Terahertz waves are positioned within the classification of electromagnetic waves* between microwaves, milliwaves, and other "radio waves" and visible light and other forms of "light" (Figure 1). This generally refers to the frequency domain from 100GHz to 10THz (terahertz) and the wavelength domain from 3mm to around 30μm, or in other words, the domain where radio waves overlap with light.
As radio waves that are close to light, terahertz waves have the properties of penetrating and being absorbed by substances. It is said that terahertz waves can be applied to sensing and imaging in a wide range of fields from the compositional analysis of substances to planetary exploration by unmanned spacecraft according to the light absorption patterns measured using these properties. Moreover, because terahertz waves have less energy than visible light, there is no risk of radiation exposure as with X-rays, and they do not require management qualifications, which is a key feature. Because they have no negative effects on the human body, terahertz waves can also be utilized for security checks such as verifying that users are not carrying any weapons or suspicious objects at airport gates or other public places, for example.
*Electromagnetic waves: one form of energy similar to motion and heat. Refers to waves that propagate while electric fields (where electrical forces act) and magnetic fields (where magnetic forces act) change.
Regarding the terahertz waves used in Beyond 5G/6G, the Federal Communications Commission, which manages communication radio waves in the US, has opened up a wide band from 95GHz to 3THz for research and experimental use. However, it is presumed that space development and other special applications are being considered as with the 94-GHz-milliwave frequency.
In Japan, research and studies are being advanced within the band used by the Beyond 5G experimental test station in the band called "sub-terahertz waves" from 90GHz to 300GHz that can primarily use a wide bandwidth that is more than 10 times that of 5G milliwaves, and the practical application of terahertz waves from 100GHz to 1THz is expected as the general-purpose band for Beyond 5G/6G, which is anticipated to be realized in the 2030s. Because compatibility through device specifications and standards is deeply involved with new radio wave standards in the global supply chain, concerns about the development of international standards are increasing with the research and development of new devices.
In 5G, Sub6 (3.7GHz/4.5GHz) radio waves with higher communication speed compared to 4G and that reach a wide area with few base stations, and milliwaves (28GHz/39GHz) with a narrow communication area with a communication speed that is about 16 times faster are used. In Beyond 5G/6G as well, efficient use of radio waves will be realized by using different bands according to the application.
As mentioned above, terahertz waves are utilized in equipment for detecting and analyzing substances, but "remote sensing" technologies are also being researched to use terahertz-wave communication networks for the detection of objects and people, or to put it another way, utilizing the network itself as a sensor.
Currently, infrared and LiDAR are used in automobiles as remote sensing technologies that use the reflection of light and radio waves. On the other hand, it is expected that terahertz waves, which are radio waves close to light, will have both broader sensing and high-speed and high-capacity data communication functions. For example, there are concepts for utilizing a wireless network composed of multiple base stations as if it were a transmission-type sensor or an image sensor through imaging with spatial resolution through the penetration and absorption properties of terahertz waves.
If remote sensing using a wireless terahertz-wave network composed of terrestrial base stations can be realized, then detailed information regarding the weather, traffic volume, obstacles on the road, and the flow of people can be obtained in a faster manner (Figure 2). Moreover, AI will forecast traffic jams, crowds, etc. in greater detail based on big data obtained from the sensors of devices communicating using remote sensing to realize functions for selecting optimal routes. In addition, because such sensors can also detect the intrusion of a person into a site, they can be used as sensors for crime prevention, among various other applications.
Furthermore, while the property of terahertz waves being easily absorbed by water sometimes becomes an issue, it is also said that this property can be used to search for trace amounts of moisture on celestial bodies. Because precipitation can be sensed through the propagation attenuation of radio waves through moisture, local rainfall and other weather information can be obtained with lower latency and higher precision. In this way, the combination of communication and sensing using a terahertz-wave network is likely to become a major step in the realization of an ultra-smart society.
There is no shortage of technological issues in the practical application of terahertz-wave wireless networks. As stated above, terahertz waves, which are close to light, have a higher radio wave straightness than milliwaves, and possess the property of being easily affected by moisture (rain and atmospheric moisture) and obstacles (buildings, trees, people, etc.). The ability to detect the amount of rainfall, objects, the presence of people, etc. from the resulting radio wave propagation attenuation and those characteristics is an advantage. However, on the other hand, such properties of terahertz waves become a major issue with radio wave propagation in wireless networks. The reason is that in using terahertz waves as radio waves for wireless communication, the loss, absorption, blockage, transmission loss, and diffused reflections due to moisture and obstacles have the disadvantage of propagating radio waves far away in terms of the objective.
Moreover, transmission and reception of Beyond 5G/6G requires communication control technologies and algorithms, device management, and AI and other system and software-side related technologies that can derive valuable information from sensing data and optimize control. At the same time, antennas, filters, amplifiers, mixers, local oscillators, etc. that support high-frequency and broadband radio waves can be cited as devices required for base stations and terminals. Developing these devices requires clearing issues due to the propagation characteristics of sub-terahertz and terahertz waves as well as miniaturization, low power consumption, heat dissipation, and stability.
Recent research and future technology innovation in Beyond 5G/6G will provide a new UX (user experience) in various industries in the 2030s while also showing significant promise as one solution to various problems that are anticipated in the future and faced by many countries, such as the working population problem, changes in the natural environment, natural disasters, etc.
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