We have looked at glass-based connectivity solutions a few times on this blog, especially the work around transparent antennas and the use of windows as part of the radio network. The idea may sound futuristic, but it is gradually moving from concept demonstrations to more practical deployment scenarios.
The latest example comes from KDDI and JR East, who have successfully demonstrated a way to expand 5G millimetre wave coverage inside JR Yamanote Line train cars. The demonstration was completed by 15 April 2026 at JR East’s Tokyo General Rolling Stock Center, with the news release published on 20 May 2026. The train cars used for the test were parked, so this should be seen as an important technical demonstration rather than a live passenger service deployment.
The challenge being addressed is easy to understand. The 28 GHz band used for 5G millimetre wave can provide high-speed, high-capacity connectivity, but it is also highly directional and more easily blocked by obstacles. Train cars are a particularly difficult environment because the metal body of the vehicle acts as a shield, making it hard for outdoor mmWave signals to reach passengers inside. This means that even where mmWave coverage is available along railway lines, on station platforms or around busy transport hubs, passengers inside the train may still not benefit from it.
KDDI and JR East tackled this by bringing the outdoor mmWave signal into the train and then re-radiating it inside the carriage. A millimetre wave-compatible glass antenna, supplied by AGC, was installed on the train window to receive the signal from an outdoor base station. The signal was then amplified using a high-gain amplifier from Kyocera, transmitted through the carriage using a low-loss dielectric waveguide from Nippon Dengyo Kosaku, and re-radiated inside the train using leaky antennas and rod antennas.
＼#山手線 の車両内を5G(ミリ波)エリア化する実証実験に成功🚃／
— KDDI広報部 (@kddipr) May 20, 2026
車両基地に留置中の山手線車両を活用し、鉄道車両内でも高速通信可能な環境構築を目指す実証に成功しました🎉… pic.twitter.com/f3KpE7dFv0
According to KDDI, this is the first attempt in Japan to bring millimetre waves from outdoor base stations into train cars and re-radiate them, based on research by KDDI and JR East as of 20 May 2026. The result was impressive. The area inside the train car capable of achieving a communication speed of 1 Gbps improved from approximately 40% to approximately 97% of the entire car.
The use of the glass antenna is especially interesting. Windows are often one of the few surfaces through which radio signals can be brought into otherwise shielded environments, whether that is a train, bus, building or vehicle. AGC’s glass antenna can be retrofitted to indoor glass surfaces and is designed to maintain transparency, which helps reduce installation constraints and avoids spoiling the interior appearance.
The dielectric waveguide is another important part of the solution. At millimetre wave frequencies, conventional cable losses can become a major problem. KDDI says the dielectric waveguide used in the demonstration has a low transmission loss of 0.5 dB per metre and reduces loss per metre by around 83% compared with conventional coaxial cable. This makes it more practical to carry the mmWave signal through the train car before re-radiating it at the required locations.
The leaky antenna and rod antenna then help distribute the signal inside the car. The leaky antenna radiates part of the millimetre wave power propagating through the waveguide into the surrounding space, while the rod antenna radiates the signal efficiently from the end of the waveguide. Together, these components allow the signal to be delivered to specific areas inside the train rather than relying on the outdoor signal to penetrate the vehicle body directly.
This demonstration also fits into KDDI’s wider work on extending mmWave coverage. The company has previously been working on wireless repeaters to improve mmWave coverage in areas with many obstructions, including urban environments and railway stations. The KDDI article mentions previous demonstrations at Shinjuku Station platform, around Takanawa Gateway Station and on the Shinkansen platform at Tokyo Station. The train car demonstration is therefore another step in trying to make mmWave useful in places where coverage would otherwise be difficult.
For passengers, the most obvious benefit would be better high-speed connectivity in busy trains. However, the longer-term value could go beyond passenger broadband. Reliable high-capacity wireless coverage inside train cars could support railway operations, onboard monitoring, real-time information systems, security services and future digital railway applications.
Of course, there are still practical questions before this kind of solution can become widely deployed. The demonstration was carried out on parked Yamanote Line train cars, and large-scale deployment would need to consider installation, maintenance, power supply, durability, train operations, cost and performance in real operating conditions. Even so, the result shows that mmWave coverage inside railway vehicles is not simply a question of adding more outdoor base stations. It may require a combination of radio engineering, materials innovation and clever use of surfaces such as glass.
This is what makes the demonstration interesting from a connectivity perspective. The future of wireless coverage will not only depend on spectrum and base stations, but also on how signals are guided, amplified, redirected and distributed in difficult environments. In this case, a train window becomes more than just a window. It becomes part of the network.
Related Posts:
- Connectivity Technology Blog: Boosting 5G Coverage with Transparent Antennas and Shared Infrastructure
- Connectivity Technology Blog: Seamless 5G Connectivity Across Germany on Deutsche Bahn
- The 3G4G Blog: A Different Approach for Mobile Network Densification
- Connectivity Technology Blog: Docomo and AGC use Metasurface Lens to Guide Millimeter Waves (mmWaves) Indoors
- Connectivity Technology Blog: Futuristic Glass Antenna by NTT Docomo and AGC
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