In 6G telecom research today, a crucial portion of wireless spectrum has been neglected: the Frequency Range 3, or FR3, band. The shortcoming is partly due to a lack of viable software and hardware platforms for studying this region of spectrum, ranging from approximately 6 to 24 gigahertz. But a new, open-source wireless research kit is changing that equation. And research conducted using that kit, presented last week at a leading industry conference, offers proof of viability of this spectrum band for future 6G networks.

In fact, it’s also arguably signaling a moment of telecom industry re-evaluation. The high-bandwidth 6G future, according to these folks, may not be entirely centered around difficult millimeter wave-based technologies. Instead, 6G may leave plenty of room for higher-bandwidth microwave spectrum tech that is ultimately more familiar and accessible.

The FR3 band is a region of microwave spectrum just shy of millimeter-wave frequencies (30 to 300 GHz). FR3 is also already very popular today for satellite Internet and military communications. For future 5G and 6G networks to share the FR3 band with incumbent players would require telecom networks nimble enough to perform regular, rapid-response spectrum-hopping.

Yet spectrum-hopping might still be an easier problem to solve than those posed by the inherent physical shortcomings of some portions of millimeter-wave spectrum—shortcomings that include limited range, poor penetration, line-of-sight operations, higher power requirements, and susceptibility to weather.

Pi-Radio’s New Face

Earlier this year, the Brooklyn, N.Y.-based startup Pi-Radio—a spinoff from New York University’s Tandon School of Engineering—released a wireless spectrum hardware and software kit for telecom research and development. Pi-Radio’s FR-3 is a software-defined radio system developed for the FR3 band specifically, says company co-founder Sundeep Rangan.

“Software-defined radio is basically a programmable platform to experiment and build any type of wireless technology,” says Rangan, who is also the associate director of NYU Wireless. “In the early stages when developing systems, all researchers need these.”

For instance, the Pi-Radio team presented one new research finding that infers direction to an FR3 antenna from measurements taken by a mobile Pi-Radio receiver—presented at the IEEE Signal Processing Society‘s Asilomar Conference on Signals, Systems and Computers in Pacific Grove, Calif. on 30 October.

According to Pi-Radio co-founder Marco Mezzavilla, who’s also an associate professor at the Polytechnic University of Milan, the early-stage FR3 research that the team presented at Asilomar will enable researchers “to capture [signal] propagation in these frequencies and will allow us to characterize it, understand it, and model it... And this is the first stepping stone towards designing future wireless systems at these frequencies.”

There’s a good reason researchers have recently rediscovered FR3, says Paolo Testolina, postdoctoral research fellow at Northeastern University’s Institute for the Wireless Internet of Things unaffiliated with the current research effort. “The current scarcity of spectrum for communications is driving operators and researchers to look in this band, where they believe it is possible to coexist with the current incumbents,” he says. “Spectrum sharing will be key in this band.”

Rangan notes that the work on which Pi-Radio was built has been published earlier this year both on the more foundational aspects of building networks in the FR3 band as well as the specific implementation of Pi-Radio’s unique, frequency-hopping research platform for future wireless networks. (Both papers were published in IEEE journals.)

“If you have frequency hopping, that means you can get systems that are resilient to blockage,” Rangan says. “But even, potentially, if it was attacked or compromised in any other way, this could actually open up a new type of dimension that we typically haven’t had in the cellular infrastructure.” The frequency-hopping that FR3 requires for wireless communications, in other words, could introduce a layer of hack-proofing that might potentially strengthen the overall network.

Complement, Not Replacement

The Pi-Radio team stresses, however, that FR3 would not supplant or supersede other new segments of wireless spectrum. There are, for instance, millimeter wave 5G deployments already underway today that will no doubt expand in scope and performance into the 6G future. That said, the ways that FR3 expand future 5G and 6G spectrum usage is an entirely unwritten chapter: Whether FR3 as a wireless spectrum band fizzles, or takes off, or finds a comfortable place somewhere in between depends in part on how it’s researched and developed now, the Pi-Radio team says.

