Networking across boundaries: enabling wireless communication through the water-air interface

Abstract

We consider the problem of wireless communication across medium boundaries, specifically across the water-air interface. In particular, we are interested in enabling a submerged underwater sensor to directly communicate with an airborne node. Today's communication technologies cannot enable such a communication link. This is because no single type of wireless signal can operate well across different media and most wireless signals reflect back at media boundaries. We present a new communication technology, translational acoustic-RF communication (TARF). TARF enables underwater nodes to directly communicate with airborne nodes by transmitting standard acoustic signals. TARF exploits the fact that underwater acoustic signals travel as pressure waves, and that these waves cause displacements of the water surface when they impinge on the water-air boundary. To decode the transmitted signals, TARF leverages an airborne radar which measures and decodes these surface displacements. We built a prototype of TARF that incorporates algorithms for dealing with the constraints of this new communication modality. We evaluated TARF in controlled and uncontrolled environments and demonstrated that it enables the first practical communication link across the water-air interface. Our results show that TARF can achieve standard underwater bitrates up to 400bps, and that it can operate correctly in the presence of surface waves with amplitudes up to 16 cm peak-to-peak, i.e., 100, 000× larger than the surface perturbations caused by TARF's underwater acoustic transmitter.