson, Qualcomm Technologies, Inc. and French aerospace company Thales announced a partnership for the first testing and validation of 5G NTN. This work aims to validate 3GPP’s assumption that 5G NTN can be supported in a smartphone form factor. Initial tests are taking place in an emulated space environment in France. 

With the possibility of successful 5G NTN just within our grasp, we can no longer just say the sky’s the limit—because the possibilities are truly out of this world.