Military spectrum management: Spectrum sharing, quantum sensors, and AI advancesStory
October 20, 2021
U.S. military spectrum management is currently undergoing many changes – from spectrum sharing to technology advances in quantum sensors and artificial intelligence (AI).
The electromagnetic spectrum (EMS) supports all kinds of civilian and military operations worldwide; it also happens to be an invisible battlespace essential to all of the U.S. Department of Defense (DoD) domains. Interruption of access to spectrum can quickly become a military nightmare.
A recent report by the U.S. GAO (Government Accountability Office) found that China and Russia are taking what the GAO calls significant steps to improve their electromagnetic warfare capabilities to challenge the U.S., meaning that the U.S. can no longer be assured of superiority within the spectrum (https://www.gao.gov/products/gao-21-440t).
The DoD is well-aware of the challenges and opportunities affecting military use of the spectrum, according to the GAO, and the department agreed with GAO’s recommendations that it should identify processes and procedures, reform governance structures, assign leadership for strategy implementation, issue an implementation plan, and develop oversight processes in the spectrum arena.
In the midst of the push to regain U.S. superiority within the spectrum, changes are underway as the DoD embarks on spectrum sharing, new quantum technologies, and a boost in speed from artificial intelligence (AI).
To help solve the growing problem of spectrum scarcity, the U.S. Federal Communications Commission (FCC) decided to open up the Citizens’ Broadband Radio Service (CBRS) band (3.5 to 3.7 GHz) of the radio-frequency (RF) spectrum, which was being underutilized, for sharing, a move that would enable multiple categories of users to safely occupy the same frequency bands.
In 2020, licenses were auctioned off to the highest bidders, which were wireless carriers, for access to this band of spectrum; even so, the U.S. Navy retained priority as the incumbent user.
“A lot of work and technology had to fall into place to even make spectrum sharing possible, and it’s finally come to fruition,” says Manuel Uhm, director of silicon marketing at Xilinx (San Jose, California), as well as chair of the board of directors of the Wireless Innovation Forum, which sets all the technical standards for the Citizens Broadband Radio Service (CBRS). “Spectrum sharing makes so much sense as we’re running out of exclusively licensed spectrum; it’s going to become more common going forward.”
The issue of the EMS and spectrum dominance is a huge priority for the DoD. “This is an extremely high priority for them – having warfighters not be jammed by enemy transmitters, preventing them from carrying out orders, sending and receiving data in a real-time manner,” Uhm adds.
For many years, the commercial strategy was “just open up the spectrum at auction, we’ll buy and own it, and no one else can use it.” But the DoD’s strategy was to maintain the spectrum it had dedicated for its purposes, and then share it for commercial purposes when not being fully utilized.
“If you carry the DoD strategy to full fruition, you can see scenarios where the DoD would also like to be able to use commercial broadband spectrum when it’s not fully utilized,” Uhm says. “Put the shoe on the other foot, where the DoD says ‘You bought this spectrum at auction but it’s underutilized so we’d like to be able to use it when you aren’t using it for commercial purposes.’ Will the commercial sector open it up too? They’re going to say they purchased it at auction and paid billions for it, but if they’re not fully using it, would they be open to spectrum sharing? As long as their commercial users have priority and are protected from interference from other users, then why not open it up to the DoD?”
While CBRS has successfully rolled out, improvements can be made: “Now that the revolution has happened, it’s about evolutionary changes,” Uhm continues. “So people are looking at CBRS and ways to streamline or improve the model for future spectrum sharing. One example is the ESC, the environmental sensing capability – that’s the system used by CBRS to try to ensure commercial broadband users don’t interfere with naval radar when it’s in use.”
The system as it stands involves a group of RF sensors set up on the coastline and in a few select spots inland. “If [the sensors] detect naval radar in operation, they notify all CBRS devices in that vicinity and force them to move off the spectrum within a five-minute time frame,” Uhm explains. “The current model works, but it’s expensive to deploy and can have errors. A sensor can go offline or miss a naval radar ping, then you have interference. Naval radar should have priority and be protected.”
An alternative proposed method is called the Incumbent Informing Capability, which is “a more proactive approach where a spectrum-coordination system uses information provided by the incumbents about when they will be using certain frequencies to avoid interference from other lower-priority users,” Uhm adds.
One technology advancement poised to shake up spectrum management lies within the realm of quantum sensors/receivers. In mid-2021, the U.S. Army reported using an Army-built quantum “Rydberg” sensor – a super-wideband radio receiver – which can analyze the full spectrum of RF and real-world signals.
The Army’s Rydberg sensor uses laser beams to create highly excited Rydberg atoms directly above a microwave circuit to boost and focus directly on the portion of spectrum being measured. These atoms are sensitive to the circuit’s voltage, so the device can be used as a sensitive probe for the wide range of signals within the RF spectrum.
“Previous demonstrations of Rydberg atomic sensors were only able to sense small and specific regions of the RF spectrum, but our sensor operates continuously over a wide frequency range,” says Kevin Cox, a researcher at the U.S Army Combat Capabilities Development Command (DEVCOM) of the Army Research Lab. “This is a really important step toward proving quantum sensors can provide a new, dominant, set of capabilities for our soldiers, who are operating within an increasingly complex electromagnetic battlespace.”
