Salt water is a conductor and as a result, currently only extremely low frequency radio based communication technologies work underwater: These have not been able support GPS, Wi-Fi, or any high data rate communications.
One of the only practical methods to carry information wirelessly undersea is through acoustic wave propagation. Acoustic waves can travel for hundreds of miles, but are five orders of magnitude slower than radio waves. This gives rise to severe reverberation and inter-symbol interference, especially over long distances. Existing acoustic modems can only achieve data rates of a few 100s of bits/second. The research team investigated the development of turbo equalization - a technique that not only mitigates inter-symbol interference but also leverages the diversity for error correction. Additionally harsh Doppler effects warp the time-scale of the waveform randomly, which can be catastrophic for communications if not compensated dynamically. This team developed a turbo equalizer that constantly tracks and compensates for Doppler effects and then built a wireless underwater modem prototype integrating this algorithm. The modem prototype has undergone extensive US Navy field-tests demonstrating that it is both highly reliable and 100 times faster than existing commercial modems.
The oceans cover 71% of the Earth's surface, carry 90% of international trade, are the home for over 95% of the world?s living biomass, and hold the vast majority of its mineral and fossil resources. The exploration, utilization and protection of this space is important to society, but any coordinated effort undersea will require positioning, navigation and communication. Therefore, technical innovation in underwater communication will have profound impact on many fronts: environmental monitoring, scientific exploration, marine renewable energy generation, commercial fishing, defense, shipping route protection and security, and resource discovery and production.
The military, resource discovery and production industries are among the broad user-base that would benefit greatly from this technology. This project brings together researchers from signal processing, communications, and oceanography to provide unprecedented capabilities to the underwater environment.
This project was part of the National Science Foundation I-Corps program. As a part of I-Corps, the team performed an extensive survey of all industries that have major operations underwater. These included commercial fishing, deep sea oil and gas operations, search and rescue, inspection, and a host of other industries. As a result of our study, a key need that was identified for these industries was a wirless data transmission capability. Current wireless underwater modems are advertised with data rates of only a few kbps and the oil and gas industry has found them incapable of handling video and real-time control. Through extensitve interviews with major industries operating underwater, we have identified a critical need for wireless communications capabilities that enable high bandwifth communication, such as supporting video transmission. Today, such communication underwater is almost entirely done through wired links. We have demonstrated the feasibility of wireless underwater communication at data rates greater than 1Mbps. Such data rates are capable of streaming video in realtime. As broadband acoustic signals propagate through water, they suffer extreme Doppler effects. Different propagation paths experience different Doppler and the level of Doppler on each path is highly time-variant. In our work, time-varying Doppler is explicitly modeled, tracked and compensated. In this paper, we provide the results from our acoustic communication experiments conducted in a 50m long wave tank. Our resampling equalizer reliably achieved 1.2Mbps over a distance of 12m.
