MIT researchers have demonstrated the first ultra-low-power underwater networking and communication system that can transmit signals over kilometers.

The technique, which scientists began developing a few years ago, uses about one millionth the power of existing underwater communication methods. By extending the communication range of their battery-free system, the researchers made the technology more feasible for applications such as aquaculture, coastal hurricane forecasting and climate change modeling.

“What started as a very exciting intellectual idea a few years ago – underwater communication with a million times less power – is now practical and real. There are still some interesting technical challenges to be solved, but there is a clear path from where we are now to implementation,” says Fadel Adib, assistant professor in the Department of Electrical Engineering and Computer Science and director of the Signal Kinetics Group. MIT Media Lab.

Underwater backscatter enables low-power communication by encoding data in sound waves that they reflect or scatter back to the receiver. These innovations allow for more precise targeting of reflected signals to their source.

This “retrodirectivity” results in less signal scattering in the wrong direction, resulting in a more efficient and longer connection.

After being tested in a river and ocean, the retro-directive device showed a communication range that was more than 15 times greater than previous devices. However, experiments were limited by the length of docks available to researchers.

To better understand the limits of underwater backscatter, the team also developed an analytical model to predict the maximum range of the technology. The model, which they validated using experimental data, showed that their retrodirectional system could communicate over distances on the scale of kilometers.

The researchers shared these findings in two papers to be presented at this year’s ACM SIGCOMM and MobiCom conferences. Adib, senior author of both articles, joins SIGCOMM paper co-authors Aline Eid, a former postdoc who is now an assistant professor at the University of Michigan, and Jack Rademacher, a research assistant; as well as research assistants Waleed Akbar and Purui Wang and postdoc Ahmed Allam. The MobiCom paper also written by one of the main authors Akbar and Allam.

Communication with sound waves

Underwater backscatter communication devices use a series of nodes made of “piezoelectric” materials to receive and reflect sound waves. These materials generate an electrical signal when subjected to mechanical force.

When sound waves hit the nodes, they vibrate and convert mechanical energy into electrical charge. Nodes use that charge to scatter some of the acoustic energy back to the source, transmitting data that the receiver decodes based on the sequence of reflections.

However, since the backscattered signal travels in all directions, only a small fraction reaches the source, reducing signal strength and limiting communication range.

To overcome this challenge, the researchers used a 70-year-old radio device called a Van Atta array, in which symmetrical pairs of antennas are connected so that the array reflects energy back in the direction it came from.

However, combining piezoelectric assemblies to form a Van Atta array reduces their efficiency. The researchers avoided this problem by placing a transformer between pairs of connected nodes. A transformer, which transfers electricity from one circuit to another, allows the nodes to reflect the maximum amount of energy back to the source.

“Both nodes receive and both nodes reflect, so it’s a very interesting system.” When you increase the number of elements in that system, you create an array that allows you to achieve much longer communication ranges,” Eid explains.

Additionally, they used a technique called cross-polarity switching to encode the binary data in the reflected signal. Each node has a positive and negative terminal (like a car battery), so when the positive terminals of two nodes are connected and the negative terminals of two nodes are connected, the reflected signal is “one”.

However, if the scientists reverse the polarity, and the negative and positive terminals are connected to each other, then the reflection is “slightly zero”.

“It is not enough just to connect the piezoelectric assemblies. By changing the polarity between the two nodes, we can transmit the data back to the remote receiver,” explains Rademacher.

When designing the Van Atta array, the researchers found that if the connected nodes were too close, they would block each other’s signals. They developed a new design with spaced nodes that allow signals to reach the array in any direction. With this scalable design, the more nodes an array has, the greater its communication range.

They tested an array of more than 1,500 experimental tests in the Charles River in Cambridge, Massachusetts, and in the Atlantic Ocean off the coast of Falmouth, Massachusetts, in collaboration with the Woods Hole Oceanographic Institution. The device achieved a communication distance of 300 meters, more than 15 times longer than before.

However, they had to cut the experiments short because they ran out of space on the dock.

When simulating the max

This inspired the researchers to develop an analytical model to determine the theoretical and practical communication limits of this new underwater backscatter technology.

Building on their group’s work with RFID, the team carefully developed a model that captured the effect of system parameters such as the size of the piezoelectric assemblies and signal input power on the device’s underwater operating range.

“It’s not a traditional communication technology, so you have to understand how you can quantify the reflection.” What is the role of different components in this process? Akbar says.

For example, the researchers needed to create a function that would capture the amount of signal reflected from an underwater piezoelectric assembly of a certain size, which was one of the biggest challenges of developing the model, he adds.

They used these insights to create a plug-and-play model in which the user could input information such as input power and dimensions of the piezoelectric assembly and receive an output showing the expected range of the system.

They evaluated the model against their experimental test data and found that it can accurately predict the range of backscattered acoustic signals with an average error of less than one decibel.

Using this model, they demonstrated that an underwater backscatter array can achieve a communication range of kilometers.

“We are developing a new ocean technology and promoting its implementation in the field of 6G cellular networks. It is very beneficial for us because now we are starting to see it very close to reality,” says Adib.

The researchers plan to continue studying the underwater Van Atta arrays, possibly using boats, to assess longer communication distances. Together, they intend to release tools and datasets so that other researchers can build on their work. At the same time, they are starting to move towards the commercialization of this technology.

“Limited range has been an open problem with underwater backscatter networks, preventing them from being used in the real world.” “This paper is a significant step forward in the future of underwater communications, enabling them to operate with minimal energy and reach long distances,” said Omid Abari, assistant professor of computer science at the University of California, Los Angeles, who was not involved. with this work. “This paper is the first to implement the Van Atta Reflector array technique in an underwater backscatter setup and demonstrate its benefits in improving communication range. This could bring battery-free underwater communication one step closer to reality, enabling applications such as underwater climate change monitoring and coastal surveillance.

This research was funded in part by the Office of Naval Research, a Sloan Research Fellowship, the National Science Foundation, the MIT Media Lab, and the Doherty Chair in Ocean Utilization.