“We’re at this tipping point where researchers and academics actually are empowered by the combination of this cutting-edge hardware with open-source software,” Mezzavilla says. “And that will enable the testing of new features for communications in these new frequency bands.” (Mezzavilla credits the National Telecommunications and Information Administration for recognizing the potential of FR3, and for funding the group’s research.)

By contrast, millimeter-wave 5G and 6G research has to date been bolstered, the team says, by the presence of a wide range of millimeter-wave software-defined radio (SDR) systems and other research platforms.

“Companies like Qualcomm, Samsung, Nokia, they actually had excellent millimeter wave development platforms,” Rangan says. “But they were in-house. And the effort it took to build one—an SDR at a university lab—was sort of insurmountable.”

So releasing an inexpensive open-source SDR in the FR3 band, Mezzavilla says, could jump start a whole new wave of 6G research.

“This is just the starting point,” Mezzavilla says. “From now on we’re going to build new features—new reference signals, new radio resource control signals, near-field operations... We’re ready to ship these yellow boxes to other academics around the world to test new features and test them quickly, before 6G is even remotely near us.”

This story was updated on 7 November 2024 to include detail about funding from the National Telecommunications and Information Administration.

A new, tunable smart surface can transform a single pulse of light into multiple beams, each aimed in different directions. The proof-of-principle development opens the door to a range of innovations in communications, imaging, sensing, and medicine.

The research comes out of the Caltech lab of Harry Atwater, a professor of applied physics and materials science, and is possible due to a type of nano-engineered material called a metasurface. “These are artificially designed surfaces which basically consist of nanostructured patterns,” says Prachi Thureja, a graduate student in Atwater’s group. “So it’s an array of nanostructures, and each nanostructure essentially allows us to locally control the properties of light.”

The surface can be reconfigured up to millions of times per second to change how it is locally controlling light. That’s rapid enough to manipulate and redirect light for applications in optical data transmission such as optical space communications and Li-Fi, as well as lidar.

“[The metasurface] brings unprecedented freedom in controlling light,” says Alex M.H. Wong, an associate professor of electrical engineering at the City University of Hong Kong. “The ability to do this means one can migrate existing wireless technologies into the optical regime. Li-Fi and LIDAR serve as prime examples.”

Metasurfaces remove the need for lenses and mirrors

Manipulating and redirecting beams of light typically involves a range of conventional lenses and mirrors. These lenses and mirrors might be microscopic in size, but they’re still using optical properties of materials like Snell’s Law, which describes the progress of a wavefront through different materials and how that wavefront is redirected—or refracted—according to the properties of the material itself.

By contrast, the new work offers the prospect of electrically manipulating a material’s optical properties via a semiconducting material. Combined with nano-scaled mirror elements, the flat, microscopic devices can be made to behave like a lens, without requiring lengths of curved or bent glass. And the new metasurface’s optical properties can be switched millions of times per second using electrical signals.

“The difference with our device is by applying different voltages across the device, we can change the profile of light coming off of the mirror, even though physically it’s not moving,” says paper co-author Jared Sisler—also a graduate student in Atwater’s group. “And then we can steer the light like it’s an electrically reprogrammable mirror.”

The device itself, a chip that measures 120 micrometers on each side, achieves its light-manipulating capabilities with an embedded surface of tiny gold antennas in a semiconductor layer of indium tin oxide. Manipulating the voltages across the semiconductor alters the material’s capacity to bend light—also known as its index of refraction. Between the reflection of the gold mirror elements and the tunable refractive capacity of the semiconductor, a lot of rapidly-tunable light manipulation becomes possible.

“I think the whole idea of using a solid-state metasurface or optical device to steer light in space and also use that for encoding information—I mean, there’s nothing like that that exists right now,” Sisler says. “So I mean, technically, you can send more information if you can achieve higher modulation rates. But since it’s kind of a new domain, the performance of our device is more just to show the principle.”

Metasurfaces open up plenty of new possibilities

The principle, says Wong, suggests a wide array of future technologies on the back of what he says are likely near-term metasurface developments and discoveries.

