KDDI Successfully Demonstrates 5G Millimeter-Wave Coverage Inside a JR Yamanote Line Train

While 5G millimeter-wave enables high-speed, high-capacity communication utilizing the wide 28GHz band, its strong signal directivity and susceptibility to obstacles pose challenges for area deployment. To address this, KDDI has been expanding millimeter-wave coverage, mainly in outdoor areas with many obstacles, such as urban areas with many buildings, using wireless repeaters [2] that autonomously form and optimize communication coverage areas. In a joint effort with JR East, following the successful demonstration at Shinjuku Station's platform in April 2025 [3], KDDI expanded millimeter-wave coverage around Takanawa Gateway Station in October 2025 and on the platforms of Tokyo Station's Shinkansen lines in March 2026. Even as millimeter-wave coverage in outdoor and station areas has been expanding, some locations, such as the metallic bodies of train cars which block the signals, are difficult to cover using only existing base stations and repeaters. In this Demonstration, a new system combining multiple technologies was implemented on a Yamanote Line train car to validate its effectiveness in providing millimeter-wave coverage inside train cars, where signals do not easily penetrate.

For this Demonstration, a system to re-radiate millimeter-wave signals received from a base station was installed inside a stationary Yamanote Line train car at JR East's Tokyo General Rolling Stock Center. The system involves receiving signals with a glass antenna on the window, amplifying them, transmitting them via a dielectric waveguide, and re-radiating them from desired locations using leaky antennas and a rod antenna. This allows for efficient coverage of necessary areas within the train car.

The Demonstration confirmed that by re-radiating radio waves from the leaky and rod antennas installed on the train car's ceiling, the impact of shielding was minimized and the communication area within the train car was efficiently expanded. The 1Gbps coverage area improved from 40% to 97% of the entire train car. This demonstrated that the previously discrete coverage areas, caused by signal attenuation from the train car's metal structure, were expanded to cover nearly the entire interior.

Due to their metal structures, train cars are generally considered more challenging for millimeter-wave coverage than typical indoor environments. Leveraging the knowledge gained from this Demonstration, KDDI will work to expand the use of millimeter-wave technology to a wide range of indoor environments, further improving communication quality and customer convenience.