Underwater Communication System Goes the Distance

Underwater wireless communication mainly relies on acoustic waves rather than radio frequency (RF) or other electromagnetic methods for a number of reasons. The underwater environment has unique challenges that make acoustic communication a more suitable choice. First and foremost, water is a highly attenuating medium for RF signals, which means that electromagnetic waves have difficulty propagating effectively through it due to high signal absorption and scattering. In contrast, acoustic waves, which are mechanical vibrations through water, experience less attenuation, making them a more practical choice for underwater communication.

Although acoustic signals are a viable option for underwater communication, there are some drawbacks to consider. One major limitation is the relatively high power consumption required to generate and transmit acoustic waves effectively. Acoustic transducers, which convert electrical signals into sound waves and vice versa, require a significant amount of energy to operate, which can be a challenge for battery-powered underwater devices. Additionally, the dispersion and scattering of acoustic waves in water can lead to signal degradation and interference, affecting the reliability of communication. Furthermore, acoustic signals have shorter transmission ranges than RF signals in the air or space. As water is denser and more viscous than air, acoustic waves dissipate energy more rapidly, reducing their effective range.

A promising communication method, called underwater backscatter, has emerged in recent years. By encoding data in sound waves that are reflected back towards a receiver (rather than generating the signals itself), this technology has proven that it is capable of very low-power transmission and reception of acoustic signals. It can easily be a million times more energy-efficient than other underwater communication methods. Unfortunately, the range of an underwater backscatter system is typically limited to tens of meters — hardly useful for most real-world applications.

Some clever thinking by researchers at MIT may get around this communication range limitation, however. They have proposed some changes to existing underwater backscatter methods that allow for low-power, battery-free operation, and at long distances — perhaps even up to a kilometer. Such a communications system could unlock new applications in aquaculture, hurricane prediction, and climate modeling.

To achieve longer-distance communications, the researchers needed to deal with the fact that existing underwater backscatter systems transmit signals in every direction, leaving only a small percentage to return to the source. Towards this goal, they revived an old radio technology called a Van Atta array. These arrays consist of a symmetric set of antennas with interconnections that cause signals to be reflected back only in the direction from which they were received. As such, energy is not scattered in every direction, and transmission distances can be much, much longer.

The efficiency of the Van Atta array was further enhanced by careful placement of transformers between pairs of connected nodes. With the hardware in place, they devised a means to encode binary digits into the reflected signals, then the system was ready for testing.

Over 1,500 experiments were carried out in a river, and in the Atlantic Ocean. These trials demonstrated that the system could transmit signals at least 300 meters, but they ran out of space at the dock, preventing them from finding the limit. Next, they developed a model to predict the device’s maximum range. This model was tested against the results of the previous experiments to validate it, and it was found to be highly accurate. If this model is to be trusted, it suggests that the theoretical range of the novel underwater backscatter technology may exceed one kilometer.

Moving forward, the team plans to test their system using boats so that they can perform some real-world tests validating the theoretical range limit. They are also planning to release their data and a suite of tools to allow other research groups to expand upon their work.