26-03-2012, 12:28 PM
Underwater Acoustic Communications: Design Considerations on the Physical Layer
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INTRODUCTION
Underwater acoustic channels are generally recognized as
one of the most difficult communication media in use today.
Acoustic propagation is best supported at low frequencies,
and the bandwidth available for communication is extremely
limited. For example, an acoustic system may operate in a
frequency range between 10 and 15 kHz. Although the total
communication bandwidth is very low (5 kHz), the system
is in fact ultra-wideband, in the sense that bandwidth is
not negligible with respect to the center frequency. Sound
propagates underwater at a very low speed of 1500 m/s, and
propagation occurs over multiple paths. Delay spreading over
tens or even hundreds of milliseconds results in a frequencyselective
signal distortion, while motion creates an extreme
Doppler effect. The worst properties of radio channels–poor
physical link quality of a mobile terrestrial radio channel
and high latency of a satellite channel–are combined in an
underwater acoustic channel.
II. ATTENUATION AND NOISE
Perhaps the most distinguishing property of acoustic channels
is the fact that path loss depends on the signal frequency.
This dependence is a consequence of absorption, i.e. transfer
of acoustic energy into heat. In addition to the absorption
loss, signal experiences a spreading loss which increases with
distance.
IV. THE DOPPLER EFFECT
Motion of the transmitter or receiver contributes additionally
to the changes in the channel response. This occurs through
the Doppler effect which causes frequency shifting as well as
additional frequency spreading. The magnitude of the Doppler
effect is proportional to the ratio a = v/c of the relative
transmitter/receiver velocity to the speed of sound. Because the
speed of sound is very low as compared to the speed of electromagnetic
waves, motion-induced Doppler distortion of an
acoustic signal can be extreme. The only comparable situation
in radio communications occurs in the Low Earth Orbiting
satellite systems, where the relative velocity of satellites flying
overhead is extremely high. (The channel there, however, is
not nearly as dispersive.) Autonomous underwater vehicles
(AUVs) move at speeds that are on the order of few m/s, but
even without intentional motion, underwater instruments are
subject to drifting with waves, currents and tides, which may
occur at comparable velocities.
V. DESIGN CONSIDERATIONS FOR ACOUSTIC NETWORKS
Implications of acoustic propagation extend beyond the
physical layer, affecting all the layers of a network architecture.
We have already seen that the bandwidth-distance
dependence builds a strong case for underwater multi-hopping.
In addition to the bandwidth, there are two major factors that
influence the design of network protocols: the transmission
power and the low speed of sound.