Ocean Connectivity at Ultra-Low Power

Purui Wang, Jack Rademacher 

Van-Atta Acoustic Backscatter (VAB) is the first technology for ultra-low-power underwater networking and communication, which can transmit signals across kilometer-scale distances. 

The technology builds on a technique we invented—called underwater backscatter—which communicates at 1 million times less power than existing underwater communication methods. 

The newest generation of our technology is the first that can communicate at ultra-low power and over distances long enough to support practical applications in coastal monitoring, underwater climate change monitoring, weather prediction, disaster response, aquaculture, and much more.

How does it work? 

Underwater backscatter nodes communicate by reflecting underwater acoustic signals. One challenge with our earlier generations is that they were not very efficient: they reflected signals in all directions. The key innovation in our newest design is that it can retrodirect its signal. Specifically, if sound arrives from a specific angle, it is re-transmitted back towards that same angle that it arrived from—this is retrodirectivity. The special configuration of piezoelectric transducers is known as a Van Atta array, and it sends incoming sound back in the direction that it came from instead of towards undesired directions, regardless of the array's orientation. This property enables the system to focus sound energy where it matters most, allowing the retroreflected sound to travel more efficiently and across longer distances. The operation of Van-Atta Acoustic Backscatter (VAB) is shown in the figure below.

How was this tested?

We performed over 1,500 trials of VAB in both the Charles River and the Atlantic Ocean. We carefully evaluated the system at multiple ranges, and demonstrated its robustness and reliability under highly variable environmental conditions such as rain, high winds, and even snow. Shown below is an image of the 4x2 version of our Van Atta array. 

Over what distances can it operate?

Over the course of the 1,500 trials, we found that VAB was capable of reliably communicating at a range of 150 meters. However, this distance was limited by our testing environment, in fact the MIT Sailing Pavilion dock is only 155 meters long! Below is a birds-eye view of our testing environment at the Charles River. 

We demonstrated that VAB is capable of transmitting bits with 99.8% accuracy up to 150 meters, which is sufficient for reliable communication. Prior work at only 10 meters of range achieves 97% accuracy, and fails to accurately transmit 50% of the bits beyond 10 meters. VAB outperforms prior work by more than 15 times, and demonstrates for the first time sufficiently long ranges for practical sensing applications in the ocean. 

We've also rigorously developed the first comprehensive theoretical and empirical analysis of the underwater backscatter channel, including the downlink and uplink of end-to-end backscatter. This work serves as the theoretical basis for designing and implementing long-range underwater backscatter. In this work we show that underwater backscatter communication is possible over kilometer-scale distances, enabling applications in deep-sea exploration, under-ice navigation, and early-warning systems for disaster response.