12-11-2020 | | By Robin Mitchell
Recently, MIT announced their development of an underwater communication system that not only allows for positioning but also for battery-less operation. What challenges do underwater communication systems face, why is GPS practically impossible underwater, and how have researchers overcome these challenges?
Wireless communication can be achieved using many different techniques, including sound, light, and induction. However the most popular by far is via radio. Radio waves have many advantages when compared to other wireless methods, and the wide range of available frequencies allows for fine-tuning for each application. For example, radio waves of long wavelengths can bounce off the atmosphere and propagate very long distances but generally have low bandwidth and thus are ideal for use in radio stations and long-range communication. Microwaves have wavelengths much shorter than typical radio waves which means that their range is reduced, but their bandwidth is much greater and thus are often found in IT applications such as Bluetooth and Wi-Fi.
However, radio is only advantageous in either a vacuum or in gasses, if we move the transmission medium to water (such as the ocean), then the story changes dramatically. The first issue faced with underwater communication is that water is a strong absorber of radio waves (especially those in the 2.4GHz range). This means that radio waves cannot propagate far underwater (unless they are extremely low frequency, but this results in bits per second speeds). The second issue faced is that the surface of the water can result in strong reflections, thus allowing for echoes to interfere with signals. This is why many underwater communication systems often use acoustic methods as sound travels incredibly far, thanks to the dense nature of water.
The Importance of Positioning, and how Underwater Vehicles Struggle
Data communication is not the only challenge faced by underwater systems; positioning can be incredibly challenging too. While surface craft can take full advantage of GPS, the underwater craft cannot as radio waves used by GPS are readily absorbed by water. This makes many underwater challenges tricky including asset tracking, recoding locations of underwater points of interest (such as shipwrecks), and environment avoidance. Underwater positioning is further complicated when needing to track, and position assets that may be hard to gain access to as any power source used would have to be portable and survive for long periods of time. If acoustic systems are used for communication and positioning, then this further puts a strain on any power source as acoustic systems are often power-hungry.
Understanding these challenges, researchers from MIT have developed an underwater communication system that not only allows for positioning but also for battery-less operation. In 2019, a team of researchers from MIT first developed a method for underwater communication that utilises piezo elements but doesn’t require a power source. Essentially, the system developed utilises a master/slave system whereby a master device (who has a power source), emits a strong acoustic signal. Slave devices, which could include asset trackers or beacons, detect this sound wave and use their piezo transducers to generate power from the acoustic signal. Once powered, these devices can then either choose to remain silent (i.e. sending a 0), or retransmit the signal back to the master (i.e. sending a 1). Researchers demonstrated this system, and the pair were able to operate up to 10 meters away while transmitting data at 3KB per second.
However, the researchers continued to work on the system to try to integrate a positioning scheme. One year one, and researchers from MIT have done just this, and their system utilises a new technique called Underwater Backscatter Localisation (UBL). Simply listening to a signal underwater cannot tell you much with regard to positioning as echoes hitting underwater obstacles, as well as the water surface, can cause large amounts of interference. To get around this issue, the researchers deployed frequency hopping whereby a transmitter emits sounds on different frequencies and then compares the incoming phases of received signals to position a distant device accurately. The use of different frequencies works well thanks to how different frequencies reflect off objects and surface differently. But the system was still able to use the same battery-less technology used in the underwater communication system developed in 2019. However, researchers have had to drop the data rates to hundreds of bits per second when used in shallow water, a problem still being worked on.