The lab’s Rydberg spectrum analyzer and other quantum sensors show potential to unlock a new frontier of Army sensors for spectrum awareness, electronic warfare (EW), sensing, and communications – all part of the Army’s modernization strategy. (Figure 1.)
[Figure 1 | Exciting rubidium atoms to high-energy Rydberg states. Atoms interact strongly with the circuit’s electric fields, enabling detection and demodulation of any signal received into the circuit. U.S. Army illustration.]
In this vein, DARPA [Defense Advanced Research Projects Agency] has launched what it calls its Quantum Apertures (QA) program to try to develop a fundamentally new way of receiving radio frequency waveforms to improve both sensitivity and frequency agility for defense applications. DARPA’s former Quantum-Assisted Sensing and Readout program (which ran from 2010 to 2018) recognized the potential to sense electronic fields using highly excited Rydberg quantum states.
The newly launched QA program is expected to run for 56 months; research is expected to begin in late 2021, with research team members Honeywell, Northrop Grumman, ColdQuanta, and SRI International on board.
QA’s goal is to develop RF antennas or apertures via quantum techniques to alter the way RF spectrum is accessed. To get there, portable and directional RF receivers are needed with greater sensitivity, bandwidth, and dynamic range than any classical receiver available today.
“Commercial wireless infrastructure, the construct of spectrum use, and beyond have been dictated by a hundred years of antenna theory, originally developed by German physicist Heinrich Hertz,” says John Burke, the program manager leading the QA program. “With the introduction of quantum, we have the ability to replace the existing fundamental limits placed on antenna technology with a whole new set of rules. Quantum Apertures seeks to create a paradigm shift in the way we process and use the spectrum.”
Rydberg sensors offer significant advantages over classic antenna-based receivers. First, these sensors aren’t plagued by sensitivity challenges because they don’t need to contend with thermal noise. Moreover, Rydberg sensors have no size or shape limitations with respect to the received RF frequency wavelength. This decoupling of the aperture shape and RF frequency enables Rydberg sensors to be programmed over a large frequency range – from MHz to THz.
The target system of the QA program is to directionally receive low-intensity, modulated RF signals and operate over a large spectral range, from 10 MHz to 40 GHz or beyond. This span will enable users to see a large swath of spectrum with one antenna, particularly the portions that are relevant to military applications.
Researchers will also attempt to develop a sensor element and its associated electronics within a one-cubic-centimeter package that can successfully operate across various frequencies, a feat that DARPA says will break the tradeoff between frequency range and size that exists with classic antennas. This setup also means that the QA sensor will rely on lasers instead of cable for wiring, achieving better resilience to high-power effects and higher tolerance of microwave radiation.
“Recent demonstrations of Rydberg atomic sensors show it’s possible to access large portions of the RF spectrum, but QA aims to go beyond those efforts by continuously connecting these demonstrations across the spectrum,” says Burke. “We’re going from simple demonstrations of one functionality to a device that can be programmed to do almost anything and do most of it better than a classical receiver could. This includes speeding up the time to tune the sensor – improving sensitivity to small signals, enhancing dynamic range, and expanding compatibility with modern signals.”
AI and the edge
Currently used systems scan a wide swath of spectrum to identify signals of interest and check them against a database to determine if action needs to be taken.
“Advances in AI are providing far better options that can respond much faster and with greater intelligence,” Xilinx’s Uhm says. “It can help identify foreign or interfering signals within a particular band of spectrum. And it can be used to help identify signals that would otherwise be classified as unknown signals because they’re not necessarily in the database being referenced. So there are a lot of AI technologies being put forward to manage the spectrum and EMS spectrum dominance for defense parties.”
Without question, AI is changing spectrum management, but so too is the concept of the edge, which can mean many things to different people.
“From a defense perspective, ‘the edge’ refers to the tactical edge, which can range from a pointy-nosed jet to a Humvee to remote sensors, but the trend toward the edge from a commercial perspective is also helping at the tactical edge where everything needs to get smaller and has tight constraints around heat dissipation and thermal management, basically the size, weight, and power,” Uhm explains.
“The edge” is an overused term, Uhm points out. “There are many requirements both in terms of compute performance in size, weight, and power within the commercial space, so when people don’t define the edge it becomes a nebulous concept like ‘the cloud,’” he says.
Having both semiconductor technology and systems focused on the edge generally means smaller, rugged form factors that don’t consume as much power, can survive longer in the field, and don’t overheat as quickly.
“If you need to go back to the cloud, there’s latency associated with doing that and your responsiveness is impacted,” Uhm says. “The goal of the edge is distributing intelligence so it’s not all in one place, but is actually distributed closer to the information being gathered to allow you to take action faster.”
This reality is important for commercial scenarios like autonomous vehicles or robotic surgery, Uhm notes, but from a defense and military perspective it’s actually huge, because you need to act very quickly on intelligence and shorten the kill chain when necessary.
“Having the intelligence right there at the scene is so critical to being agile and nimble – as opposed to having to go back and wait for instructions from central command,” he adds. “Warfighters don’t have time for that.”
Security is an often-overlooked aspect of the spectrum. But it’s garnering more attention now within the commercial space because more hacks and malicious attacks are being made public.
“If you hack a system, you can take control of it. Security is of paramount importance – all systems need to be secure and have multiple levels of security,” Uhm notes. From the waveform “all the way down to the individual chip level every key component needs to be secure, because hacks can occur in a number of different ways, including via the spectrum.”