By combining new and old technologies, a team of researchers from the Massachusetts Institute of Technology was able to develop a long-range underwater communications system with extremely low power consumption.
Underwater wireless communication systems typically use acoustic waves instead of radio waves, and there are a number of reasons for this. The underwater environment presents its own unique challenges that make the use of acoustic waves more feasible. First of all, it is worth noting that water is a medium with a high degree of attenuation of radio frequency signals – this means that it is quite difficult for electromagnetic waves to propagate through the water column due to the high absorption and scattering of the signal. Conversely, acoustic waves, which are mechanical vibrations in water, are characterized by less attenuation, which makes them a more practical choice for underwater communications.
However, despite the fact that acoustic signals are a fairly reliable means of underwater communication, we should not forget about some of their disadvantages. One of their main limitations is the relatively high power consumption required for efficient generation and transmission of acoustic waves. Acoustic transducers, which convert electrical signals into sound waves and vice versa, require significant amounts of energy to operate, which can be a problem for battery-powered underwater devices. In addition, dispersion and scattering of acoustic waves in water can lead to deterioration in signal quality and the occurrence of interference that affects communication reliability. In addition, acoustic signals have a shorter transmission range than radio frequency signals in air or space.
In recent years, a new promising communication method has emerged, called “underwater backscattering”. By using data encoding in sound waves that are reflected back to the receiver (rather than generating the signals themselves), this technology allows very low power acoustic signals to be transmitted and received. This method could easily be a million times more energy efficient than other underwater communication methods. Unfortunately, however, the range of an underwater backscatter system is typically limited to tens of meters, which is unlikely to be useful for most real-world application scenarios.
However, some interesting developments by researchers from the Massachusetts Institute of Technology may well allow one to bypass this limitation of communication range. The scientists are proposing some changes to existing underwater backscatter techniques that would make it possible to transmit signals over long distances—perhaps even up to a kilometer—over low-power systems without a battery. Such a communications system could pave the way for new applications for aquaculture, hurricane forecasting services and climate modeling.
To make the communication system work with the ability to propagate signals over long distances, the researchers needed to overcome one problem, which was that existing underwater backscatter systems transmit signals in all directions, leaving only a small percentage to return to the source. Some rethinking of an old technology called the Van Atta matrix helped them achieve this goal. We are talking about arrays consisting of a symmetrical set of antennas with connections that ensure that signals are reflected only in the direction from which they were received. This way, energy is not dissipated in all directions, and transmission distances can become much, much longer.
The performance of the Van Atta matrix was further enhanced by careful placement of transformers between pairs of connected nodes. Using hardware, the researchers developed a principle for encoding binary digits into reflected signals, after which the system was ready for testing.
To date, researchers have conducted more than 1,500 experiments in rivers and the Atlantic Ocean. These tests demonstrated the system’s ability to transmit signals over at least 300 meters, but there was not enough space for the docking station, leaving the researchers unable to find the system’s operating limits. So they developed a model to predict the device’s maximum range. This model was tested based on the results of previous experiments that were designed to confirm its performance, and as a result it was found to be highly accurate. If this model is to be believed, then in theory the range of action of the new underwater backscatter technology could exceed one kilometer.
Next, the research team plans to test their system using boats, which will allow them to perform some real-world tests testing the theoretical range limit. The scientists also plan to publish their data and toolkit to allow other research groups to improve on their proposed design.
We previously reported that Japanese scientists were able to transmit electric current and data over a distance of more than 10 km through one fiber optic cable.