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Accelerating the utilization of millimeter waves:
NEC's distributed MIMO system

Featured Technologies

April 21, 2023

Expectations are high for "Beyond 5G/6G" as the next-generation communication protocol to support metaverse and digital twins. Millimeter waves are used with 5G and are expected to be used in full-scale for 6G. However, they are vulnerable to blockages and prone to interruptions in communication, which constitute the reasons for why immediate adoption is discouraged. NEC conducted collaborative demonstration experiments*1 with Nippon Telegraph and Telephone Corporation ("NTT") and NTT DOCOMO, INC. ("DOCOMO") up to October 2022, and succeeded for the first time in the world*2 to demonstrate a technology that predicts the statuses of mobile terminals and dynamically switches distributed antennas at base stations to transmit. We interviewed the researchers about this technology that greatly improved the chances of millimeter waves being utilized in future communication technologies.

Overcoming the challenges of millimeter waves for 6G

Advanced Network Research Laboratories
Research Manager
Kazushi Muraoka

― What kind of technology is this one, which overcomes the challenges for 6G?

Muraoka: This technology compensates for the shortcomings of millimeter waves which will be used for 6G. Millimeter waves are high-frequency radio waves that enable high-speed, large-capacity communications required for metaverse and digital twins. They are expected to be effective in realizing an innovative future with transmission of the five senses and future forecasting with digital twins.

However, on the other hand, millimeter waves have a feature of strong straightness, which is a disadvantage when the radio waves need to reach behind buildings and other obstacles. This would cause sudden interruptions in communications with humans and vehicles once they move into areas where radio waves do not propagate. This problem has been a hindrance in applying millimeter waves.

To address this issue, NEC has been developing a technology called "distributed MIMO" with NTT and DOCOMO. Distributed MIMO is a technology where a number of antennas are installed in the service area of a base station for wireless communication with mobile terminals. Covering the service area with a larger number of antennas eliminates radio wave blind spots, thus minimizing the impact of blockages.

Takeuchi: Then, we encountered another problem, which happens in the switching of antennas used for communication. For example, if a mobile terminal is communicating with antenna A while on the move and its radio waves get blocked, it then needs to switch over to antenna B, which also covers the same area. Normally, radio waves have some reach even with some obstacles, which allows for controllability and continued transmission even after blockage. However, this is not the case with millimeter waves, which have rapid fluctuations in radio waves. There is a risk of interruption in communication even if it is momentary. What solved this problem is the newly developed antenna coordination technology based on terminal mobility prediction.

Integrating NEC's wireless communication know-how and AI technologies

Advanced Network Research Laboratories
Lead Research Engineer
Toshiki Takeuchi

― What kind of technology is the antenna coordination technology based on terminal mobility prediction?

Takeuchi: This is a technology where the AI in the base station predicts the future location of the mobile terminal from its current location, selects the optimal antenna, and automatically switch to another antenna to transmit. Instead of switching antennas after the radio waves have been blocked, antennas are switched before blockage to prevent interruption of transmission. The effectiveness of this technology was verified by the collaborative demonstration experiments conducted with NTT and DOCOMO up to October 2022. This enables stable communication using millimeter waves, which we believe would contribute to widespread adoption of the 28-GHz millimeter wave communication used in 5G and the millimeter waves expected to be used for 6G.

NEC is a company that has been working on the wireless system business for many years. In the course of such business efforts, we have continued to accumulate wireless signal processing, implementation technology, and know-how of system integration. Furthermore, NEC now possesses numerous world-class core technologies in the area of AI. This technology was an approach achieved by such NEC with its strengths in communication and AI.

The business unit's early development of a demonstration experiment system for distributed MIMO also worked as a great advantage in the research of this technology.

Maruta: The business unit has been focusing on the Massive MIMO technology since 2014, with a large number of antennas installed at base stations, and working on R&D and commercializing it in collaboration with the research laboratories. In 2019, to apply this know-how to millimeter waves, the business unit developed the demonstration experiment system in order to develop distributed MIMO, which had been garnering attention in academic circles. This time, we applied the research laboratories' antenna coordination technology based on terminal mobility prediction to this system for demonstration experiments. The collaboration with NTT and DOCOMO, through which we shared the development of element technologies, was also a key factor in accelerating research.

Muraoka: Distributed MIMO is realized as a total system. It functions on many element technologies, with the antenna coordination based terminal mobility prediction being one of them. One example of other technologies used is the technology for avoiding interference, another one of NEC's developments.

Shikida: Because distributed MIMO requires a large number of antennas to be installed in a service area, it comes with the shortcoming of being prone to mutual interference. Therefore, we developed a technology that avoids interference through inter-antenna coordination. Using the measurement data of radio wave reception quality of each antenna, the system determines which combination of antennas is least likely to result in interference. While technology for working around interference has been researched from a number of different approaches, our technology envisions how distributed MIMO will be applied and is designed to function with high accuracy even in the presence of obstacles. This technology has been adopted in and presented at the 2022 IEEE 96th Vehicular Technology Conference (VTC2022-Fall), a top international conference in the field of mobile communications.

Advanced Network Research Laboratories
Senior Researcher
Jun Shikida

Aiming for an early application, starting from the 28-GHz band to 5G

Wireless Access Development Department
Senior B5G/6G Engineer
Yasushi Maruta

― What future plans are you envisioning?

Muraoka: First of all, we would like to gear this distributed MIMO system toward more practical use. The ideal is to enable stress-free, standard use of the large-capacity communication of millimeter waves. To this end, we will deepen collaborations with other in-house teams, as well as external partners, and further promote R&D.

Maruta: While this system was developed with 6G in mind, millimeter waves are already starting to be used for 5G. While still few in number, 5G stations that use 28-GHz millimeter waves are being set up in high-traffic areas. It is also probable that the 28-GHz band will be used for local 5G. As a vehicle to accelerate the use of millimeter waves, we are currently working on R&D with an eye to deploy this technology in the run-up to 6G.

Distributed MIMO is a technology where a number of antennas are installed in the service area of a base station for MIMO transmission with mobile terminals. It has been spotlighted as an effective method to compensate for the nature of millimeter waves that they are vulnerable to obstacles despite allowing for high-speed, large-capacity communications. Nevertheless, it had the issue of interruption in communication when blockage occurred between the antenna and the mobile terminal. The straightness of millimeter radio waves is so strong that they cannot propagate behind obstacles, which results in a broken link with the terminal at that point. Distributed MIMO is a system that connects the terminal to other distributed antennas when communication with the connected antenna is broken. However, because millimeter waves have rapid radio-wave fluctuations, switching to another antenna after the blockage is too late, resulting in temporary disconnection. In contrast, NEC's newly developed technology adopts an AI in the base station to predict the near-future travel location of the connected terminal and assigns an optimal antenna. NEC became the first in the world to succeed in the demonstration experiment of this technology. It is expected to contribute to the widespread application of millimeter waves in the 28-GHz band for 5G and millimeter waves for 6G.

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