Aiming for High-speed, High-capacity Wireless Communication Systems through Practical Application of Terabit-class Wireless Transmission Technology

Creating new fundamental technologies for wireless transmission through integration of various technology fields

—First, what kind of research are you conducting on photonics-electronics-convergence wireless transmission technology?

With a frequency more than 1000 times higher than that of radio waves, light can handle a huge amount of information efficiently. My research team aims to take advantage of these characteristics of light by combining electromagnetic and photonics (optical) technologies and knowledge with electronics (electrical) technology and knowledge that support the current wireless transmission infrastructure in a manner that creates new fundamental technologies for wireless transmission that achieve extremely low power consumption and high capacity (Fig. 1). Although we are still in the conceptual stage and research into the application of light will continue in depth from now onwards, hereafter, I’ll introduce one approach for increasing the capacity of wireless communication systems, namely, using the physical property of electromagnetic waves “spatial modes” based on electromagnetism.

Fig. 1. Future outlook for photonics-electronics-convergence wireless transmission technology.

Increasing yearly, the volume of wireless communication traffic has increased more than 100 times since the launch of fourth-generation mobile communication systems (4G) and LTE (Long-Term Evolution) services. In preparation for the future increase in demand for wireless communications, it is urgently needed to increase the speed and capacity of wireless communications. Regarding wireless communications, three main resources can be used: frequency, space, and power.

Radio-frequency resources are currently in very limited supply. In reality, however, due to current technology, costs, and other factors, only certain available frequency bands (ranging from a few hundred megahertz to a few gigahertz) are congested. Let us consider the current 5G and look ahead to 6G in the 2030s and beyond. If we can develop hardware and system technologies that can efficiently use a wide range of frequency resources that are not currently in use—from frequency bands of 30 GHz or more (millimeter waves) to even higher frequency bands of 100 GHz or more (terahertz (THz) waves)—we will be able to prepare for further increases in wireless demand.

With regard to space, it is easy to imagine that by forming a large number of narrow beams in different spatial directions (like the blue beams depicted in Fig. 1), it would be possible to multiplex and transmit signal streams simultaneously in the same frequency band without them interfering with each other. If it becomes possible to multiplex signal streams in the same frequency band by applying the principle of spatial modes known as orbital angular momentum (OAM) multiplexing transmission, which has been actively investigated over the past decade, it will be possible to further increase transmission capacity in accordance with the number of signal streams to be spatially multiplexed, thus enable efficient use of frequency resources without waste.

To meet the massive increase in wireless communication traffic expected to occur from 2030 onwards due to new communication services such as 6G, my research team aims to develop ultra-wideband radio-frequency resources such as the millimeter waves and THz waves mentioned above. We aim to construct wireless systems that use spatial resources in a manner that maximizes the efficient use of those frequency resources by achieving high capacity (spatial-mode multiplexing) and multiple simultaneous connections (ultra-high-density beamforming) with high energy efficiency (Fig. 1).

Increasing the capacity of wireless communications is a never-ending technical challenge, and to deliver the vast amounts of information required for new services such as artificial intelligence, augmented reality (AR), and virtual reality (VR), which are currently experiencing rapid progress, to general users wirelessly, even faster and more stable wireless communication systems will be required. For example, if it becomes possible to provide inexpensive, stable wireless communication systems that are a hundred-, thousand-, or even-more times faster than today’s standards, it may become possible to create the kind of futuristic services that are only seen in science fiction.

—What specific technological research are you conducting toward the world’s first terabit-class wireless transmission?

To enable terabit-class wireless transmission, we have been researching and developing spatial-mode-multiplexing transmission technology in the sub-THz (150 GHz) band, which is a frequency band that is currently not widely used. Specifically, we are researching the following three technologies.

(1) OAM-MIMO multiplexing transmission technology

With this technology, which focuses on the OAM of radio waves, the wavefront of radio waves is precisely controlled to form multiple, independent spatial modes (so-called OAM modes). Each OAM mode is modulated with a different signal stream in a manner that enables multiple signal streams to be multiplexed at the same frequency and in the same direction (Fig. 2(a)). Although we must still overcome many issues before this technology can be put to practical use, in theory, OAM multiplexing transmission can increase the number of multiplexed signal streams infinitely. OAM-MIMO multiplexing transmission technology combines multiplexing transmission using these OAM modes with conventional digital-signal-processing-based spatial multiplexing (MIMO: multiple-input multiple output) transmission technology, thus dramatically increasing the number of spatially multiplexed signal streams [1].

