Aiming to eliminate interference between antennas

Various wireless communication methods, such as smartphones, Wi-Fi, and Bluetooth, are currently used depending on the application. The frequency bands that can be used for each method are regulated by the Radio Act. However, the rapid increase in the volume of information has highlighted the critical challenge of effectively and efficiently using each frequency band. In response to this challenge, Professor Tamura and his team have been studying a method known as “in-band duplex communication”.

“For instance, we can all picture how, if inbound and outbound lanes are not separated on a highway, then cars will collide with each other and cause accidents.However, our method enables the simultaneous use of the same frequency for transmission and reception.

That said, challenges remain. Using the same frequency means that transmitting and receiving antennas placed close to each other will pick up each other’s radio waves, causing interference. Eliminating this inter-antenna coupling is the goal of my research. This method can be used not only for in-band duplex communication, also for multiple-input multiple-output (MIMO), a technology which uses multiple antennas to increase the speed, reliability, and flexibility of wireless communication. Even in MIMO systems,  interference between the antennas can lead to significant reductions in communication speed and quality”, explains Professor Tamura.

In practice, what specific ideas did the research team explore? Conventional methods for avoiding interference involve physically separating antennas, tilting them 90 degrees to form a cross, or using other spatial arrangements to prevent interference.

“Regrettably all these methods would inevitably take up too much space. Accordingly, we started researching how to create a circuit that would prevent radio interference even with a small distance between the antenna by applying filter theory. In this case, two resonant circuits function as filters. They generate two attenuation poles near the operating frequency of the antennas, achieving wideband attenuation and suppressing interference”.

However, this method introduced complications, including an increased number of chip components, not only for the antennas, but also for the resonant and transmitting circuits, as well as those ensuring their consistency. Moreover, manufacturing variations across chips made assembly more time-consuming as efforts to fine-tune performance increased.

“What I realized at this point was that the antennas were functioning as resonators”, Professor Tamura recalls.

Finding inspiration from the words of senior colleagues

“What antennas do is  to receive and transmit radio waves within the desired frequency band by creating a resonance phenomenon. From this arose the idea of using the antenna itself as a resonator”.

Professor Tamura stated that the seeds of this insight originated from advice given to him by two key figures. One of these was Yohei Ishikawa, at that time Director of the Murata Manufacturing R&D Center, with whom Professor Tamura had become acquainted over the course of academic conferences and other events while working at Panasonic. The other was the late Professor Tatsuo Itoh of UCLA, who served as a mentor in joint research projects.

“Both of them firmly believed that the key to understanding radio and electromagnetic wave phenomena is to  first grasp the physical phenomenon by expressing  them as an equivalent circuit , and only then to proceed with creation. In other words, all physical phenomena can be described using R (resistor), L (inductor), and C (capacitor). Following this advice, I drew the equivalent circuit and noticed that antennas resemble resonators. That’s when it hit me, that I could reduce the number of elements!”

Nevertheless, it took extensive trial and error to develop the method. The patch antenna was designed to target the 2.4–2.5 GHz  band, commonly used by wireless LANs such as Wi-Fi. When spacing the two antennas close together down to 0.2 mm, considering integration into smartphones and small sensors, simulations showed that approximately half the power from one antenna was absorbed by the adjacent one, preventing long-distance transmission.

“Solving this problem required increasing the resonance distance between the two antennas while monitoring the admittance (ease of power flow) between them. This may get a bit technical, but we analyzed the characteristics of the two antennas using a method called even-odd mode. We devised new ways to connect transmission lines and add capacitors such that the admittance would be zero, effectively cutting the line”, explains Professor Tamura.

This enabled the transmittance of nearly all the power sent to the antenna as radio waves into space without loss. The research team also successfully reduced the number of elements from 14 in earlier designs to 6. This design theory served as the foundation for creating and measuring an actual circuit, yielding results consistent with simulations using a very simple configuration. The reduced number of elements also minimized the impact of component variation, allowing the research team to develop a circuit completely free of inter-antenna interference.

“The electromagnetic waves that can be transmitted and received are currently limited to a narrow range of approximately 2.45 GHz. Thus, we are currently conducting joint research with companies to expand this bandwidth”, explains Professor Tamura. Their research is accelerating toward practical application.

Wide-ranging research: from smart factories to power supply to underwater drones

Professor Tamura’s laboratory has also been working with Aichi Prefecture and local companies on basic research into smart factories, which aim to eliminate all wiring in factories and handle all aspects from information transmission to power supply.

“Smart factories aim to monitor robot operations and production processes using sensors to improve work efficiency and reduce product defects . However, wiring failures and the need for sensor battery replacement have been identified as challenges. Therefore, we are researching how to use metal meshes (safety fences) surrounding production equipment as pseudo-shielded spaces to confine radio waves and wirelessly power the equipment. In this setup, radio waves at communication frequencies can be transmitted and received by them outside the wire mesh. This enables power supply and information communication without requiring human entry into the smart factory”, says Professor Tamura.

His laboratory is also researching charging and data transmission for underwater drones, which are used for deterioration diagnosis of bridges and dams, environmental monitoring in aquaculture, and marine resource exploration.

“Charging is a challenge here as well. In particular, marine resource exploration requires 7–8 hours just for diving and surfacing, leaving only approximately 3–4 hours for actual exploration. Therefore, we have been exploring options for placing a charging station underwater, initiating power supply when the drone lands on the bottom, and simultaneously transmitting exploration data to the station. If implemented, this would allow drones to be stationed underwater, enabling remote operation”.

The highlight of Professor Tamura’s research is the use of lightweight copper tape rather than coils as electrodes to avoid interfering with the drone’s buoyancy.

“With seawater, we had to be creative, as it is rich in ions, which can cause electricity to diffuse. Therefore, we used a cushion to absorb the impact of the drone hitting the sea floor, while trapping seawater to prevent electrical diffusion. This led to the development of a technology that uses high-frequency ion currents to transmit electricity with over 90% efficiency—the first of its kind worldwide. I think this is a much-needed technology that can really come into its own in places that are inaccessible to people," says Professor Tamura proudly.　The research team is also working on wireless power supply to implantable medical devices such as pacemakers. This would eliminate the need for battery replacement through surgery, significantly reducing the burden on patients. Like all of Professor Tamura’s research, it is a step towards developing technology that responds to the real needs of society. We look forward to the swift practical implementation of these technologies.

Reporter's Note

Professor Tamura explains that, during his university years, he reflected on his desire to do work that helps people. One day in class, a professor told him, “Come to my lab if you want to know why we can talk on cell phones”. This intrigued him and led him to pursue research. He went on to complete his master’s degree and joined a company, an experience he says was highly valuable in his later career as a researcher. “Even then, I felt this significant gap between the research conducted at universities and the research and development at companies aiming for practical application. Thus, I have a clear vision of how my research will be used, and I conduct it with the goal of giving back to society.” This interview clearly showed why Professor Tamura is a sought-after partner for joint research with companies.