Solving the scarcity of radio-frequency resources through research on radio-wave propagation—namely, analyzing and mathematically modeling how radio waves travel in accordance with their frequency and environment
—Would you tell us about the research you are currently conducting?
My basic field of research is radio-wave propagation, a field in which we analyze and mathematically model how radio waves travel in accordance with their frequency and environment. This research field can be broadly categorized into three areas: channel models for simulation to design and evaluate wireless devices and systems; interference-evaluation models to investigate interference propagation in the same system and between different systems for sharing radio-frequency resources; and propagation models to design wireless zones and links. The first two, channel models for simulation and interference-evaluation models, are related to system development and system sharing and evaluation and serve as a foundation for evaluating various technologies side by side. They are therefore techniques targeted for standardization. The third area, propagation models, is related to system operation, so it is considered an area of differentiation in a competitive environment.
Regarding my current research themes, I first imagined the world around 2050, in which ground terminals, crewed aerial vehicles (flying cars), drones, and other terminals are connected to wireless networks and move around the three-dimensional space of cities (evolved from two-dimensional mobility to three-dimensional mobility). In such a world, to provide wireless communication services that will keep all terminals connected in a stress-free manner as demanded in the Innovative Optical and Wireless Network (IOWN) era, I defined the evolutionary goal of wireless communication systems as “transformation from two-dimensional area coverage to provision of optimal quality to individual terminals as well as freedom from radio-resource constraints.” I then designated the technologies needed to reach this goal as “technology for determining and predicting—in real time—wireless quality, which changes in a complex manner according to location and time, and optimally controlling radio resources such as frequency while assuring freedom from interference.”
Against this background, my research goal is to enable wireless communication that is essentially free from radio-resource constraints through the optimization of available radio resources and exploration of new radio resources based on the radio-wave-propagation technologies that we have established thus far. My specific research themes are real-time wireless-quality-estimation platform, dynamic radio-resource-control technology, and exploration of new radio resources.
The real-time wireless-quality-estimation platform applies a cyber-physical system, which optimally controls the real space by reproducing the real world in a virtual space and feeds back the results of simulations conducted in the virtual space to the real space, to the wireless world to demonstrate and establish the world’s first real-time wireless-quality estimation (Fig. 1). Cyber-physical systems in the wireless world are being examined for beyond 5th-generation mobile communication system (5G) and 6G networks. However, when radio waves are transmitted from the rooftop of buildings in urban areas, the propagation of the radio waves changes constantly due to unevenness on the surfaces of buildings, reflections and obstructions by roadside trees, and movement of the receiving side, such as cars passing along the road, so the quality of wireless communications also varies. To simulate this radio-wave propagation environment in a virtual space, it is necessary to model minute unevenness of the buildings, positions of windows, movements of cars, etc., which faces considerable challenges. Even if such modeling was possible, simulation of city-size models would require computation time in the order of several days to months, which is a major hurdle to real-time simulation.
Fig. 1. Real-time wireless-quality-estimation platform.
Therefore, my research colleagues and I focused on the amount of energy attenuation along each of the countless propagation paths that radio waves take from transmission to reception and reduced the radio-wave propagation simulation to the problem of searching propagation paths that minimize the amount of energy attenuation. We thought this makes it possible to use the method of solving a combinatorial optimization problem through quantum annealing, which in turn would dramatically speed up simulations.
We were able to advance this high-speed wireless-quality-estimation technology to the point that the time required by simulation could be reduced by more than one millionth compared with the conventional radio-wave-propagation simulation method. I believe that two types of information, amplitude and phase, are necessary to estimate wireless quality and achieve freedom from radio-resource constraints. We currently can handle amplitude, but we are taking the challenge to establish new technology that can handle phase.
—Could you explain dynamic radio-resource-control technology and exploration of new radio resources?
