Advances in Underwater Wireless Communications

The IEEE Communications Magazine, January 2009, has a interesting section on Underwater Wireless Communications for those who may be interested.

Underwater networks of sensors have the potential to enable unexplored applications and to enhance our ability to observe and predict the ocean. Unmanned or Autonomous Underwater Vehicles (UUVs, AUVs), equipped with underwater sensors, are also envisioned to find application in exploration of natural undersea resources and gathering of scientific data in collaborative monitoring missions. These potential applications will be made viable by enabling communications among underwater devices. UnderWater Acoustic Sensor Networks (UW-ASNs) will consist of sensors and vehicles deployed underwater and networked via acoustic links to perform collaborative monitoring tasks.

The single most killer application for Underwater Wireless Communication would be in disaster prevention. Sensor networks that measure seismic activity from remote locations can provide tsunami warnings to coastal areas, or study the effects of submarine earthquakes (seaquakes).

There are major challenges in the design of underwater acoustic networks that include:

• The available bandwidth is severely limited;
• The underwater channel is impaired because of multipath and fading;
• Propagation delay is five orders of magnitude higher than in Radio Frequency (RF) terrestrial channels, and variable;
• High bit error rates and temporary losses of connectivity (shadow zones) can be experienced;
• Underwater sensors are characterized by high cost because of a small relative number of suppliers (i.e., not much economy of scale);
• Battery power is limited and usually batteries cannot be recharged;
• Underwater sensors are prone to failures because of fouling and corrosion.

There has been intensive research on MAC protocols for ad hoc and wireless terrestrial sensor networks in the last decade. However, due to the different nature of the underwater environment and applications, existing terrestrial MAC solutions are unsuitable for this environment. In fact, channel access control in UW-ASNs poses additional challenges due to the peculiarities of the underwater channel, in particular limited bandwidth, very high and variable propagation delays, high bit error rates, temporary losses of connectivity, channel asymmetry, and extensive time-varying multipath and fading phenomena. Existing MAC solutions are mainly focused on Carrier Sense Multiple Access (CSMA) or Code Division Multiple Access (CDMA). This is because Frequency Division Multiple Access (FDMA) is not suitable for UW-ASN due to the narrow bandwidth in UW-A channels and the vulnerability of limited band systems to fading and multipath. Moreover, Time Division Multiple Access (TDMA) shows a limited bandwidth efficiency because of the long time guards required in the UW-A channel. Furthermore, the variable delay makes it very challenging to realize a precise synchronization, with a common timing reference.

The next challenge is to select a routing protocol from a range of protocols that will work in the best manner with the MAC solution selected.

Another challenge is to select the correct transport layer protocols. A transport-layer protocol is required to achievereliable transport of event features and to perform flow and congestion control. Most existing Transport Control Protocol (TCP) implementations are unsuited for the underwater environment because the flow control functionality relies on window-based mechanisms that require an accurate estimate of the round trip time (RTT).

If you dont have access to IEEE Comms Mag then you can still read one of the papers online here.