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Feature Article

New Concepts in High-Data-Rate, Cable-Free Underwater Networks
Network Development for High-Frequency Hydroacoustic Modems Would Allow High-Data-Rate, Long-Range Communication

By Craig Benson
Electrical Engineer
Underwater Networks Group
University of New South Wales
Canberra, Australia

While submarine cables provide excellent long-range, high-capacity communications, their point-to-point nature is limiting in many underwater applications that require maneuvering, such as real-time data distribution from sensor networks, and real-time tasking of AUVs, divers and cooperative swarms of vehicles. Unlike in the terrestrial environment, wireless communication through water, particularly saltwater, is problematic.

Methods of cable-free communication can be broadly classified into those that use light, those that use radio waves and those that use acoustic signals. In clear conditions, underwater optical signaling can provide high data rates at relatively low cost via the use of LEDs and photodetectors. Optical signaling is, however, greatly affected by turbidity, which can curtail communication to a range of a single arm's length. Unfortunately, radio waves also do not propagate well in saltwater due to the conductivity of the medium, limiting underwater radio communication to low frequencies and very short ranges.

Most underwater communication systems utilize acoustic signals because they propagate reliably in all water conditions. However, acoustic waves are subject to absorption in water, and the rate of absorption increases as the acoustic signal frequency increases. Consequently, the highest acoustic carrier frequency that can be used for communication reduces in proportion to distance. Long-range communication links can only be achieved at low frequencies, with limited signal bandwidth, which means that the achievable data rate is low. Commercial modems tend to provide robust connections in the range of tens to a few thousand bits per second over several kilometers. Less robust communication can be achieved over similar distances by research modems or commercial modems in slower modes at a data rate up to approximately 25 kilobits per second, with carrier frequencies around 25 kilohertz.

Such results are hard-won by advanced signal processing. Further advances will allow more reliable data rates in more difficult conditions. Because of the limited bandwidth available at low carrier frequencies, it is unlikely that substantial increases in data rates will be achieved at commercial modem frequencies and link ranges.

Higher data rates up to 500 kilobits per second have been reported for very short acoustic links, at a carrier frequency of 1 megahertz. Also noteworthy is the Hermes modem developed in the last few years by Pierre-Phillipe Beaujean that has achieved 87 kilobits per second in the difficult harbor environment. These high-data-rate systems operate at carrier frequencies in the hundreds of kilohertz or higher and have signal bandwidths much broader than possible at low frequencies. However, these modems operate only over distances of 50 to 200 meters.

From a user's perspective, there are situations where a short link in the 100-meter range is useful, but in many cases, longer communication distances are required. Such long communication ranges do not mean that the individual communication links must be long-range, in the same way that an international call can be placed from a cell phone even though the base station to which the phone is directly connected is only a few kilometers away. High-data-rate communications can be made available to underwater systems through a network where each of the links is short-range; it is highly desirable that each link has the highest data rate and the longest range possible.

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Craig Benson is an electrical engineer with the Underwater Networks Group at the University of New South Wales, Canberra, Australia. The aim of the group is to provide underwater users with affordable high-data-rate, cable-free communications.

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