Since 5G began its rollout in 2018 or 2019, fifth-generation wireless networks have spread across the globe to cover hundreds of millions of users. But while it offers lower latency than precursor networks, 5G also requires more base stations. To avoid installing unsightly equipment on more and more shared spaces, Japanese companies are developing transparent glass antennas that allow windows to serve as base stations that can be shared by several carriers.

Because 5G networks include spectrum comprising higher frequencies than 4G, base stations for 5G networks serve a smaller coverage footprint. Which means more base stations are needed compared to 4G. Due to a lack of installation spots and the high cost of rolling out 5G networks, carriers in Japan have been sharing mobile infrastructure.

Last month the Tokyo-based communications company JTower announced the deployment of the new glass antenna, created in part by glassmaker AGC (one of the world’s largest) and the mobile carrier NTT Docomo. The first was installed on a window in Tokyo’s Shinjuku district.

The product is “the world’s first antenna that turns a window into a base station that can be attached to a building window inside and turn the outdoors into a service area without spoiling the cityscape or the exterior appearance of the building,” says Shota Ochiai, a marketing manager at AGC.

NTT Docomo reports that it uses transparent conductive materials as the basis for its antenna, sandwiching the conductive material along with a transparent resin, the kind used in laminated windshields, in between two sheets of glass.

“I don’t think the idea for using transparent conductive materials as an antenna existed before,” said AGC’s Kentaro Oka in a company statement. “The durability of the antenna was significantly increased by placing the conductive materials between glass.”

The transparent antenna can be engineered according to the thickness of the glass to reduce the attenuation and reflection of the radio signals being absorbed and emitted by the window-sized device. “The glass antenna uses our proprietary technology to smooth out the disruption in the direction of radio waves when they pass through a window,” says Ochia.

A brief history of the window antenna

Branded WAVEANTENNA, the antenna is installed on the interior surface of windows. Apart perhaps from its cabling, the WAVEANTENNA is an otherwise inconspicuous piece of equipment that is often tucked out of sight, placed near the top or otherwise at the edges of a window.

It is compatible with frequencies in the 5G Sub6 band—meaning signals that are less than 6 gigahertz (GHz). Sub6 antennas represent critical portions of a 5G deployment, as their lower frequency ranges penetrate barriers like walls and buildings better than the substantially higher-bandwidth millimeter-wave portions of the 5G spectrum.

An earlier version of the product was launched in 2020, while a version that could handle sharing by multiple cell networks was introduced last year, according to AGC. The company says its antenna is optimized for frequencies between 3.7 and 4.5 GHz bands, which still allows for substantial bandwidth—albeit not comparable with what an ideal millimeter-wave 5G deployment could reach. (Millimeter waves can deliver typically between 10 and 50 GHz of bandwidth.)

The glass antenna can help expand 5G coverage as infrastructure sharing will become more important to carriers, AGC says. Besides increasing the number of locations for base stations, the device makes it easier to select the appropriate installation height, according to Ochiai.

AGC has also applied 5G glass antennas to automobiles, where they can help reduce dropped signals. The company reports that users include Halo.Car, an on-demand EV rental service in Las Vegas that relies on high-speed networks for remote drivers to deliver cars to customers.

The proliferation of IoT technology has made chatterboxes out of everyday hardware and new gadgets too, but it comes with a downside: the more devices sharing the airwaves the more trouble they have communicating. The nearly 30 billion connected devices expected by 2030 will be operating using different wireless standards while sharing the same frequency bands, potentially interfering with one another. To overcome this, researchers in Japan say they have developed a way to shrink the devices that filter out interfering signals. Instead of many individual filters, the technology would combine them onto single chips.

For smartphones to work with different communications standards and in different countries, they need dozens of filters to keep out unwanted signals. But these filters can be expensive and collectively take up a relatively large amount of real estate in the phone. With increasingly crowded electromagnetic spectrum , engineers will have to cram even more filters into phones and other gadgets, meaning further miniaturization will be necessary. Researchers at Japanese telecom NTT and Okayama University say they’ve developed technology that could shrink all those filters down to a single device they describe as an ultrasonic circuit that can steer signals without unintentionally scattering them.

The ultrasonic circuit incorporates filters that are similar to surface acoustic wave (SAW) filters used in smartphones. SAW filters convert an electronic RF signal into a mechanical wave on the surface of a substrate and back again, filtering out particular frequencies in the process. Because the mechanical wave is thousands of times shorter than the RF wave that creates it, SAW filters can be compact.

Today’s filters screen out unwanted RF signals by converting them to ultrasonic signals and back again. New research could lead to a way to integrate many such filters onto a single chip.NTT Corporation

“In the future IoT society, communication bandwidth and methods will increase, so we will need hundreds of ultrasonic filters in smartphones, but we cannot allocate a large area to them,” because the battery, display, processor and other components need room too, says Daiki Hatanaka a senior research scientist in the Nanomechanics Research Group at NTT. “Our technology allows us to confine ultrasound in a very narrow channel on a micrometer scale then guide the signal as we want. Based on this ultrasonic circuit, we can integrate many filters on just one chip.”

Valley Pseudospin-dependent Transport

Guiding ultrasonic waves along a path that changes direction can cause backscattering, degrading the signal quality. To counter this, Hatanaka and colleagues tapped Okayama University’s research into acoustic topological structures. Topology is mathematics concerned with how different shapes can be thought of as equivalent if they satisfy certain conditions—the classic example is a donut and a coffee mug being equivalent because they each have just one hole. But as highlighted by the 2016 Nobel Prize in Physics, it’s also used to explore exotic states of matter including superconductivity.

