MIT researchers have demonstrated the first ultra-low-power underwater networking and communications system that can transmit signals over a kilometer range. The technology, which researchers began developing several years ago, uses about a million times less power than existing underwater communication methods. By extending the communication range of battery-free systems, researchers are making the technology more feasible for applications such as aquaculture, coastal hurricane prediction and climate change modeling.
"Underwater communications at a million times less power was a very exciting idea just a few years ago, but now it's feasible. While there are still some interesting technical challenges that need to be solved, there is a clear path from where we are now to deployment," said Fadel Adib, associate professor in the Department of Electrical Engineering and Computer Science and director of the Signal Dynamics Group at the MIT Media Lab.
The device is a piezoelectric sensor array that enables battery-free underwater communications. Image source: Provided by researchers
Underwater backscatter enables low-power communications by encoding data in sound waves that are then reflected or scattered back to the receiver. These innovations allow reflected signals to be directed more precisely to the source.
Because of this "reverse directivity," less signal is scattered in the wrong direction, allowing for more efficient, longer-range communications. When tested in rivers and oceans, the reverse-direction device communicated at more than 15 times the range of previous devices. However, the experiments were limited by the length of dock available to the researchers.
To better understand the limits of underwater backscattering, the team also developed an analytical model to predict the technology's maximum range. They validated the model using experimental data, showing that their reverse-directed system can communicate over a kilometer range.
The researchers shared these findings in two papers that will be presented at this year's ACMSIGCOMM and MobiCom conferences. Adib is the senior author of these two papers. He co-authored the SIGCOMM paper with former postdoctoral fellow Aline Eid, now an assistant professor at the University of Michigan, and research assistant Jack Rademacher, as well as research assistants Waleed Akbar, Purui Wang and postdoctoral fellow Ahmed Allam. The co-first authors of the MobiCom paper are also Akbar and Allam.
Three team members conduct experiments at Woods Hole Research Institute. Image source: Provided by researchers
Communicate using sound waves
Underwater backscatter communications devices utilize arrays of nodes made of "piezoelectric" materials to receive and reflect sound waves. These materials produce electrical signals when acted upon by mechanical forces.
When sound waves hit the nodes, they vibrate and convert mechanical energy into electrical charges. The node uses electrical charges to scatter part of the acoustic energy back to the source, transmitting data, and the receiver decodes the data based on the sequence of reflections. However, because backscattered signals propagate in all directions, only a small portion reaches the sound source, reducing signal strength and limiting communication range.
To overcome this problem, the researchers took advantage of a 70-year-old radio device called the Van Atta array, in which a symmetrical pair of antennas are connected in such a way that the array reflects energy back in the direction of the signal's source.
However, connecting piezoelectric nodes to form a Van Atta array reduces its efficiency. The researchers avoided this problem by placing a transformer between pairs of connected nodes. Transformers transfer electrical energy from one circuit to another, allowing nodes to reflect maximum energy back to the source.
"Both nodes are receiving, and both nodes are reflecting, so it's a very interesting system," Ed explains. "As the number of elements in the system increases, you can build an array that allows for longer communication distances."
Additionally, they used a technique called cross-polarity switching to encode binary data in the reflected signal. Each node has a positive and a 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 "1 bit".
But if the researchers reverse the polarity and connect the negative and positive poles to each other, the reflected signal is "zero."
"It's not enough to simply connect piezoelectric nodes together. By alternating the polarity of the two nodes, we can transmit data back to the remote receiver," Rademacher explains.
When building the VanAtta array, the researchers discovered that if the connected nodes were too close, they would block each other's signals. They devised a new design in which the nodes are interleaved so that signals can reach the array from any direction. With this scalable design, the more nodes in the array, the greater the communication range.
Working with the Woods Hole Oceanographic Institution, they conducted more than 1,500 experimental tests of the array on the Charles River in Cambridge, Massachusetts, and in the Atlantic Ocean off the coast of Falmouth, Massachusetts. The device has a communication range of 300 meters, which is more than 15 times longer than what they have previously demonstrated.
However, due to insufficient dock space, they had to shorten the experiment.
Simulation maximum
This inspired the researchers to build an analytical model to determine the theoretical and practical communication limits of this new underwater backscatter technology. Building on their group's research into radio frequency identification (RFID), the research team crafted a model to capture the impact of system parameters, such as the size of the piezoelectric nodes and the input power of the signal, on the device's underwater operating range.
"This is not a traditional communications technology, so you need to understand how to quantify reflections. What are the roles of the different components in this process?" Akbar said. For example, researchers needed to derive a function that captures the amount of signal reflected from an underwater piezoelectric node of a specific size, which was one of the biggest challenges in developing the model.
They used these insights to create a plug-and-play model where users can enter information such as input power and piezoelectric node size and get an output that shows the expected range of the system.
They evaluated the model against experimental data and found that the model could accurately predict the range of inverse acoustic signals with an average error of less than 1 dB. Using this model, they found that underwater backscatter arrays have the potential to achieve communication distances of kilometers long.
"We're creating a new ocean technology and pushing it into the realm of 6G cellular networks that we've been doing," Adib said. "It's a very meaningful thing for us because we're now starting to see this technology very close to reality."
The researchers plan to continue studying the underwater backscatter VanAtta array, perhaps using ships, so they can evaluate longer communication ranges. At the same time, they also plan to release tools and datasets so that other researchers can build on them. At the same time, they are also beginning to move toward commercializing the technology.
"Limited range has been an open issue for underwater backscatter networks, hindering their use in real-world applications." Omid Abari, assistant professor of computer science at UCLA, said: "This paper enables underwater communications to achieve long-distance transmission while operating with minimal energy, thus enabling future underwater communications." This paper takes an important step forward in the field of communication. This paper introduces VanAttaReflector array technology into an underwater backscatter environment for the first time and demonstrates the advantages of this technology in increasing communication range by several orders of magnitude. This can bring battery-free underwater communication one step closer to reality, enabling applications such as underwater climate change monitoring and coastal monitoring."