Analysis and Protocols to Establish Downlink for Communication Across the Air-Water Interface

Abstract

In underwater wireless networks (UWNs), conventionally there is no direct communication between an underwater node and a remote command center, because there is no known physical signal that propagates well in both the water and air mediums. Radio signals are the popular choice in the air while acoustics are the prime means for communication underwater. Hence, the typical architecture of an UWN includes surface nodes or gateways which have dual modems, an acoustic modem to communicate with underwater nodes and a radio modem to interact with terrestrial command centers. However, such an architecture is logistically complicated and could expose the network to security threat. Therefore, it is desirable to establish a direct communication from air to underwater without having any intermediate surface node. This dissertations opts to fulfill such a need by pursuing two viable solutions, namely, using visual light communications (VLC) and exploiting the optoacoustic (photoacoustic) effect. Through extensive analysis and experiments we show that VLC is an effective means for scenarios where the distance between the airborne and underwater nodes is relatively short. We first analyze the coverage area and intensity inside the water for a light transmitter placed in the air. We then provide guidelines for using single and multiple light sources to establish robust VLC links under rough environmental conditions like high water current and turbidity. Furthermore, we investigate effective modulation techniques that suit VLC. For long distance communication, VLC is not a viable option since light gets absorbed at an exponentially growing rate with the increase of water depth. Photoacoustic (PA) energy transfer mechanism is the promising method for long distance communication. Although the use of the photoacoustic mechanism is quite common in medical imaging, little progress has been made on building the communication protocol stack for air-to-underwater communication. In our research work, at first, we have studied the channel characteristics of the PA based air-to-underwater communication. We show that by carefully choosing the relative position of the airborne unit and underwater node, we can generate a narrowband acoustic signal which is very crucial for long distance communication. Moreover, we develop suitable modulation and demodulation schemes for PA communication.