Debatable Frequencies: Where Does the Terahertz Band Start?

March 25, 2021

Written by Caio Castro
Question mark displayed on a smartphone

During the 6GSymposium last October, a group of panellists discussed using the Terahertz frequency for communications. At some point, one of the experts said that this band starts at 300 GHz, raising the immediate question among attendees: Wasn’t Terahertz supposed to begin at 100 GHz?

It’s easy to find examples of people using either 100 GHz or 300 GHz to define the Terahertz band. An article published by researchers at Brown University and Rice University, for instance, considered THz to begin at 100 GHz. Others, as you can see in this article from IEEE, adopt 300 GHz as a threshold. 

After all, where do Terahertz waves start? 100 GHz or 300 GHz? And more important: Does it matter?

According to Rahim Tafazolli, Head of the Institute for Communication Systems at Surrey University, the Terahertz frequency talked about for a potential 6G network starts at 100 GHz – or 0.1 THz – and goes all the way up to 10 THz. The range between 100 GHz and 300 GHz is also known as low Terahertz, while some call it sub-THz.

Nonetheless, Tafazolli believes that rigid definitions would limit the 6G conversation right now. “People talk about potential frequency band for 6G, which has not been defined yet. We are at the very early research stage to see which frequency band is more suitable for short- or long-range communication,” he said.

“So you don’t need to be exact at this stage because everything is for research and investigation until the next couple of years, at least.”

Gerhard Schoenthal, Chief Operating Officer at Virginia Diodes, has a similar understanding. He also sees the Terahertz range going from 100 GHz to 10 THz but adds other reasons why the definition is still open.

“It is looser than some of the more concrete definitions. For instance, a centimetre wave is very specifically a 3 GHz to 30 GHz. Millimetre wave goes from 30 GHz to 300 GHz, and sub-millimetre wave is 300 GHz to 3 THz. That’s because the way these terms have evolved, but also because you focus on those particular wavelengths that you get from the speed of light equation,” Schoenthal said.

The speed of light equation Schoenthal mentions is the math to determine a given frequency. That’s why he says that above 10 THz, we stop using frequency-related terms (hertz, for instance) and shift to the wavelength lexicon.

The Practical Differences

Although definitions may be flexible at the first stages of research, there are practical differences between using 100 GHz or 300 GHz and above for communications.

One of the distinctions lies in energy consumption and infrastructure cost. “What is important is how much power you require to have a certain range. The higher the frequency is, the more the signal gets attenuated, weakened for long-distance using the same amount of power,” Tafazolli explained. “If you want to cover a certain area, that means you need to have more base stations or more masts.”

Another aspect is the state of electronics. Because signal detection works differently in higher frequencies, the equipment needs to keep up with the changes. Which raises a question that remains unanswered: When will this equipment be commercially available?

According to Tafazolli, the industry has the know-how to develop tools for the low-Terahertz band. However, the way to detect signals alters considerably above approximately 200 GHz.

“After we do all the research, tests, measurements, et cetera then we would know more clearly what state of the art in terms of technology is. What technology do we need to develop? What is state of the art in that and how will Terahertz be utilised for 6G?” he predicts.

6G in Terahertz Frequencies

While many researchers believe the next generation of mobile connectivity will push up into the Terahertz band – the Hexa-X vision published in March 2021 is a recent example – there’s still room for development in this field.

“I think that in ten years Terahertz won’t necessarily be used for communications, but there’s a good opportunity for it to happen, especially in fixed point-to-point [communications],” said Schoenthal.

“I also think that there is a very good likelihood that we will see frequencies above 95 GHz used in automotive radar and other radar-type applications for commercial and consumer interests.”

 Tafazolli thinks that it’s unlikely that all use cases will depend upon a single frequency band. Instead, in his view, the industry will set different ranges for different use cases.

“I personally believe that 6G will be a network of networks,” he stated. “It will have to be different frequency bands. I believe that, in the future, we’ll see more short-range communication in that frequency – the components will be smaller, so it probably would be easier for wearables, or you can deploy the network inside buildings, inside the offices.”

At the end of the day, more than the terminology, what matters the most is whether researchers manage to understand and take advantage of the Terahertz band or not in this next decade. “Some people choose to start Terahertz at 100 GHz. That’s fine by me. If others choose to start at 300 GHz, that’s fine with me too,” Schoenthal concludes.

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