Engineers at university and industry labs perform research into frequencies that could become part of 6G. Now it’s time for use cases and standards.

Conferences covering the technologies that could become part of 6G continue to focus on THz frequencies. Research at universities around the world has demonstrated that wireless data transfer is possible and at data rates higher than those we’ve seen from 5G’s mmWave signals.

Realizing the potential for THz signals, ETSI announced the formation of a working group to study use cases. The ISG THz group will at first focus on two categories of use cases. The first will include mobile applications with high data rate requirements such as virtual and augmented reality, applications for in-flight and in-train entertainment, and vehicular and satellite communications. The second category will cover applications that require both communication and sensing functionalities, such as holographic telepresence, and interactive and cooperative robotics.

EE World spoke with ISG THz chair Thomas Kürner, professor at Technische Universität Braunschweig, who explained the functions and goals of the group.

EE World: Why start the THz group now?
Kürner: Therahertz communication is well into research for the upcoming 6G. That brings the frequency range into the focus of 3GPP. The time is right for ETSI to start modeling activities. That gives us 2 to 2½ years to prepare.

EE World: One of the group’s goals is to define the frequency range for 6G. Current research is focusing on frequencies between 100 GHz and 300 GHz. Is that the frequency range of interest to the group? Should we call it “sub-THz” as many do?
Kürner: There are many definitions for THz. When we started research here at Technische Universität Braunschweig almost 20 years ago. We started with research around 300 GHz, which I consider that start of the THz band. Some people consider the band to start at 1 THz, but that is not mature just now.

Research projects have operated between 200 GHz and 300 GHz and today there is research at around 150 GHz. There are many measurements at such frequencies, and many people consider that to be THz research. We should refer to 0.1 THz to 1 THz as “sub-THz.” Some papers refer to work at 60 GHz as being 1 THz. I don’t agree with that.

EE World: The ETSI press release states, “define the target scenarios and the concrete frequency bands of interest on THz communications. Of major interest to the members of ISG THz will be the analysis of specific radio propagation aspects for THz communications, such as molecular absorption; effect of micro-mobility; specific considerations for scattering, reflections, and diffractions; and considerations for near-field propagation.” What do you mean by “micromobility?”
Kürner: Because of the path loss at these frequencies, you need “pencil beam” antennas. Transmit and receive antennas need to be perfectly aligned. If you have antennas with just a 1° or 2° half-power angle, even small users of mobile devices can cause antenna misalignment. We conducted some experiments asking a use to move around so we could measure impact of those movements.

EE World: Does that mean the locations of the antennas have to be stable?
Kürner: I wouldn’t phrase it that way, but I will say that the applications are mostly for fixed access such as in a data center. For those applications, the technology is mature. Today’s research is focused on bringing mobility into the game. Antennas will have to search for each other’s beams. That’s already an issue at mmWave frequencies but is more of an issue at higher frequencies. I can’t predict when this will be solved. We must investigate this issue if we’ve going to work with 3GPP.

EE World: That will involve using phased arrays and beam steering, correct?
Kürner: Yes, that’s correct.

EE World: In November 2021, we published a video interview covering reconfigurable intelligent surfaces. Is work still going on in that field?
Kürner: Yes, work is ongoing. ETSI has another working group looking into that. It’s a hot topic for the THz frequency range because at those frequencies, you can’t transport energy through diffraction. As soon as you lose line-if sight, the only choice is to use reflection or scattering. In that context, reflecting surfaces are quite interesting.

EE World: How are researchers performing channel sounding in THz frequencies?
Kürner: To characterize channels at these frequencies, we use spectroscopy, the THz time domain. We look at reflections whereby you send a short laser pulse and the by Fourier Transform, we can get reflection properties. That’s the first thing we do. To characterize a complete channel, we use vector-network analyzers for stable environments. For dynamic environments, we use time-domain channel sounding where you send a pseudorandom bit sequence (PRBS). Universities have equipment that can operate at 300 GHz.

EE World: Research at NYU has shown that the attenuation rate of increase for rain gets smaller at about 100 GHz. Is work continuing on that subject?
Kürner: Work is continuing. There are also peaks in the curve at certain frequencies. These are resonance frequencies from molecules. Researchers at University of Oulu and University of Oklahoma have done measurements.

EE World: The group’s goal is to have recommendations ready 2025 based on 3GPP. Is that correct?
Kürner: Whenever we develop a new wireless standard, work on channel models stops at the time that standards begin to develop. There’s not enough time to properly develop these channel models. Because of that time pressure, you have to make compromises. We have about 2 to 2½ years to finish our work. Our goal is to have proper models in place when they will be needed.