In their experiments, the researchers in Japan fashioned a waveguide made up of arrays of periodic holes with three-fold rotational symmetry. Where two arrays with holes that were rotated 10 degrees apart from each other met, a topological property called valley pseudospin arises. At this edge, tiny ultrasonic vortexes “pseudospin” in opposite directions, generating a unique ultrasonic wave known as valley pseudospin-dependent transport. This propagates a 0.5 GHz signal in only one direction even if there is a sharp bend in the waveguide, according to NTT. So the signal can’t suffer backscattering.

“The direction of the polarization of the valley states of ultrasound automatically forces it to propagate in only one direction, and backscattering is prohibited,” says Hatanaka. “

NTT says the gigahertz topological circuit is the first of its kind. The research team is now trying to fabricate a waveguide that connects 5 to 10 filters on a single chip. The initial chip will be about 1 square centimeter, but the researchers hope to shrink it to a few hundred square micrometers. In the second stage of research, they will try to dynamically control the ultrasound, amplify the signal, convert its frequency, and integrate these functions into one system.

The company will consider plans for commercialization as the research proceeds over the next two years. If the research becomes a commercial product the impact on future smartphones and IoT systems could be important, says Hatanaka. He estimates that future high-end smartphones could be equipped with up to around 20 ultrasonic circuits.

“We could use the space saved for a better user experience, so by using ultrasonic filters or other analog signal components we can improve the display or battery or other important components for the user experience,” he says.

Skies over Tokyo are thick with air traffic these days amid an influx of international tourists. But one plane recently helped revive the dream of airborne Internet access for all. Researchers in Japan announced on 28 May that they have successfully tested 5G communications equipment in the 38 gigahertz band from an altitude of 4 kilometers.

The experiment was aimed at developing an aerial relay backhaul with millimeter-wave band links between ground stations and a simulated High-Altitude Platform Station (HAPS), a radio station aboard an uncrewed aircraft that stays aloft in the stratosphere for extended periods of time. A Cessna flying out of Chofu Airfield in western Tokyo was outfitted with a 38 GHz 5G base station and core network device, and three ground stations were equipped with lens antennas with automatic tracking.

With the Cessna as a relay station, the setup enabled communication between one ground station connected to the 5G terrestrial network and a terrestrial base station connected to a user terminal, according to a consortium of Japanese companies and the National Institute of Information and Communications Technology.

“We developed technology that enables communication using 5G [New Radio] by correctly directing 38 GHz beams toward three ground stations while adapting to the flight attitude, speed, direction, position, altitude, etc. during aircraft rotation,” said Shinichi Tanaka, a manager in broadcaster SKY Perfect JSAT’s Space Business Division. “We confirmed that the onboard system, designed for the stratosphere, has adequate communication and tracking performance even under the flight speed and attitude fluctuations of a Cessna aircraft, which are more severe than those of HAPS.”

The sharpest beam width of the ground station antenna is 0.8 degrees, and the trial demonstrated a tracking method that always captures the Cessna in this angular range, Tanaka added.

A Cessna [top left] carried a 38 GHz antenna [top right] during a flight, functioning as a 5G base station for receivers on the ground [bottom right]. The plane was able to connect to multiple ground stations at once [illustration, bottom left].NTT Docomo

Millimeter wave bands, such as the 38 GHz band, have the highest data capacity for 5G and are suited for crowded venues such as stadiums and shopping centers. When used outdoors, however, the signals can be attenuated by rain and other moisture in the atmosphere. To counter this, the consortium successfully tested an algorithm that automatically switches between multiple ground stations to compensate for moisture-weakened signals.

Unlike Google’s failed Loon effort, which focused on providing direct communication to user terminals, the HAPS trial is aimed at creating backhaul lines for base stations. Led by Japan’s Ministry of Internal Affairs and Communications, the experiment is designed to deliver high-speed, high-capacity communications both for the development of 5G and 6G networks as well as emergency response. The latter is critical in disaster-prone Japan—in January, communication lines around the Noto Peninsula on the Sea of Japan were severed following a magnitude-7 earthquake that caused over 1,500 casualties.

“This is the world’s first successful 5G communication experiment via the sky using the Q-band frequency,” said Hinata Kohara, a researcher with mobile carrier NTT Docomo’s 6G Network Innovation Department. “In addition, the use of 5G communication base stations and core network equipment on the aircraft for communication among multiple ground stations enables flexible and fast route switching of the ground [gateway] station for a feeder link, and is robust against propagation characteristics such as rainfall. Another key feature is the use of a full digital beamforming method for beam control, which uses multiple independent beams to improve frequency utilization efficiency.”

Doppler shift compensation was a challenge in the experiment, Kohara said, adding that the researchers will conduct further tests to find a solution with the aim of commercializing a HAPS service in 2026. Aside from SKY Perfect JSAT and NTT Docomo, the consortium includes Panasonic Holdings, known for its electronics equipment.

The HAPS push comes as NTT Docomo announced it has led another consortium in a $100 million investment in Airbus’ AALTO HAPS, operator of the Zephyr fixed-wing uncrewed aerial vehicle. The solar-powered wing can be used for 5G direct-to-device communications or Earth observation, and has set records including 64 days of stratospheric flight. According to Airbus, it has a reach of “up to 250 terrestrial towers in difficult mountainous terrain.” Docomo said the investment is aimed at commercializing Zephyr services in Japan, including coverage of rural areas and disaster zones, and around the world in 2026.