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- Sep 19, 2024
- Blog
- Wireless
Terahertz series: Part 1 What is terahertz?
#6G, #FSOC/Terahertz
In the field of mobile telecommunications, the first generation (1G) mobile communication system was introduced in the 1980s. Since then, generations have evolved approximately every 10 years.These generational shifts in mobile communication are driven by advancements in the underlying technologies. The first generation (1G) relied on FDMA (Frequency Division Multiple Access), the second generation (2G) adopted TDMA (Time Division Multiple Access), and the third generation (3G) used CDMA (Code Division Multiple Access).
As of 2024, in Japan, the mainstream services are the fourth generation, LTE, and the fifth generation, 5G. Both generations employ OFDMA (Orthogonal Frequency Division Multiple Access) technology, and have also introduced MIMO (Multi-Input Multi-Output) systems, significantly improving frequency efficiency with each iteration. Each new generation has also brought with it new frequency allocations: for example, 3G used the 2 GHz band, 4G expanded into the 2.5 GHz band (dedicated to BWA in Japan) and the 3.5 GHz band, while 5G utilizes the 4.5 GHz band and millimeter waves.
Looking ahead, around 2030—approximately a decade after the arrival of 5G—the sixth generation (6G) of mobile communication is expected to become a reality. Global research and standardization efforts are already underway. This article, we will provide a detailed explanation of terahertz communication, which is expected to be put into practical use in the 6G era, in a three-part series. In this article, we will explain the theme of "What is terahertz?"
1. What is terahertz communication?
”The term "terahertz" combines the prefix "tera," which follows kilo, mega, and giga, with the unit "hertz," which measures frequency. Technically, 1 THz equals 1000 GHz. However, in the telecommunications industry, frequencies from 100 GHz to 10 THz are sometimes referred to as the terahertz band.
Frequencies below 1 THz, while not strictly terahertz, are often described as "sub-terahertz."
Millimeter waves, which are also used in 5G, refer to frequencies with wavelengths between 1 mm and 10 mm. Specifically, frequencies from 30 GHz to 300 GHz fall under the category of millimeter waves. On the other hand, frequencies with wavelengths between 0.1 mm and 1 mm are known as sub-millimeter waves.
As a result, a frequency like 100 GHz can have multiple names depending on the context, including "terahertz wave," "sub-terahertz wave," and "millimeter wave." The terminology varies across industries and academic fields, so it is important to be mindful of these differences. In this article, we will refer to frequencies above 100 GHz as "terahertz waves."
The internationally defined upper limit for radio waves is 3 THz. Beyond this point lies the realm of light. In the light spectrum, frequency is often expressed in terms of wavelength. When converted to frequency, visible light ranges from approximately 405 THz to 790 THz. The range of infrared radiation, which has longer wavelengths (lower frequencies) than visible light, is classified as follows: near-infrared (384 THz - 100 THz), mid-infrared (50 THz - 100 THz), far-infrared (20 THz - 50 THz), and extreme infrared (0.3 THz - 20 THz).
Terahertz waves occupy a unique space, acting as both radio waves and a type of infrared radiation, which gives them properties of both radio waves and light. Due to their extremely high frequencies, terahertz waves have not been used in communications up until now. However, the next section will explore what might be possible when these waves are actually applied to telecommunications.
図1. Terahertz Band Definition
2. How terahertz Has Been Used So Far
Terahertz waves have existed in nature for a long time, but it is only recently that humanity has been able to actively generate and use them for communication. Before that, they were only received passively. In radio astronomy, for example, cooled superconducting receivers and large parabolic antennas are used to observe terahertz waves, helping researchers study the distribution of dust in primitive galaxies, the formation of planetary systems, and the evolution of the universe through molecular line observations. In Earth observation and remote sensing, terahertz waves are capable of simultaneously detecting molecules such as water, oxygen, and carbon dioxide. This precise observation of greenhouse gases using terahertz technology contributes to tackling urgent issues like global warming.
Terahertz waves are also being applied in tomography, where their penetrative properties and spectral techniques are used to analyze thin layers in paintings. This is made possible by the wideband terahertz waves generated from light pulses. In addition, terahertz imaging technology, such as terahertz cameras, can detect hidden metal objects beneath clothing by receiving the natural terahertz waves emitted by humans, contributing to advancements in security screening.
While terahertz waves have been utilized in various fields, recent technological advancements have reached the stage where frequencies are being precisely allocated for communication purposes.
Research is now underway to explore how these terahertz waves can be used for data transmission.
Credit: ALMA (ESO/NAOJ/NRAO)
（Source (Japanese only): Document 3 "State Survey of Plaster using Terahertz Spectroscopy and Imaging" from the 2nd Meeting of the Review Committee on Preservation and Utilization of Kofun (Ancient Burial Mound) Mural Paintings）
3. Applying terahertz to Telecommunications
As of 2024, the fifth generation of mobile communications (5G) is becoming widely adopted. In Japan, 5G coverage is expanding nationwide, and as the use of millimeter waves (introduced in 5G) increases, it is expected that ultra-fast communication will become even more commonplace.
However, mobile data traffic is growing beyond expectations, and by the time the sixth generation of mobile communication (6G) is introduced, even with millimeter waves, there is a risk of network congestion. As a result, future generations of mobile communication beyond 5G will require even broader frequency bands. This is why the terahertz band is gaining attention as a promising candidate for new frequencies in 6G.
The International Telecommunication Union Radiocommunication Sector (ITU-R), which oversees global radio spectrum management, holds the World Radiocommunication Conference (WRC) every four years. At WRC-19, held in 2019, discussions took place regarding the use of frequencies above 275 GHz. As a result, a total of 137 GHz of spectrum was made available for communication services across different countries.
To date, the total frequency allocated to the four mobile network operators (as of 2024) is approximately 3 GHz. When combined with the 97.5 GHz of frequencies identified before WRC-19, a total of 234 GHz of spectrum could potentially be used for communication in the future.
The availability of such a broad spectrum for communications would greatly alleviate concerns about network congestion and could also lead to the invention of new mobile applications.
In the next article, we will focus on the use of terahertz waves in communication, exploring the history of research and development in this area, as well as the features and challenges of using terahertz for communication.
Sep 19, 2024
Blog
New Ways to Use Radio Waves in the 6G Era - Integrated Sensing and Communication -
#6G, #FSOC/Terahertz