Since the radio frequencies used for wireless communications are a finite resource, radio-resource shortages have become a major problem as mobile communications are undergoing a rapid increase in users, users are being concentrated in specific areas, and higher speeds and broader bandwidth are becoming more common. Solving this problem includes expanding the reuse of currently available frequency resources and exploring new frequency bands.
The dynamic radio-resource-control technology is for reuse of frequencies. By avoiding and suppressing the radio-frequency interference between radio waves by using positional information and the amplitude/phase information obtained through high-speed wireless-quality estimation on the aforementioned real-time wireless-quality-estimation platform, it will achieve the ultimate reuse of radio resources and provide stable wireless-transmission quality.
Radio-frequency interference, which occurs when different radio waves with frequencies in the same frequency band conflict, has always been a serious problem in regard to maintaining constant-quality wireless communications.
As shown in Fig. 2, frequency-resource reuse, which enables multiple systems to share the same frequencies, involves three processes: (i) interference avoidance by restricting radio waves so that they do not reach terminals other than the desired one; (ii) interference suppression by removing unnecessary radio waves from the received radio waves through the use of radio waves of the same amplitude but opposite phase; and (iii) frequency sharing by controlling the time of use of certain radio waves to create “time gaps” in the frequency resource. To implement these three processes, real-time location information and amplitude/phase information about the radio waves at the receiving terminal are required, and that information can be obtained by using the real-time wireless-quality-estimation platform.
Fig. 2. Dynamic radio-resource-control technology.
Radio frequencies are globally allocated and registered according to their intended use by the International Telecommunication Union - Radiocommunication Sector (ITU-R). Almost all frequencies available for current technologies have been used, so it is difficult to allocate frequencies to new systems and services worldwide. In Japan, almost no frequencies below 30 GHz are available (Fig. 3). The objective of the exploration of new radio resources is to enable ITU-R to allocate new frequencies to wireless communication systems by developing international-standard propagation models for high-frequency bands—particularly, the sub-terahertz band (above 100 GHz) under consideration for 6G use—that are not being fully used and where radio-wave-propagation characteristics are not yet understood.
Fig. 3. Usage status of radio waves in Japan.
Our main activity for the exploration of new radio resources is to incorporate radio-wave-propagation-related technologies of the NTT Group into the key recommendations of ITU-R. We are also conducting propagation measurements in the sub-terahertz band, which is the basis for those technologies. As frequency increases, the factors that disturb radio-wave propagation multiply, and propagation distance tends not to increase. I believe that if used properly, higher frequencies can be used for wireless communications. However, it has been experimentally shown that radio waves are more significantly attenuated by diffraction in situations in which their line-of-sight is blocked, such as behind buildings.
A phenomenon unique to the sub-terahertz band is that because the wavelengths in this band are very short, radio waves in the frequency bands conventionally used for mobile phones that would be reflected by flat surfaces, such as the walls of buildings, will be reflected by the uneven surfaces of the walls. It has thus been found that radio waves in the sub-terahertz band are scattered even by normal (uneven) walls, and despite the lack of diffraction, radio waves can propagate further than expected. I believe there may be use cases that are unique to the sub-terahertz band that take advantage of this characteristic.
Since 2006, I have been a member of the Japanese delegation to ITU-R meetings and have participated in various discussions in the Working Parties (WPs) under ITU-R Study Group (SG) 3 (radio wave propagation). I have served as vice-chair of WP 3K (point-to-area propagation) and chair of its Sub Working Group 3K3 (short-range indoor/outdoor propagation). I have also expanded activities to WP 5D (IMT (International Mobile Telecommunications) systems) under SG 5 (terrestrial services) and have been discussing the opening up of new frequencies for 5G and 6G. Through these activities, I hope to contribute to the continuous exploration of new radio resources.
—What do you think is the value of your research?
