February 21, 2024 TOPICS

[Ambitious Graduate Students] Pushing the Limits of Underwater Acoustic Communication—Development of Robust Acoustic Communication Technology for Marine Robots Operating at Maximum Speed—

Taichi Fujita, Takumi Yoshimura, and Kouichi Hashimoto (2nd-year master's program students, Graduate School of Science and Engineering)

The combined area of Japan's territorial waters and exclusive economic zone are the sixth largest in the world, and researchers are proactively exploring the marine environment and seabed resources in these vast waters. This kind of oceanographic research cannot happen without marine robots.

Underwater marine robots are operated from ships, so their lifeline is communication. The success or failure of oceanographic research depends on the extent to which communications with marine robots can be stabilized as well as their mobility in the ocean, where the environment is highly variable.

To dramatically increase the range of activity of marine robots, Taichi Fujita, Takumi Yoshimura, and Kouichi Hashimoto, three second-year master's students in the Graduate School of Science and Engineering who belong to Professor Hiroshi Kubo's Wireless Signal Processing Laboratory, have taken up the challenge of developing the latest acoustic communication technology.

In this edition, we take a look at the story of their successful development of a robust acoustic communication technology that can handle speeds of up to three knots (about 5.5 km/h), which is said to be the maximum speed for marine robots, in an underwater environment that makes wireless communication tough.

What is underwater acoustic communication?

Underwater acoustic communication, which uses sound waves to communicate underwater, faces numerous issues that need to be resolved. The propagation speed of sound waves in water is about 1/200,000th of that of radio waves. This increases the Doppler shift (Doppler effect) and the delay spread, resulting in a significant degradation of communication quality. In addition, the frequency bandwidth available for sound waves is only a few millionths of that for radio waves, making high-speed, high-capacity communications difficult. When the natural factors of the ocean are added to the mix, the problem becomes even more difficult to solve.

Fujita: Communication is highly stable when using a wired connection. However, cables can become entangled in unexpected obstacles on the seafloor, leading to cable cuts, or in the worst-case scenario, the loss of the marine robot itself. On the other hand, stable underwater acoustic communication often requires a large number of receivers, leading to an increase in equipment scale. Operation was prone to instability when using a small number of receivers, so you had to communicate with the marine robot while regulating its speed.

The challenge that Fujita and his team faced was how to achieve stable underwater acoustic communication at three knots, which is said to be the maximum speed for marine robots, while drastically reducing the number of receivers installed.

They conducted their most recent experiment in a shallow-water environment, which imposes harsh operational conditions for underwater acoustic communication. Also, since cost prohibited the use of an actual marine robot, it was substituted with a small vessel towing receivers.


Challenges the team has encountered in their experiments

Fujita and his colleagues each assumed different roles in the experiment. Yoshimura oversaw the management of experiment, while Fujita developed the acoustic communication technology and analyzed the transmitted and received data. Meanwhile, Hashimoto controlled the receivers and performed propagation measurements.

Yoshimura: We conducted this experiment in cooperation of many people, including employees at OKI Com-Echoes Co., Ltd. and the captain of the small vessel towing the receiver. I was in charge of coordinating with the people who cooperated with the experiment as well as managing the experiment on the day of the test, including monitoring the progress of the experiment. As opposed to the laboratory, unexpected things can happen in marine experiments. Therefore, I tried to control the whole process calmly, including the timing of communication, in order to obtain reliable results.

Fujita: My research focuses on underwater acoustic communication technologies effective in harsh environments and the testing of those technologies. Then, I operated a device that sends signals from a transmitter to a receiver and retrieves the data, and then I analyzed that data. This research is based the outcomes that have been accumulated in Professor Kubo’s laboratory over the years, and it made full use of the knowledge of Mr. Yoshimura, Mr. Hashimoto, and their colleagues.

Hashimoto: I rode on the vessel that towed the receiver, where I communicated closely with the captain while controlling the equipment. I also analyzed the propagation environment to determine how the sound waves reached the receiver, and I made minor improvements to our experimental methods.

The experiment the team conducted was particularly meaningful in that the members not only leveraged their respective research findings, they also mobilized all of the knowledge that has been accumulated over the past 12 years in Professor Kubo’s laboratory.

Yoshimura: Since its inception, our laboratory has been working toward the goal of communicating in a mobile environment at a speed of three knots. Our predecessors conducted many experiments, starting with a communication test at a distance of five meters on Lake Biwa about 10 years ago, followed by a long-distance test at 370 meters off the coast of Izu, and a low-speed travel test at one knot. In this process, we have inherited many things from alumni, such as simulators for evaluating communication methods in advance, methods for measuring the actual environment and analyzing its impact on communication, and know-how on conducting experiments.

The team adjusts equipment just before the experiment. Left: Transmitter
Right: Receiver

The experiment, which was truly a team effort, was conducted in September 2023. However, an unexpected problem arose, even though the preliminary simulation should have been more than enough to prepare them— bubbles!