(2) THz-waveguide-circuit technology

We have designed and prototyped a THz-waveguide circuit that can precisely control the wavefront of spatial modes over an ultra-wide bandwidth of 135 to 170 GHz in the 35-THz band and simultaneously form and separate multiple spatial modes (Fig. 2(b)). This waveguide circuit can simultaneously generate and separate eight different OAM modes with high precision. It can also spatially multiplex up to two signal streams within each mode, making a total of up to 16 signal streams. This waveguide circuit is composed of passive elements that do not require digital signal processing or external control; thus, it can significantly reduce barriers to practical application such as the development of new ultra-high-speed digital-signal-processing devices related to dedicated spatial-mode-multiplexing processing and additional operating costs (power consumption, etc.).

(3) Imaging-reflector-antenna technology

When practicality is considered, the transmission distance over which high-speed transmission is possible is also an important issue. Radio waves transmitted by OAM multiplexing tend to spread more easily over distance than normal radio waves, so the antenna used for transmission and reception must handle this spread. In general, the larger the effective aperture diameter of the antenna, the more efficiently the radio waves can be concentrated, and the result of that concentration is the formation of a beam that can be transmitted over long distances. When spatial-mode multiplexing is being applied, however, the wavefront of the radio waves has a spatial phase distribution that is controlled with extremely high precision, so it is necessary to enlarge the effective antenna aperture so as not to disturb the spatial modes (OAM modes). To satisfy that necessity, we devised imaging-reflector-antenna technology, by which a magnified image of a small circular array antenna is formed without distortion on a main reflector. It thus becomes possible to increase the effective antenna-aperture diameter and extend transmission distance (Fig. 2(c)).

Fig. 2. Key technologies for spatial-mode-multiplexing transmission.

For example, the aforementioned waveguide circuit has an antenna with an aperture diameter of 6 cm, which enables terabit-class wireless transmission over a distance of about 1 m. By using the reflector to expand one of the antennas (the transmitting side) by 7.5 times (to 45 cm), we increased the distance over which equivalent high-speed transmission is possible by 7.5 times. In other words, if the aperture diameter of the antenna is increased by 10 times—on both the transmitting and receiving sides—the equivalent transmission distance can be increased by 100 times.

By integrating these three component technologies into a system, we were able to demonstrate, as a world’s first in a laboratory environment, ultra-high-speed wireless transmission exceeding 1 Tbit/s (1.58 Tbit/s). We also demonstrated that by using the imaging-reflector-antenna technology, which enables us to expand the effective antenna diameter without disturbing the spatial modes (OAM modes), it is possible to extend the transmission distance possible with spatial-mode multiplexing in proportion to the effective antenna diameter on the transmitting or receiving side (Fig. 3).

Fig. 3. Transmission experiment in a laboratory environment.

This system provides ultra-wideband, high-speed transmission comparable to that provided by commercial optical transmission systems, and I believe it will contribute to the construction of future flexible wireless networks in ways such as complementing optical networks connecting wireless base stations (wireless backhaul/fronthaul) and high-capacity wireless-relay transmission. We plan to evaluate these technologies in a real environment through field-transmission experiments at distances of more than 100 m.

—What were some of the difficulties you encountered in your research and what are some of the challenges you will face in the future?

Although theoretical innovation and progress are important in academic research, it is also important to demonstrate the effectiveness and practicality of the entire system and demonstrate its value to the world. In my teams’ research on terabit-class wireless transmission, in addition to building a sufficient experimental system and improving efficiency through automation to produce world-leading results, my entire team had to make meticulous plans and extensive preparations, including complying with legal systems such as the Radio Law, when conducting transmission experiments outdoors.

Since the experimental environment is so complex, it is only natural that things do not always work as predicted by theory. Conversely, thanks to this unpredictability, we have made many discoveries that can be fed back to update theory. During our experiments, we are naturally required to make quick and flexible decisions, revise theories, and verify hypotheses, and I remember that it was a constant, daily struggle to take responsibility for completing the goals that I had declared for myself.

This type of experience, however, is extremely important for honing the technical wisdom that is indispensable not only for the ongoing process of verifying new technologies but also for evaluating the feasibility and practicality of research, including research in slightly different fields, from different perspectives and discussing new research themes.

Regarding future challenges, it is necessary to establish new fundamental technologies for wireless transmission, and to meet that necessity, as mentioned above, it will be necessary to acquire knowledge of a wide range of technical fields, including electromagnetics, electronics (high-frequency devices, etc.), photonics (optical devices, etc.), in addition to wireless and optical transmission technologies. Therefore, in addition to improving my own and my team’s knowledge and sensibilities, collaboration across other fields both inside and outside the organization will be ever more important. Fortunately, NTT has many highly specialized researchers in each field and is internationally acknowledged as a company that provides communications infrastructure. I want to further expand the scope of collaboration with other fields of technology.

