You can wirelessly communicate with people on the other side of the world in real time, but it’s still impossible for a submarine to talk to an airplane. They both use systems designed for their environment, and those systems aren’t compatible. At least, they weren’t until now. Researchers from MIT have developed a technology to link the underwater world with the open air.
Most wireless communication on earth happens via radio waves, which propagate easily through the air. However, they dissipate quickly after hitting the water. Meanwhile, submarines and other underwater device rely on sonar — sound waves that can travel through water over long distances. These two systems normally don’t connect. Fadel Adib and Francesco Tonolini of MIT Media Lab have created an interlink called Translational Acoustic-RF communication, or TARF.
The system consists of two parts. On the underwater side, there’s a speaker that pumps out sound (similar to sonar). These pressure waves expand outward until they reach the surface. It can use different frequencies to transmit data. For example, a 0 could be a wave in the 100Hz range, and a 1 could be a 200Hz wave.
The sound waves cause minuscule vibrations as they break the surface, but these ripples are just a few micrometers in height. This is usually where the disconnect would happen, but the team built a special radar apparatus that beams signals onto the water’s surface in the millimeter wave range between 30 and 300 gigahertz. This is the same frequency range that will form the backbone of 5G mobile networks. The signals bounce back to the radar cones, but vibrations from sound waves reaching the surface cause small modulations in the signal. By decoding these signals, TARF can assemble the message sent by the underwater speaker.
TARF can support higher data rates by transmitting multiple signals at the same time, using a technique called orthogonal frequency-division multiplexing. This allows a single burst of sound to carry hundreds of bits of data, and the radar receiver is surprisingly good at reading that. The team tested TARF in a water tank and a swimming pool with active swimmers. Even small waves in the surface can be thousands of times larger than the vibrations TARF reads, but the frequency of these waves is much lower. The researchers developed a system that zeroes in on the high frequency, low amplitude signals and filters out everything else.
Early tests have been successful, but TARF is currently limited to a few meters of separation. The team hopes that TARF could eventually let underwater vehicles and sensors communicate with aircraft as they fly overhead.