Successful Construction of Outdoor Areas Using "Terahertz" Frequency for the 6G Era ~ Realizing Areas for Future Vehicle Communications ~ | SoftBank Research Institute of Advanced Technology | SoftBank | About Us | SoftBank (2024)

１．Terahertz: The New Frequency in the 6G Era

Today, communication devices are not limited to smartphones and IoT devices; robots and vehicles also rely on communication technologies. We anticipate that this demand will continue to grow, necessitating even higher communication capacity and speed. In light of this, we have constructed terahertz communication areas designed for future connected cars, envisioning a use case where these vehicles utilize terahertz communication while in motion.

2. Designing of Terahertz Areas for Vehicles in the 6G Era

In the future, as autonomous and connected vehicles become more prevalent, there will likely be a need for high-speed, high-capacity communication systems for the vehicles themselves. Current wireless networks are expected to face congestion issues as they may struggle to handle the vast amounts of data generated by these vehicles. For instance, vehicles will need to upload large volumes of data to the network or download high-resolution maps specific to different regions. By utilizing terahertz communication, we can significantly increase network capacity, meeting these demands effectively.
Additionally, while the high propagation loss of terahertz waves presents a challenge, limiting the coverage to straight roads can minimize signal dispersion, allowing for relatively larger coverage areas. This approach offers a practical solution to harness the benefits of terahertz communication for connected vehicles.

3. Technical Challenges for the Practical Use of Terahertz Communication

Traditionally, omnidirectional antennas have been used in devices like smartphones. However, with the high propagation loss of terahertz frequencies, using omnidirectional antennas results in excessive signal spread, making it difficult to maintain the necessary power for communication. To address this, beamforming technology, which directs a concentrated beam of radio waves towards the device and tracks its movement to maintain communication, has typically been employed. However, as the frequency bandwidth increases, the beam tracking calculation becomes significant, posing a challenge. To design the coverage without the beam control, we developed an antenna that minimizes power dispersion while covering as wide an area as possible.

4. Development of Cosecant Beam Antenna

To address the challenges mentioned in the previous section, we explored designing the areas with "cosecant-squared characteristics." This technology, used in aviation radar, ensures that the received power at both the base station and the terminal remains constant regardless of the horizontal distance between the transmitting and receiving antennas, even with elevation differences. Specifically, by making the product of the antenna gains of the base station and the terminal proportional to the cosecant squared of the angle θ formed by the antennas, we can counteract the attenuation due to distance.
Traditionally, this characteristic could be achieved simply by making the antenna directional pattern a cosecant-squared beam. However, due to the short wavelength of the terahertz band, it is difficult to realize omnidirectional antennas. Therefore, we designed areas with cosecant-squared characteristics by configuring both the base station and terminal antennas with cosecant beam patterns.
As a result, we were able to maintain an antenna gain of approximately 20 dBi for both the base station and terminal, while achieving an almost constant received power from near the base station to the edge of the coverage area.

Figure 5 shows the cosecant beam antenna (hereafter referred to as the "cosecant antenna") that we developed. Given the very short wavelength of 300GHz, approximately 1mm, we were able to significantly reduce the antenna size, successfully developing a palm-sized version.

5. Outdoor Demonstration Experiments Towards the Realization of 6G

To confirm the feasibility of forming a 300GHz wireless area using a cosecant antenna, we conducted an outdoor demonstration experiment near SoftBank's headquarters in Minato-ku, Tokyo. In this experiment, we converted 5G NR signals to 300GHz to enable area measurements using equipment designed for 5G, and made the area at 300GHz.
We confirmed the area coverage by converting terahertz signals to 5G frequencies using equipment mounted on a measurement vehicle and demodulating the broadcast information transmitted from the 5G base station. The transmitting antenna on the base station side was installed on a pedestrian deck approximately 10 meters above the ground, while the receiving antenna on the terminal side was mounted on top of the measurement vehicle. The area measurements were conducted while driving the vehicle through the designated area.

As a result, we successfully demodulated the broadcast signal within an area ranging from 10 to 140 meters horizontally from the base station. Although the measurements in this experiment were limited to 140 meters due to road constraints, the received power levels indicate that there is sufficient margin to extend the communication range even further. This suggests the potential for achieving longer-distance area coverage.

6. Conclusion

In this measurement, we proposed a new use case for terahertz communication, which has traditionally been considered for fixed wireless access and short-range communication applications. Specifically, we introduced the use case of high-speed communication for fixed and short-range communication applications.
Moving forward, we will continue to validate various use cases for 6G technology and gather valuable insights through our technical research. By doing so, we aim to contribute to the realization of a future digital society.

References

[1] Beyond 5G／6Gに向けて、テラヘルツ波を活用した屋外での通信エリア構築の検証に成功
[2] Our Initiatives to Utilize the Terahertz Band in Mobile Communications