“The metasurface [can] be flat, ultrathin, and lightweight while it attains the functions normally achieved by a series of carefully curved lenses,” Wong says. “Scientists are currently still unlocking the vast possibilities the metasurface has available to us.

“With improvements in nanofabrication, elements with small feature sizes much smaller than the wavelength are now reliably fabricable,” Wong continues. “Many functionalities of the metasurface are being routinely demonstrated, benefiting not just communication but also imaging, sensing, and medicine, among other fields... I know that in addition to interest from academia, various players from industry are also deeply interested and making sizable investments in pushing this technology toward commercialization.”

While the technology world awaits NIST’s latest “post-quantum” cryptography standards this summer, a parallel effort is underway to also develop cryptosystems that are grounded in quantum technology—what are called quantum-key distribution or QKD systems.

As a result, India, China, and a range of technology organizations in the European Union and United States are researching and developing QKD and weighing standards for the nascent cryptography alternative. And the biggest question of all is how or if QKD fits into a robust, reliable, and fully future-proof cryptography system that will ultimately become the global standard for secure digital communications into the 2030s. As in any emerging technology standard, different players are staking claims on different technologies and implementations of those technologies. And many of the big players are pursuing such divergent options because no technology is a clear winner at the moment.

According to Ciel Qi, a research analyst at the New York-based Rhodium Group, there’s one clear leader in QKD research and development—at least for now. “While China likely holds an advantage in QKD-based cryptography due to its early investment and development, others are catching up,” says Qi.

Two different kinds of “quantum secure” tech

At the center of these varied cryptography efforts is the distinction between QKD and post-quantum cryptography (PQC) systems. QKD is based on quantum physics, which holds that entangled qubits can store their shared information so securely that any effort to uncover it is unavoidably detectable. Sending pairs of entangled-photon qubits to both ends of a network provides the basis for physically secure cryptographic keys that can lock down data packets sent across that network.

Typically, quantum cryptography systems are built around photon sources that chirp out entangled photon pairs—where photon A heading down one length of fiber has a polarization that’s perpendicular to the polarization of photon B heading in the other direction. The recipients of these two photons perform separate measurements that enable both recipients to know that they and only they have the shared information transmitted by these photon pairs. (Otherwise, if a third party had intervened and measured one or both photons first, the delicate photon states would have been irreparably altered before reaching the recipients.)

“People can’t predict theoretically that these PQC algorithms won’t be broken one day.” —Doug Finke, Global Quantum Intelligence

This shared bit the two people on opposite ends of the line have in common then becomes a 0 or 1 in a budding secret key that the two recipients build up by sharing more and more entangled photons. Build up enough shared secret 0s and 1s between sender and receiver, and that secret key can be used for a type of strong cryptography, called a one-time pad, that guarantees a message’s safe transmission and faithful receipt by only the intended recipient.

By contrast, post-quantum cryptography (PQC) is based not around quantum physics but pure math, in which next-generation cryptographic algorithms are designed to run on conventional computers. And it’s the algorithms’ vast complexity that makes PQC security systems practically uncrackable, even by a quantum computer. So NIST—the U.S. National Institute of Standards and Technology—is developing gold-standard PQC systems that will undergird tomorrow’s post-quantum networks and communications.

The big problem with the latter approach, says Doug Finke, chief content officer of the New York-based Global Quantum Intelligence, is PQC is only believed (on very, very good but not infallible evidence) to be uncrackable by a fully-grown quantum computer. PQC, in other words, cannot necessarily offer the ironclad “quantum security” that’s promised.

“People can’t predict theoretically that these PQC algorithms won’t be broken one day,” Finke says. “On the other hand, QKD—there are theoretical arguments based on quantum physics that you can’t break a QKD network.”

That said, real-world QKD implementations might still be hackable via side-channel, device-based, and other clever attacks. Plus, QKD also requires direct access to a quantum-grade fiber optics network and sensitive quantum communications tech, neither of which is exactly commonplace today. “For day-to-day stuff, for me to send my credit card information to Amazon on my cellphone,” Finke says, “I’m not going to use QKD.”

