27-06-2013, 04:33 PM
Underwater Wireless Communications: Current Achievements and Research Challenges
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INTRODUCTION
While wireless communication technology today has become part of our daily life, the idea of
wireless undersea communications may still seem far-fetched. However, research has been active
for over a decade on designing the methods for wireless information transmission underwater.
Human knowledge and understanding of the world’s oceans, which constitute the major part of our
planet, rests on our ability to collect information from remote undersea locations. The major
discoveries of the past decades, such as the remains of Titanic, or the hydro-thermal vents at bottom
of deep ocean, were made using cabled submersibles. Although such systems remain indispensable
if high-speed communication link is to exists between the remote end and the surface, it is natural to
wonder what one could accomplish without the burden (and cost) of heavy cables. Hence the
motivation, and our interest in wireless underwater communications. Together with sensor
technology and vehicular technology, wireless communications will enable new applications
ranging from environmental monitoring to gathering of oceanographic data, marine archaeology,
and search and rescue missions.
The signals that are used to carry digital information through an underwater channel are not radio
signals, as electro-magnetic waves propagate only over extremely short distances. Instead, acoustic
waves are used, which can propagate over long distances. However, an underwater acoustic channel
presents a communication system designer with many difficulties. The three distinguishing
characteristics of this channel are frequency-dependent propagation loss, severe multipath, and low
speed of sound propagation. None of these characteristics are nearly as pronounced in land-based
radio channels, the fact that makes underwater wireless communication extremely difficult, and
necessitates dedicated system design.
Ensemble of channel impulse responses (magnitudes)
Within this limited bandwidth, the signal is subject to multipath propagation, which is particularly
pronounced on horizontal channels. In shallow water, multipath occurs due to signal reflection from
the surface and bottom, as illustrated in Figure 1. In deep water, it occurs due to ray bending, i.e. the
tendency of acoustic waves to travel along the axis of lowest sound speed. Figure 2 shows an
ensemble of channel responses obtained in deep water. The multipath spread, measured along the
delay axis, is on the order of 10 ms in this example. The channel response varies in time, and also
changes if the receiver moves. Regardless of its origin, multipath propagation creates signal echoes,
resulting in intersymbol interference in a digital communication system. While in a cellular radio
system multipath spans a few symbol intervals, in an underwater acoustic channel it can spans few
tens, or even hundreds of symbol intervals! To avoid the intersymbol interference, a guard time, of
length at least equal to the multipath spread, must be inserted between successively transmitted
symbols. However, this will reduce the overall symbol rate, which is already limited by the system
bandwidth. To maximize the symbol rate, a receiver must be designed to counteract very long
intersymbol interference.