12-11-2012, 06:22 PM
Underwater acoustic sensor networks: research challenges
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Introduction
Underwater sensor networks are envisioned to
enable applications for oceanographic data collection,
pollution monitoring, offshore exploration,
disaster prevention, assisted navigation
and tactical surveillance applications. Multiple
unmanned or autonomous underwater vehicles
(UUVs, AUVs), equipped with underwater sensors,
will also find application in exploration
of natural undersea resources and gathering of
scientific data in collaborative monitoring missions.
To make these applications viable, there
is a need to enable underwater communications
among underwater devices. Underwater sensor
nodes and vehicles must possess self-configuration
capabilities, i.e., they must be able to
coordinate their operation by exchanging configuration,
location and movement information,
and to relay monitored data to an onshore
station.
Underwater acoustic sensor networks: communication architecture
In this section, we describe the communication
architecture of underwater acoustic sensor
networks. In particular, we introduce reference
architectures for two-dimensional and threedimensional
underwater networks, and present
several types of autonomous underwater vehicles
(AUVs) which can enhance the capabilities of
underwater sensor networks.
The network topology is in general a crucial
factor in determining the energy consumption, the
capacity and the reliability of a network. Hence,
the network topology should be carefully engineered
and post-deployment topology optimization
should be performed, when possible.
Basics of acoustic propagation
Underwater acoustic communications are
mainly influenced by path loss, noise, multi-path,
Doppler spread, and high and variable propagation
delay. All these factors determine the temporal
and spatial variability of the acoustic channel,
and make the available bandwidth of the Under-
Water Acoustic channel (UW-A) limited and
dramatically dependent on both range and frequency.
Long-range systems that operate over several
tens of kilometers may have a bandwidth of
only a few kHz, while a short-range system operating
over several tens of meters may have more than
a hundred kHz of bandwidth. In both cases these
factors lead to low bit rate [14], in the order of tens
of kbit/s for existing devices.
Physical layer
Until the beginning of the last decade, due to
the challenging characteristics of the underwater
channel, underwater modem development was
based on non-coherent frequency shift keying
(FSK) modulation, since it relies on energy detection.
Thus, it does not require phase tracking,
which is a very difficult task mainly because of
the Doppler-spread in the UW-A channel, described
in Section 4. In FSK modulation schemes
developed for underwater communications, the
multi-path effects are suppressed by inserting time
guards between successive pulses to ensure that the
reverberation, caused by the rough ocean surface
and bottom, vanishes before each subsequent
pulse is received.