I believe that the research that I have undertaken thus far, in particular, applying radio-wave-propagation-related technologies used for the area design and system/interference evaluation to wireless transmission and radio-wave propagation for the fusion of real space and simulation space will lead to (i) the development of new fields in which radio-wave-propagation technologies can be used for wireless transmission, (ii) establishment of new wireless-transmission methods by shifting from the conventional measurement-based approach to a data-driven approach, and (iii) overcoming wireless-performance limits by eliminating the constraints on radio resources. I believe that they will significantly contribute to invigorating the wireless research community. I also want to contribute to the expansion of the entire communications-related market by upgrading wireless communication services from the conventional “best-effort” service to a “highly reliable” service and enabling wireless communications that are not restricted by radio resources.
In terms of frequency resources, it is said that the lower frequencies, called the “platinum band,” have higher economic value owing to its superior propagation characteristics. I believe that research into radio-wave-propagation-related technologies will make it possible to dynamically free these platinum-band frequency resources from interference and fully use them, and expand their use from surface areas on land to spatial regions in the sky, thereby increasing the overall economic value of radio frequencies (including high frequencies).
Identifying the goal of your research, organizing the necessary technologies to achieve it, including involvement of people outside your expertise, and taking on new challenges
—What do you keep in mind as a researcher?
When I experience an inconvenience, such as a slow response from a home appliance, I try to focus on whether there is some way to solve the problem with technology. I then imagine possible solutions for solving the problem with the mindset “maybe I can solve it in this way.” If a solution seems possible, I consider what technology would be needed to implement that solution. Even if a possible solution exists, not all problems can be solved. However, I believe this mindset is important in regard to research activities because I can define the goal of a research theme and select the best approach in terms of time and resources as I organize my approach toward that goal. This mindset has been very useful for me because time and resources available for my research activities are limited.
I am also aware of trends in areas outside my expertise. Quantum annealing—a component of the real-time wireless-quality-estimation platform—is one example of this awareness. I have been interested in quantum annealing and studying it despite it not being my field of expertise, and when I was investigating the approach to achieving real-time wireless-quality estimation, I realized that it could be used for estimating wireless quality. Much research cannot be completed within a specific field of expertise alone, so it is essential to be aware of external trends to combine one’s expertise with other technologies. It is also important to interact with people outside one’s field and company by participating in academic conferences and other events. In addition to gathering information on external trends, borrowing the knowledge of external experts and promoting collaboration make it possible to receive positive stimulation from each other in a manner that leads to new ideas.
I’m also conscious of taking on new challenges and sticking with them. I have been involved in standardization at ITU-R for almost 20 years, and my involvement started when my boss asked me to participate in it. At first, I knew nothing about standardization, and since it wasn’t the same as competing to get the best research result, I wasn’t interested in it and thought I’d just attend about three or so meetings. However, as I continued my involvement, after five then ten meetings, meeting outcomes were achieved. Consequently, people around me in the standardization community began to recognize me, and I felt like my profile was growing. Even if I wasn’t interested in something at first, by sticking with it, my interest grew as the result of approaching it in a positive manner. Therefore, I will make sure to keep this mindset at all times.
Imagining the future and enjoying research that makes it a reality
—What is your message to junior researchers?
I want you to imagine the future you want and take up research that will create and make it a reality. It’s fine if the future you imagined is achieved after you retire as a researcher, and fine if only part of it is achieved. Research is fundamentally not about what is in front of you now but about things that will happen many years from now. In other words, if you don’t imagine the future, you won’t be able to find your goal. Above all, research aims to contribute to the future, not the present.
I hope you will enjoy your research while keeping a work-life balance in mind. When you are immersed in your research, you may find yourself thinking about it even in your free time, but if that situation continues, you may become narrow-minded and reach a dead end. By taking time to refresh yourself in your private time, you will be able to receive positive stimulation from those around you, which will enable you to see your research subject from a different perspective, and that perspective may lead to the birth of new ideas. By taking a positive approach to your research, you can make the most of your leisure and the stimulation that comes from a good work-life balance. Doing so is vital to enjoying your research.
