How would sub-Terahertz waves behave in the real world to deliver 6G? That’s the question researchers at Japanese mobile operator NTT tried to answer in an article featured in April’s edition of the International Journal of Microwave and Wireless Technologies, published by the Cambridge University Press.

According to a team of experts from the NTT Access Network Service Systems Laboratories and the company’s 6G-IOWN Promotion Department, several properties of waves above 71 GHz are mostly unknown, especially from 100 GHz and beyond.

The researchers investigated how signals in these frequencies behave in three scenarios: Human blockage, scattering from rough surfaces, and path-loss characteristics in urban environments. The experiments, carried out in Tokyo, had different setups depending on the situation tested.

The first trial had a human placed between two Tx and Rx antennas at several distances. The researchers measured results from ten frequencies between 0.8 GHz and 150 GHz, mixing bandwidths used for 5G and those expected for 6G.

The study found that the human body can block more than 30 decibels at 150 GHz, while blockage loss drops to ten decibels – in the worst-case scenario – at 0.8 GHz. That means that as we go higher in frequency, signal loss due to the presence of people in the environment increases. They also discovered that the blockage area decreases as the frequency increases.

Another finding is that signal scattering also varies according to the frequency used. In a second experiment, the NTT researchers tested how much signal was scattered or reflected in several situations. They threw waves at 2.2 GHz, 26.4 GHz, 66.5 GHz, and 97.5 GHz at a metal wall with 6 mm grooves in it.

The final result: 66.5 GHz and 97.5 GHz bands had more diffused scattering than in lower frequencies.

“Since buildings in an urban environment have various irregularities and rough surfaces, scattering due to them could be more diffused than in this environment and affect the path-loss characteristics,” the paper reads.

One last conclusion from the research involves a global standardisation body. In 2017, the ITU introduced a guideline for evaluating radio interface technologies for 5G called M.2412-0. Among others, the document outlines models to measure radio frequencies in different situations.

When testing urban microcell (UMi) path-loss characteristics, the NTT team found that although these calculation models were valid for Line of Sight (LOS) transmission, there was a significant difference in the Non-Sight of Light (NLOS) environment.

“In addition, the measured power angular profiles indicate that reflection waves from the surrounding buildings and structures are the dominant paths in an NLOS environment, and the path-loss frequency dependence increases as the power of reflection waves decreases,” the experts said in the paper.

New Topology for 6G

The study was conducted based on a new topology for 6G suggested by NTT in a white paper published in 2020. According to the company, transmissions will transition from today’s base stations’ coordinated positions to an overlapping or a moving setup with multiple connection paths.

Featured image by Dick Thomas Johnson