Key Technology In 6G: Terahertz

Terahertz is known as one of the top ten technologies that will change the future. It is a hot research technology in the world today and has significant performance advantages in space science, passive remote sensing, security detection, biomedicine and astronomical observation.

Terahertz waves can be seen everywhere in nature, and the thermal radiation of most objects around us is terahertz waves. It is an electromagnetic wave located in the transition region between microwaves and infrared short waves. In the field of electronics, this electromagnetic wave is called millimeter waves and submillimeter waves. In the field of optics, it is also called far-infrared rays. Generally, we call the frequency range 0.1-10THz and the electromagnetic wave with a wavelength of 3mm-30um as terahertz wave. In some scenarios, it specifically refers to 0.3-3THz. In other cases, the generalized terahertz wave can be defined to 100THz.

In the early days, due to the lack of high-efficiency emission sources and sensitive detection instruments, this section of electromagnetic waves did not attract much attention and no in-depth research was conducted. It is also called the terahertz gap (THz Gap) in the electromagnetic spectrum. With the development of optoelectronic technology and semiconductor technology, there has been more in-depth research on terahertz technology. Because of its unique position in the electromagnetic spectrum, researchers generally conduct research from two directions when studying terahertz technology: 1. From the electromagnetic or electronics field in the long-wave direction ; 2. the optical field in the short-wave direction. The physical properties of these two directions are also significantly different.

On the one hand, the emission of low-frequency electromagnetic waves originates from the macroscopic movement of charges. As the electromagnetic frequency becomes larger and larger, the influence of some low-frequency negligible effects will become more and more obvious, such as the stray capacitance effect. Therefore, some classic electromagnetic wave sources and electronic components cannot be adapted to the terahertz band, and faster and smaller electronic components must be developed to meet the needs of this band. Light waves originate from the quantum transition of electrons. The difference between the two energy levels of terahertz radiation photons is only millielectronvolts. This energy level difference is even smaller than the energy of most crystal optical phonons. Therefore, it will be seriously affected by the thermal relaxation effect. To obtain a high-efficiency light source in this band, the influence of the thermal relaxation effect must be effectively avoided.

On the other hand, for low-frequency electromagnetic waves, since the wavelength of the electromagnetic wave is generally larger than the size of the components, when dealing with the interaction between the electromagnetic field and the components, the electromagnetic field can be regarded as uniformly distributed. On the contrary, the size of optical components is much larger than the wavelength of light. Therefore, when studying optical problems, we can no longer think that the light field is uniformly distributed, but that the medium is uniform within the optical wavelength range.

This means that for terahertz, we cannot just approximate research through electromagnetics and optics. We need to develop new technologies based on different situations and combine the characteristics of both directions. This is why there has been a terahertz gap in the electromagnetic spectrum for a long time.

We know that millimeter waves with larger bandwidth will be introduced in 5G.
Compared with millimeter wave, the terahertz spectrum is richer. Terahertz has wider bandwidth resources, high information dissemination capacity and strong transmission capability. In theory, it can achieve high-speed data transmission of more than 100Gbit/s.
Its beam is narrower, its directivity is better, its anti-interference ability is strong, its wavelength is shorter, and it is easy to achieve miniaturization. Although terahertz waves also have certain optical properties, unlike optical communications, they are not affected by environmental noise generated by ambient light sources, and they also have good performance under adverse climate conditions such as fog and dust. In comparison, terahertz has the characteristics of large transmission capacity, high security, and good penetration. It has significant advantages in the field of wireless communications and is widely considered by the industry to be a candidate technology for 6G.

At the World Radiocommunication Conference (WRC-19) in 2019, passed the 275 GHz-296 GHz, 306 GHz-313 GHz, 318 GHz-333 GHz and 356 GHz-450 GHz frequency bands, a total of 137 GHz bandwidth resources can be used for fixed and land mobile business applications without restrictions. This conference also clarified for the first time the spectrum resources available for terrestrial active radio service applications in the terahertz frequency band above 275 GHz, and raised the upper limit of available spectrum resources for active services to 450 GHz.

6G communication has the vision of intelligent connection of all things and digital twins. Terahertz technology can be used as a very important interface technology solution, which can apply terahertz to a variety of communication application scenarios.

In terrestrial communication applications, terahertz technology can realize ultra-high-speed wireless mobile scenarios, such as holographic communication, integrated communication and perception, virtual reality and augmented reality, metaverse and other large data transmission applications. 5G uses millimeter wave technology to introduce fixed wireless access scenarios. This scenario is also applicable to terahertz technology, which can be used as a supplement to millimeter waves. In addition, the high-speed transmission of terahertz also provides conditions for high-speed wireless backhaul technology.
Wireless backhaul can effectively reduce the cost of fiber deployment and make base station deployment more flexible, especially in environments with complex natural conditions such as rivers, deserts, and mountains. At the same time, wireless data centers can also be built using terahertz technology, which will greatly reduce deployment space and deployment costs.

In addition to ground communications, terahertz waves can also be used to achieve integrated air, space, sea and ground communications. Terahertz communication equipment can be built and operated on space-based platforms such as satellites, drones, and airships. This is due to the nearly lossless propagation of terahertz waves in outer space, allowing signals to travel long distances. This will achieve multi-in-one signal coverage in space, sky, ocean, and remote ground.

With the continued breakthroughs in terahertz technology in the future, terahertz technology can be used to miniaturize communication systems, and devices can be made into micron or even micronano sizes. With the development of production processes, wireless communication among nodes on the chip can be achieved. In addition, with the development of new materials and technologies, it has become possible to construct a local area network in the human body.

In the context of today's rapid development of communication technology, the importance of terahertz technology is self-evident. Mastering terahertz technology will take the initiative and avoid various passive situations caused by information searches around the world.

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Key Technology In 6G: Terahertz