Terahertz Series: Part 3 - SoftBank Pioneering Terahertz Communication Technology - Challenge for 6G Mobile Communications

1. Challenges of 6G Terahertz Communication

The world's communication infrastructure is rapidly evolving, and even more advanced communication technologies are expected to be necessary in the 6G era. As societal infrastructures change, such as with the realization of smart cities and the widespread adoption of autonomous vehicles, there will be a demand for improvements in data transmission speeds, ensuring connection reliability, and providing uninterrupted services even in high-density urban environments or during high-speed mobility.

On the other hand, the volume of communication traffic, mainly driven by smartphones, is increasing every year. It is foreseen that existing 4G and 5G technologies will eventually be insufficient to meet future communication demands. The evolution from 4G to 5G significantly improved communication speeds and the number of simultaneous connections. In addition to these advancements, research is being conducted in 6G to enhance frequency utilization efficiency to further improve communication speeds, as well as to utilize even higher frequency bands. One such focus is the terahertz band (0.1 to 10 THz). Terahertz communication possesses an extremely wide bandwidth, enabling faster data transmissions.

For a deeper dive into terahertz technology, please refer to our past blog posts.

Sep 19, 2024

Blog

Terahertz series: Part 1 What is terahertz?

#6G #FSOC/Terahertz #Terahertz #Frequency

Dec 06, 2024

Blog

Terahertz Series: Part 2 Communication Standards and Potential of Terahertz Technology

#6G #FSOC/Terahertz #Terahertz

Challenges of Terahertz Communication

While terahertz (THz) communication offers numerous advantages, it also presents several technical challenges that must be addressed for practical implementation. The key challenges in technology development include the following.

The problem of Atmospheric Attenuation:
As illustrated in Figure 1, terahertz waves are more readily absorbed by atmospheric water vapor, oxygen molecules, and other particles compared to the radio waves traditionally used in communication. This means that the attenuation of the signal can vary significantly within the terahertz band, and this characteristic must be carefully considered. The communication distance may be limited to relatively short ranges, thus necessitating technical measures such as increasing the antenna gain to mitigate this issue.

Line-of-Sight and Obstructions:
Terahertz waves have a high degree of straight-line propagation, which means they are easily obstructed by obstacles, leading to interruptions in communication. This characteristic limits the applicable scenarios for terahertz communication. Therefore, it is crucial to effectively utilize terahertz waves in appropriate and suitable environments.

Implementation of Communication Devices :
The wavelength of terahertz waves is very short, approximately 1mm, roughly 1/100th the wavelength of those used in current mobile communications. While this allows for the miniaturization of components such as antennas to 1/100th of their conventional size, it also presents challenges in manufacturing and implementation, requiring highly precise fabrication technologies. In semiconductor technology, ongoing R&D focuses on developing high-power and high-sensitivity devices for receiving signals, both of which are essential for the practical application of terahertz communication.

At SoftBank, we are actively conducting research and development to overcome these technical challenges and bring terahertz communication to practical implementation.

Figure 1. Absorption rate of Terahertz Band（Data: NICT https://smiles-p6.nict.go.jp/thz/jp/decay.html）

2. SoftBank's Initiatives to Date

SoftBank has been actively conducting research and development with the aim of utilizing terahertz waves for 6G mobile communication. This initiative began in 2018 with a joint research project with the National Institute of Information and Communications Technology (NICT).

From the beginning, the goal was to apply terahertz communication technology to smartphones. In 2020, SoftBank, in collaboration with Gifu University and NICT, successfully developed an ultra-compact dielectric antenna (DCA: Dielectric Cuboid Antenna) specifically designed for smartphones. Figure 2 shows the actual DCA. This antenna supports a wide band ranging from 220 GHz to 330 GHz and is characterized by its high gain per unit area compared to other terahertz antennas. In 2021, laboratory tests using this DCA successfully transmitted data at speeds of 17 Gbps.

While it may still be some time before smartphone manufacturers can integrate this DCA into their devices, we have gone beyond merely evaluating its technical performance and have envisioned numerous potential use cases for terahertz technology in smartphones.

Figure 2. Ultra-Small Antenna announced in 2020（DCA: Dielectric Cubic Antenna）

Figure 3. Data Communication test with DCA in 2021

In 2022, SoftBank obtained a trial license for outdoor field measurements and conducted experiments around areas in Tokyo.

3. Outdoor demonstration experiment of Terahertz Waves

Figure 4. Outdoor Field Measurement（Left: Measurement Field, Right: Schematic diagram）

SoftBank aims to apply terahertz waves to mobile communication and is conducting not only indoor experiments but also outdoor communication tests to evaluate performance in real-world environments. One of the most critical challenges is verifying the communication coverage area. In an outdoor experiment conducted by SoftBank in 2022, a propagation test was carried out to investigate how far the signal could reach and how much coverage could be secured, assessing the characteristics of terahertz wave transmission.

In this propagation experiment, 300 GHz radio waves were used, and the test was conducted with unmodulated waves to observe distance characteristics. The experiment was set up with a transmitting vehicle and a receiving vehicle, each equipped with terahertz equipment on its roof. The transmitting vehicle remained stationary, while the receiving vehicle gradually moved away to measure distance-dependent characteristics (Figure 4).