China’s early QKD lead dwindling

According to Qi, China may have originally picked QKD as a focal point of their quantum technology development in part because the U.S. was not directing its efforts that way. “[The] strategic focus on QKD may be driven by China’s desire to secure a unique technological advantage, particularly as the U.S. leads in PQC efforts globally,” she says.

In particular, she points to ramped up efforts to use satellite uplinks and downlinks as the basis for free-space Chinese QKD systems. Citing as a source China’s “father of quantum,” Pan Jianwei, Qi says, “To achieve global quantum network coverage, China is currently developing a medium-high orbit quantum satellite, which is expected to be launched around 2026.”

That said, the limiting factor in all QKD systems to date is their ultimate reliance on a single photon to represent each qubit. Not even the most exquisitely-refined lasers and fiber optic lines can’t escape the vulnerability of individual photons.

QKD repeaters, which would blindly replicate a single photon’s quantum state but not leak any distinguishing information about the individual photons passing through—meaning the repeater would not be hackable by eavesdroppers—do not exist today. But, Finke says, such tech is achievable, though at least 5 to 10 years away. “It definitely is early days,” he says.

“While China likely holds an advantage in QKD-based cryptography due to its early investment and development, others are catching up.” —Ciel Qi, Rhodium Group

“In China they do have a 2,000-kilometer network,” Finke says. “But it uses this thing called trusted nodes. I think they have over 30 in the Beijing to Shanghai network. So maybe every 100 km, they have this unit which basically measures the signal... and then regenerates it. But the trusted node you have to locate on an army base or someplace like that. If someone breaks in there, they can hack into the communications.”

Meanwhile, India has been playing catch-up, according to Satyam Priyadarshy, a senior advisor to Global Quantum Intelligence. Priyadarshy says India’s National Quantum Mission includes plans for QKD communications research—aiming ultimately for QKD networks connecting cities over 2,000-km distances, as well as across similarly long-ranging satellite communications networks.

Priyadarshy points both to government QKD research efforts—including at the Indian Space Research Organization—and private enterprise-based R&D, including by the Bengaluru-based cybersecurity firm QuNu Labs. Priyadarshy says that QuNu, for example, has been working on a hub-and-spoke framework named ChaQra for QKD. (Spectrum also sent requests for comment to officials at India’s Department of Telecommunications, which were unanswered as of press time.)

“A hybrid of QKD and PQC is the most likely solution for a quantum safe network.” —Satyam Priyadarshy, Global Quantum Intelligence

In the U.S. and European Union, similar early-stage efforts are also afoot. Contacted by IEEE Spectrum, officials from the European Telecommunications Standards Institute (ETSI); the International Standards Organization (ISO); the International Electrotechnical Commission (IEC); and the IEEE Communications Society confirmed initiatives and working groups that are now working to both promote QKD technologies and emergent standards now taking shape.

“While ETSI is fortunate to have experts in a broad range of relevant topics, there is a lot to do,” says Martin Ward, senior research scientist based at Toshiba’s Cambridge Research Laboratory in England, and chair of a QKD industry standards group at ETSI.

Multiple sources contacted for this article envisioned a probable future in which PQC will likely be the default standard for most secure communications in a world of pervasive quantum computing. Yet, PQC also cannot avoid its potential Achilles’ heel against increasingly powerful quantum algorithms and machines either. This is where, the sources suggest, QKD could offer the prospect of hybrid secure communications that PQC alone could never provide.

“QKD provides [theoretical] information security, while PQC enables scalab[ility],” Priyadarshy says. “A hybrid of QKD and PQC is the most likely solution for a quantum safe network.” But he added that efforts at investigating hybrid QKD-PQC technologies and standards today are “very limited.”

Then, says Finke, QKD could still have the final say, even in a world where PQC remains preeminent. Developing QKD technology just happens, he points out, to also provide the basis for a future quantum Internet.

“It’s very important to understand that QKD is actually just one use case for a full quantum network,” Finke says.

“There’s a lot of applications, like distributed quantum computing and quantum data centers and quantum sensor networks,” Finke adds. “So even the research that people are doing now in QKD is still very, very helpful because a lot of that same technology can be leveraged for some of these other use cases.”