Adding photonics to electronics opens the door to a new, yet unseen, era

—What are the prospects for your future research?

We are currently conducting large-scale transmission experiments over 100 m, which is a significantly extended distance compared with the 7.5 m that we previously experimentally demonstrated. If this transmission is successful, it will become possible to practically use the sub-THz (150 GHz) band as a backhaul for wireless base stations. The outdoor setup of a 40-GHz-band field experiment for transmission over 100 m that we conducted in 2020 is shown in Fig. 4.

Fig. 4. Field experiment conducted in the 40-GHz band in 2020.

In the future, it should also be possible to increase the speed and capacity of wireless communications between buildings in urban areas or within stadiums, and use the above-mentioned technologies for advanced AR and VR (Fig. 5).

Fig. 5. Practical application and future prospects of terabit-class wireless communication systems.

For example, by combining new technologies such as three-dimensional projectors with Vocaloid (singing-voice-synthesizer software) live performances, it may become possible to virtually and simultaneously watch a soccer match being played overseas in a stadium in Tokyo. I think it would be very interesting if we could recreate the actual performance of famous overseas players in a nearby stadium, including the parts that are not shown on the television screen.

By using light, which has a frequency more than 1000 times higher than that of radio waves, it is possible to handle extremely large amounts of information efficiently. Our goal is to combine photonics (optical) technology with the electronics technology that supports the current wireless transmission infrastructure, thus create new fundamental technologies for wireless transmission that achieves extremely low power consumption and high capacity.

I hope that my research will contribute to a variety of NTT businesses in the following ways: (i) drastically increasing capacity, lowering cost, and reducing power consumption by enabling ultra-wideband, highly energy-efficient wireless system control; (ii) lowering the barrier to the practical application of a terabit-class wireless communication system by enabling seamless connection with the All-Photonics Network currently being discussed mainly at the IOWN (Innovative Optical and Wireless Network) Global Forum; and (iii) promoting the introduction of NTT optical device technologies (coherent digital-signal-processor technology, photonics-electronics-convergence device technology, etc.) into wireless systems.

—What is your impression of NTT Network Innovation Laboratories, where you work?

I have been affiliated with NTT Network Innovation Laboratories ever since I joined NTT, and at the Laboratories, my research and development aims to bring innovation and high added value to the information transmission systems of the future by using all physical waves, namely, light, radio waves, and sound waves. One such area is high-capacity communication technologies such as OAM. I feel that if you join our Laboratories, create your vision from a basic idea, and have it recognized as something an essential technology, you will find an environment that supports you in taking on various challenges until the technology develops over the medium to long term.

The hurdles to becoming the world’s first or best are not easy to overcome overnight. In reality, many component technologies must be devised, refined, and combined, an experimental environment must be created, and those technologies must be experimentally demonstrated before success is accomplished. Needless to say, each of these steps requires a great deal of time and effort. I believe that NTT Network Innovation Laboratories, which provides ample support for such new challenges, is an extremely favorable environment for researchers.

—Finally, what is your message to researchers, students, and business partners?

I particularly feel that researchers who work at the forefront of creating innovation have knowledge and experience in their fields that are second to none. However, when I actually meet and talk with them, I often find that they excel at looking at things from multiple perspectives, even in fields outside their specialty. Above all, I think they are working on their own research because they find it interesting. The word “interesting” used by these researchers goes beyond mere academic curiosity. It is a word that brings to mind many things, such as feasibility, level of technological innovation and value, impact on society, and the world after practical application. It’s a word that requires a rich sense. I still remember how happy I was when, during my university days, a world-renowned professor said to me, “That’s interesting!” Those words came from someone who was usually very harsh, so they made a strong impression on me and those words have stuck with me ever since.

I think that to hone that rich sense that enables you to produce ideas that are both feasible, innovative, and future-oriented, you need to accumulate a wide range of knowledge, experience, and discussions. It’s fine to start with something narrow, but it’s important to strive to be proud of being at the cutting edge, discuss things with many people from various fields, and gain a multifaceted perspective.

I look forward to engaging in many discussions with current active researchers, students aspiring to become researchers, and business partners with a broad range of social perspectives and to continuing to create new, exciting technologies together with you all.

Reference

[1]	Video of OAM-MIMO Multiplexing Transmission Technology for Terabit-class Wireless Transmission, https://www.youtube.com/watch?v=5P6Igla2krg