MIT scientists have taken a step toward cracking an age-old challenge related to wireless communication: direct data transmission between airborne and underwater devices.
Presently, underwater sensors cannot transmit data to those on land, as both use various wireless signals that only operate in their respective mediums. Radio signals that travel via air die very quickly in water. Acoustic signals, or sonar, conveyed by underwater devices mostly bounce off the surface without ever breaking through. This causes ineffectiveness and other issues for a range of applications, such as submarine-to-plane communication and ocean exploration.
In a paper being presented at this week’s SIGCOMM conference, MIT Media Lab scientists have built a system that handles this problem in an innovative way. An underwater transmitter guides a sonar signal to the water’s surface, causing minute vibrations that match the 1s and 0s transmitted. Above the surface, an extremely sensitive receiver reads these tiny disturbances and decodes the sonar signal.
“Trying to cross the air-water boundary with wireless signals has been an obstacle. Our idea is to transform the obstacle itself into a medium through which to communicate,” says Fadel Adib, an assistant professor in the Media Lab, who is leading this study. He co-authored the paper with his graduate student Francesco Tonolini.
The system, referred as “translational acoustic-RF communication” (TARF), is still in its primary stages, Adib says. But it signifies a “milestone,” he says, that could pave the way to new capabilities in water-air communications. Using the system, military submarines, for example, would not have to surface to communicate with airplanes, revealing their location. Also, underwater drones that monitor marine life would not have to continually resurface from deep dives to transmit data to scientists.
Another potential application is assisting searches for planes that disappear underwater. “Acoustic transmitting beacons can be implemented in, say, a plane’s black box,” Adib says. “If it transmits a signal every once in a while, you’d be able to use the system to pick up that signal.”
Present-day’s technological workarounds to this wireless communication problem suffer from numerous downsides. Buoys, for example, have been engineered to gather sonar waves, process the data, and transmit radio signals to airborne receivers. But these can float away and vanish. Many are also required to encompass large areas, making them unfeasible for, say, submarine-to-surface communications.
TARF includes an underwater acoustic transmitter that transmits sonar signals using a regular acoustic speaker. The signals travel as pressure waves of different frequencies equivalent to different data bits. For instance, when the transmitter wants to convey a 0, it can convey a wave traveling at 100 Hertz; for a 1, it can convey a 200-Hertz wave. When the signal touches the surface, it causes minute ripples in the water, just a few micrometers in height, matching those frequencies.
To realize high data rates, the system conveys numerous frequencies simultaneously, building on a modulation scheme used in wireless communication, known as orthogonal frequency-division multiplexing. This allows the scientists to convey hundreds of bits simultaneously.
Located in the air above the transmitter is a new type of very-high-frequency radar that processes signals in the millimeter wave spectrum of wireless transmission, between 30 and 300 gigahertz. (That’s the band where the forthcoming high-frequency 5G wireless network will function.)
The radar, which resembles a pair of cones, conveys a radio signal that reflects off the vibrating surface and rebounds back to the radar. Due to the way the signal hits the surface vibrations, the signal returns with a somewhat modulated angle that matches precisely to the data bit transmitted by the sonar signal. A vibration on the water surface signifying a 0 bit, for example, will make the reflected signal’s angle to vibrate at 100 Hertz.
The radar reflection is going to vary a little bit whenever you have any form of displacement like on the surface of the water. By picking up these tiny angle changes, we can pick up these variations that correspond to the sonar signal.
Fadel Adib, Assistant Professor
Listening to “The Whisper”
A significant challenge was helping the radar identify the water surface. To accomplish that, the MIT team used a technology that detects reflections in an environment and organizes them by power and distance. As water has the strongest reflection in the new system’s environment, the radar recognizes the distance to the surface. Once that’s established, it narrows in on the vibrations at that distance, disregarding all other adjacent disturbances.
The next huge challenge was trapping micrometer waves enclosed by much larger, natural waves. The smallest ocean ripples on calm days, known as capillary waves, are just about 2 cm tall, but that’s 100,000 times larger than the vibrations. Rougher seas can form waves 1 million times larger. “This interferes with the tiny acoustic vibrations at the water surface,” Adib says. “It’s as if someone’s screaming and you’re trying to hear someone whispering at the same time.”
To crack this, the scientists developed advanced signal-processing algorithms. Natural waves take place at about 1 or 2 Hertz - or, a wave or two moving over the signal area per second. The sonar vibrations of 100 to 200 Hertz, however, are a hundred times faster. Due to this frequency differential, the algorithm zeroes in on the rapid-moving waves while disregarding the slower ones.
Testing the Waters
The scientists put TARF through 500 test runs in a water tank and in two varied swimming pools on MIT’s campus.
In the tank, the radar was positioned at ranges from 20 cm to 40 cm above the surface, and the sonar transmitter was positioned from 5 cm to 70 cm below the surface. In the pools, the radar was placed about 30 cm above surface, while the transmitter was immersed about 3.5 m below. In these experiments, the scientists also had swimmers generating waves that rose to nearly 16 cm.
In the two settings, TARF was able to precisely decode numerous data - such as the sentence, “Hello! from underwater” - at hundreds of bits per second, similar to regular data rates for underwater communications. “Even while there were swimmers swimming around and causing disturbances and water currents, we were able to decode these signals quickly and accurately,” Adib says.
In waves greater than 16 cm, however, the system could not decode the signals. The subsequent steps are, among other things, tweaking the system to operate in rougher waters. “It can deal with calm days and deal with certain water disturbances. But [to make it practical] we need this to work on all days and all weathers,” Adib says.
The MIT scientists also hope that their system could ultimately enable a plane or airborne drone flying across a water’s surface to continually pick up and decode the sonar signals as it whizzes by.
The study was supported, partly, by the National Science Foundation.