Overcoming problems to achieve more robust performance

In the experiment, a receiver towed by a small vessel was substituted for a marine robot. However, the three-knot travel speed caused the receiver to float close to the sea surface, resulting in a significant increase in noise from the bubbles generated by the vessel.

Yoshimura: The figures from the simulation did not directly indicate that bubbles would be an issue, but our partners at OKI Com-Echoes told us to watch out for bubbles. Actually, Hashimoto also suspected that bubbles might be a problem during the environmental evaluation process, and this concern was right on target.

The team had about one month until they conducted the next experiment. When we asked the three members about some new problems that have arisen, they provided some surprising responses.

Fujita: To be honest, the sense was this was not a very serious issue. On the other hand, if we could overcome this problem, it would mean that we could establish our acoustic communication technology under even harsher conditions. What makes research enjoyable is coming to understand those things you don’t know. For this reason, we engaged in a lot of dialogue to fine-tune our approach ahead of the next experiment.

Hashimoto: I also did not take the issue too seriously. I was confident that I could fine-tune the experiment based on previous simulations and the results of this experiment, so I was able to turn my sights to the next experiment without letting it become a mental burden. That being said, the toughest aspect was the drain on our physical health, seeing as we conducted the experiments in September, when it was extremely hot, and in November, when it was extremely cold. (laughs)

In the experiment in November, the team doubled the weight they attached to the receiver and extended the length of the cable to keep the receiver from floating too far upward and to minimize the effects of the bubbles. In addition, to cope with the time-varying propagation environment, which is one of the most important issues in underwater acoustic communication, the team successfully developed a signal reception method that predicts the current propagation environment based on the past environment. What’s more, they boosted their system’s tolerance to large delay spread by increasing the delay spread that can be handled by orthogonal frequency division multiplexing (OFDM), a technology that is used for digital terrestrial broadcasting and other applications.

As a result, the team achieved the remarkable feat of being able to handle travel speeds of three knots with just one or two receivers, which is far fewer than was previously possible. This marked the moment when the goal of Professor Kubo’s laboratory, which he and his students had been working on for 12 years, was realized.

A scene from the experiment conducted in November.

The team’s encounter with underwater acoustic communication

Although the three members achieved significantly better performance than the results of previous experiments, their respective encounters with acoustic communication were quite different.

Fujita: I had no interest in the communications field until high school, but before I chose which laboratory to join, it was the words of Professor Kubo that drew me to his laboratory: "Underwater acoustic communication is the last frontier.” After that, I decided to pursue a master's degree to further my study of acoustic communication.

Yoshimura: I have always been interested in marine life, so I thought that if effective acoustic communication could be established, it could increase the range of motion of marine robots and lead to the discovery of new marine creatures. I became increasingly fascinated by acoustic communication, and I have been researching underwater acoustic communication to this day.

Meanwhile, Hashimoto says it was an episode from his high school days that led him to join Professor Kubo’s laboratory.

Hashimoto: When I was in high school, I had the opportunity to experience telecommunications in an IT class. But I had trouble establishing communications back then. So, when I entered university, I was ready to tackle the challenge again. I was drawn to Professor Kubo’s laboratory by the joy everyone felt after their first successful communication test and the fact that underwater acoustic communication is an interesting field.

What all three have in common is a relentless curiosity and passion for underwater acoustic communication. The three members, who have been working hard to overcome obstacles and challenges, looked confident and satisfied when they talked about what they had accomplished.

Analyzing data on site.

Moving from telecommunications to new fields

After completing the program, all three plan to get a fresh start working in fields other than telecommunications. Fujita will go to work for an industrial equipment manufacturer, Yoshimura a medical equipment manufacturer, and Hashimoto a testing equipment manufacturer.

Fujita: During my time in Professor Kubo’s laboratory, I gained experience in theory, manufacturing, and field testing. In particular, working on this experiment taught me a valuable lesson: that it is up to me to recognize the important information contained in the data. I would like to analyze everyday phenomena while keeping my eyes open to a wide array of possibilities.

Yoshimura: By working on this experiment, I think I have gained the ability to see the big picture. Successfully completing such a difficult experiment has also given me confidence. Since I have been assigned to a production engineering position at a manufacturer, I hope I can utilize my experience from the experiment to take a bird's eye view of the whole process when handling any task and to do work that is beneficial to others.

Hashimoto: In our acoustic experiments, we found solutions to problems based on data from the experimental environment that we analyzed from various angles. This made me realize the importance of deepening mutual understanding by looking at things from perspectives other than my own and by discussing things together rather than going it alone. I would like to use this experience to find solutions to difficult problems.

Leveraging their inherent boldness and the problem-solving skills they cultivated in acoustic communication experiments, these three are sure to make breakthroughs in new fields.

L to R: Takumi Yoshimura, Taichi Fujita, and Kouichi Hashimoto

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