Figure 5 shows the results of the experiments conducted in Odaiba, Minato Ward, Tokyo.

Figure 5. Measurement Results（Left: Measurement Route, Right: Chart of Receiving Power）

The experiment was conducted on a clear day, and the results closely matched the free-space pathloss, which did not account for atmospheric moisture absorption. It was previously believed that terahertz waves are heavily absorbed by moisture and do not travel far. However, these results indicate that when used in small cells along roadways, the impact of atmospheric moisture absorption is minimal.

Additionally, when changing lanes to avoid parked vehicles while driving, continuous reception was maintained despite the strong linearity of terahertz waves, with only minor attenuation.

Typically, terahertz communication requires precise alignment between transmitting and receiving antennas, but these results suggest that at this distance, slight misalignment of the antenna axes has only a limited effect.

4. Use case demonstration of Terahertz Communication

Let's consider potential use cases for terahertz communication.

As of 2024, terahertz communication is still in the research phase. Current transmission and reception devices are still large, and power consumption remains unoptimized. It will likely take more time before it can be integrated into smartphones.

Another characteristic of terahertz waves is the high directivity of their antennas. Typically, devices used by end-users employ omnidirectional antennas that can receive signals from 360 degrees. However, in the case of terahertz waves, there is a bias in directivity to increase sensitivity to radio waves coming from specific directions.

To enable terahertz communication on smartphones, it would be necessary to either equip the device with antennas that can receive signals from all directions or require users to point their devices toward the base station, similar to how a remote control works. However, for applications such as cars and trains, where the orientation remains relatively fixed, these adjustments would not be necessary.

At SoftBank, we focused on the characteristics of vehicles—such as their fixed travel direction, the ability to accommodate larger communication modules compared to smartphones, and access to a more substantial power supply. Recognizing these advantages, we conducted demonstration experiments on terahertz communication for vehicles as an initial use case for the technology.

Overview of the Demonstration Experiment

Figure 6. Terahertz communication for vehicle（Left: view from road side, Right: view from transmitter

The demonstration experiment was conducted on a straight road next to SoftBank’s headquarters in Minato Ward, Tokyo.

In this experiment, a system was built to convert 3.9 GHz 5G signals to 300 GHz, enabling 5G-equivalent communication. A transmitting device, acting as a base station, was installed on a pedestrian deck at the height equivalent to the second floor of a building. Measurements were conducted by receiving the base station’s broadcast information using a 5G area measurement device mounted on a vehicle.

By utilizing a commonly used 5G area measurement device, it is possible to simulate and evaluate how extensively the coverage area could be expanded using terahertz technology in the future.

Cosecant Beam Antenna

To provide stable communication to terminals moving at high speeds, high-speed beam tracking capabilities are generally required. However, beamforming devices for terahertz waves have not yet commercially available, and there are concerns that constructing such a system would result in a bulky and costly system. Moreover, for stable communication, it is preferable to minimize fluctuations in the received signal level.

In this demonstration experiment, instead of using beamforming for tracking mobile terminals, we adopted an approach to cover a wide area along the roadway. However, merely expanding the coverage area would cause significant signal fluctuations. Therefore, we developed a cosecant beam antenna that maintains a stable signal level over varying distances, which we deployed in this experiment.

Figure 7. Cosecant Beam Antenna（Left: Actual Antenna, Right: Illustration of Cosecant Beam Coverage）

As a result of this demonstration experiment, we found that it is possible to make a coverage extending up to approximately 150 meters, reaching the end of the road. Discussions about terahertz communication have primarily revolved around ultra-short-range applications, such as contactless payments. However, these results remark a significant step towards real-world deployment in urban areas and on highways.

Moreover, due to the limitations of the current system, only unidirectional communication was implemented. However, if bidirectional communication is realized, it will enable real IP-based data transmission, further expanding the potential applications of terahertz technology.

5. Future Prospects: Practical Implementation of Terahertz Communication

SoftBank is further accelerating its research and development with a view to the practical application of terahertz communication. In the future, terahertz communication is expected to become widely deployed, not only enabling high-speed communication while in motion but also serving as a replacement for optical fiber connections. This would allow a wide range of everyday devices to seamlessly connect wirelessly, facilitating high-speed, high-quality data transmission.

Terahertz communication can also be integrated with emerging technologies such as autonomous vehicles and drones, offering innovative services. For example, in coordination with self-driving cars, it could provide real-time traffic information and rapid emergency response, ulrimately enhancingsafety and improving logistics efficiency.

As introduced in the first and second articles, many challenges remain in realizing terahertz communication, and skepticism exists about its feasibility as a mobile communication technology for 6G. However, based on the findings from past research, SoftBank has focused on mobile usage, and has successfully demonstrated that it is possible to make communication areas in real-world outdoor environments, as demonstrated in our field tests.

Moving forward, SoftBank will continue leveraging its accumulated technology and expertise to drive broader technological advancements and service innovations in the 6G era.