What the THF? Debate evolves over where terahertz starts

6G
Like it or not, terahertz is likely to be part of 6G - however it's defined. (Getty Images)

For anyone looking for an end-of-the-summer mind bender, this might be one to consider. Where does the terahertz (THz) spectrum range start?

The subject came up during a symposium last year, and it was covered here. Some researchers figure it starts at 100 GHz. Others say it’s 300 GHz. Suffice it to say, the debate is alive and well – depending on who you talk to.

Why should we care? For one, it helps to know what’s what when it comes to spectrum, the idea being there’s a finite amount in the lower bands. Some day, somebody will come knocking for even more spectrum, and these very high frequency ranges will come in handy when there’s nowhere else to go.

For another, it’s part of the grand plan for 6G, and even though it’s early days in a lot of ways for 5G, it’s not too early to start preparing for the next “G.” According to this Samsung white paper, it’s inevitable that mobile communications will use THz bands in future systems – it offers an enormous amount of bandwidth and could provide a way to meet the terabits per second (Tbps) data rate for 6G.

Definitions emerge 

When Josep Jornet started working on terahertz communications at Georgia Tech with the now-retired Professor Ian Akyildiz, there wasn’t much going on at any frequency above 100 GHz. The wireless community, primarily academia, was focused on millimeter wave, which at the time mostly meant 60 GHz. Along with some of the early device folks working on terahertz, “we chose the 100 GHz as the starting point,” Jornet told Fierce.

Similarly, aligned with the device-related work, “we picked 10 THz as the end. So, 100 GHz to 10 THz. This definition was backed up by agencies within DoD, at the time the main ones investing on fundamental research in this area,” said Jornet, who is now an associate professor at Northeastern University.

If you were to “go by the book,” so to speak, millimeter wave refers to signals whose frequency is in the millimeter range, where the formal definition is from 30 GHz to 300 GHz, or from 10 mm down to 1 mm. “Thus, if mmWave ends [at] 300 GHz, THz cannot start at 100 GHz. The problem is that the term mmWaves was so often used to refer to 60 GHz, that saying mmWave would many times not trigger people to think above 100 GHz,” he said.

To compensate for that, a common term that’s appearing is “upper mmWave” to refer to the frequencies between 100 GHz and 300 GHz, and THz from 300 GHz up, he noted.

“Interestingly, though, if we go by ITU recommendation, the Tremendously High Frequency (THF) or Terahertz go from 300 GHz to 3 THz,” he said, referring to this link. “If we really wanted to be strict, terahertz would mean 1 THz and up. In my group, we now refer to this as ‘true THz,’” with experimental testbeds already prepared for that.

RELATED: If mmWave sounds crazy, just wait for 6G and Terahertz: Special Report

According to Qammer Abbasi, associate professor at Scotland’s University of Glasgow, the widely accepted spectrum range of THz in academia is around 0.1 THz to 10 THz. He acknowledged there is debate about the start of THz, either 100 GHz or 300 GHz, but said he believes the debate is not that important at this early stage of 6G research. It will become clearer in the years to come, he added.

Although the term “THz” spectrum is quite recent, it has been studied in the literature with different names – such as sub-millimeter waves and far-infrared (IR), Abbasi said.

RELATED: Marek’s Take: Terahertz spectrum will pave the way to 6G

It’s also worth pointing out that the sensing community – think radar and spectroscopy – is usually ahead of the communications industry with applications and many of the names have been “given” not in light of what communications people think, Jornet noted, but what the sensing community coined. Since it started in the sensing world, it's not unusual to equate THz with advanced imaging. 

Arbitrary cut-off?

Four years ago, instructors at New York University (NYU) Wireless kicked off a series of seminars on terahertz.

The current director of NYU Wireless, Tom Marzetta, said defining the lower cutoff for the THz band appears somewhat arbitrary. “There is a natural tendency, of course, to make this cutoff lower in order to claim that you are working in the THz regime!,” he told Fierce via email.

"The same seems to have happened with mmWave, which would more accurately be called cmWave (a carrier frequency of 30 GHz is equivalent exactly to one centimeter wave-length, and 300 GHz is equivalent to one millimeter). One THz is, of course, 1000 GHz," Marzetta said. "Since the FCC licenses spectrum, I would think that everyone should abide by their nomenclature."

That makes sense for folks in the U.S., where the FCC allocates spectrum. In a way, the FCC may have been playing it safe when it adopted new rules in 2019 related to “spectrum above 95 GHz,” when it wanted to encourage the development of new technologies leading up to 6G. That order referenced frequencies between 95 GHz and 3 THz but didn’t try to define THz.

Another number that appears many times in these discussions is 275 GHz. “This was to due with the fact that spectrum allocations currently stop at 275 GHz (there are no allocations above this frequency). This is going to change soon, as the work on the ITU continues,” Jornet said.

At this stage, if you want to be safe and not trigger any question or doubt, “one thing that many of us are doing is to say ‘systems above 100 GHz,’ including upper mmWave (100 GHz to 300 GHz) and THz (300 GHz to 10 THz),” Jornet said. That seems to cover all the bases – for now.