Researchers at MIT have demonstrated an underwater network that can transmit signals across kilometer-scale distances without any battery.
According to the researchers, the network uses about one-millionth the power of existing underwater communication methods and the battery-free capability makes it feasible for applications such as aquaculture, coastal hurricane prediction and climate change modeling.
The underwater backscatter enables low-power communication by encoding data in sound waves where it reflects, or scatters, back toward a receiver. These innovations can be reflected signals precisely directed at the source, MIT said. Because of this, less signal scatters in the wrong directions, allowing for longer-range communication.
“What started as a very exciting intellectual idea a few years ago — underwater communication with a million times lower power — is now practical and realistic,” said Fadel Adib, associate professor in the Department of Electrical Engineering and Computer Science at the MIT Media Lab. “There are still a few interesting technical challenges to address, but there is a clear path from where we are now to deployment.”
Testing the system
MIT tested the system in a river and an ocean where the device exhibited a communication range more than 15 times farther than previous devices.
The issue was that the experiments were limited by the length of the docks available to researchers. So the team developed an analytical model to predict the technology’s maximum range, finding the retrodirective system could communicate across kilometer-scale distances.
How it works
The underwater device uses an array of nodes made from piezoelectric materials to receive and reflect sound waves. The materials produce an electric signal when mechanical force is applied. When sound waves hit the nodes, they vibrate and convert the mechanical energy into an electric charge. The charge is then used to scatter some of the acoustic energy back to the source where it transmits data that a receiver decodes based on the sequence of reflections.
However, only a small fraction researches the source because the backscattered signal travels in all directions, reducing the signal strength and communication range. To deal with this issue, MIT researchers used a Van Atta array, a 70-year-old radio device. The device has symmetric pairs of antennas that are connected so that the array reflects energy back in the direction it came from.
Connecting the piezoelectric nodes to the Van Atta array reduces the efficiency but using a transformer between the pairs of connected nodes allowed it to reflect the maximum amount of energy back to the source.
“Both nodes are receiving and both nodes are reflecting, so it is a very interesting system,” said Aline Eid, an assistant professor at the University of Michigan who worked on the project. “As you increase the number of elements in that system, you build an array that allows you to achieve much longer communication ranges.”
MIT tested the array in more than 1,500 trials at Charles River in Cambridge, Massachusetts, and in the Atlantic Ocean off the coast of Falmouth, Massachusetts. The device was able to achieve communication ranges of 300 meters, which is more than 15 times longer than previously demonstrated.
The full research can be found on MIT’s SIGCOMM paper.
