Acoustic Communication 15.5 Point-to-Point Links:Signal Processing 379
Part B | 15.5
once again, but this time the cleaned signal is used as
a new reference in the decision feedback portion of the
adaptive equalizer. The second step is to repeat the entire operation again and again, so that the original signal
is equalized against an ever cleaner reference signal.
The statistical properties of the acoustic channel,
the type of interleaver, error coding, and decoding have
a significant impact on turbo-equalizers. However, the
same can be said of every equalized communication
system, and overall this technique can lead to significant improvements in data transmission quality over
a more traditional equalizer approach. Therefore, turboequalizers are becoming increasingly more common in
underwater acoustic communication systems [15.38,
39, 69].
15.5.5 Adapting to the Environment
Adapting to the communication environment involves
not only receiver-end channel estimation, but transmitter adaptation as well. This task is much more
challenging in a time-varying environment which hinders the feedback from the receiver to the transmitter.
In an acoustic channel, the situation is exacerbated by
the long propagation delay which may make the feedback obsolete by the time it arrives. It is thus imperative
to identify those environmental parameters that can
withstand the feedback delay, and focus on the related
transmitter functions.
In light of the large-scale and small-scale channel
variations discussed in Sect. 15.4.4, adaptive mechanisms that have been considered include large-scale
power control and small-scale waveform control. The
first refers to adjusting the total transmitting power
regardless of the modulation/detection method used,
while the latter refers to adjusting the shape of the signal
spectrum, i. e., distribution of the total available power
over a given bandwidth. While the former targets power
savings over extended periods of time (e.g., hours or
days), the latter targets improved signal detection in
a particular communication session (several data packets).
Neither type of adaptivity has been implemented in
a commercial system, although research suggests significant benefits. Specifically, large-scale power control
was shown to have significant benefits both in the sense
of power savings [15.45] and in the sense of interference control in networked systems [15.81].
The work on small-scale adaptation has been versatile, including active time-reversal [15.82], single-mode
excitation [15.83], and adaptive modulation [15.63].
Time reversal (or phase conjugation in the frequency
domain) has in particular been extensively studied as an
adaptive matched-filtering technique for acoustic communications [15.82], both in the sense of receiver adaptivity and transmitter adaptivity. It uses a time-reversed
replica of a received signal waveform to implement
a filter matched to that waveform, and can operate either passively or actively. Passive time-reversal resides
at the receiver side only, where its role is to acquire
a probe signal and use it to perform low-complexity
front-end filtering prior to equalization or interference
cancelation [15.84, 85]. In contrast, active time-reversal
operates at the transmitter side, where its role is to
time reverse the feedback signal and use it as the
basic pulse (basic transmit waveform) that will best
match the channel. By doing so, the transmitter, typically equipped with a large array, focuses its energy not
only in time, but also in space ([15.82] and references
therein). In repeated actions of this type, both ends of
the link can focus their energy.
Adaptive modulation has been considered in the
context of single-carrier MIMO systems [15.61], and,
more recently, in the context of multicarrier systems [15.63], where adaptive power and/or rate control
(adaptive bit loading) can be implemented easily by
adjusting the amplitude and/or the modulation level
of each carrier separately. The performance improvement available from these techniques is contingent on
the quality of the channel state information that is fed
back to the transmitter. Recent results [15.63], which
report on the first experimental demonstration of this
type, suggest the possibility to isolate the more slowly
varying channel parameters (i. e., the predictable propagation path gains) from the more rapidly varying ones
(phases) and use them to design an adaptive modulation
system.
15.5.6 Networks
With the technological advances in acoustic modems,
vehicles, and sensors, the concept of an underwater
network of distributed autonomous nodes [15.14] is
coming closer to realization. Experimental deployments
are well underway, demonstrating the existing hardware capabilities in series of experimental deployments
supported by the US Navy, such as the SeaWeb that
uses telesonar modems [15.28], or deployments that use
WHOI micro-modems [15.86]. Recently, efforts have
begun to standardize acoustic network protocols with
the goal of ensuring inter-operability between different modems. In particular, an international NATO-led
effort is active in establishing JANUS [15.12], a standard that specifies data packet structures, acknowledgement (ACK) mechanisms, bit rates, etc., for use in
given frequency band/transmissison distance configurations. Simultaneously, research is steadily advancing
on the design of multiple-access methods, medium
Part B | 15.5
once again, but this time the cleaned signal is used as
a new reference in the decision feedback portion of the
adaptive equalizer. The second step is to repeat the entire operation again and again, so that the original signal
is equalized against an ever cleaner reference signal.
The statistical properties of the acoustic channel,
the type of interleaver, error coding, and decoding have
a significant impact on turbo-equalizers. However, the
same can be said of every equalized communication
system, and overall this technique can lead to significant improvements in data transmission quality over
a more traditional equalizer approach. Therefore, turboequalizers are becoming increasingly more common in
underwater acoustic communication systems [15.38,
39, 69].
15.5.5 Adapting to the Environment
Adapting to the communication environment involves
not only receiver-end channel estimation, but transmitter adaptation as well. This task is much more
challenging in a time-varying environment which hinders the feedback from the receiver to the transmitter.
In an acoustic channel, the situation is exacerbated by
the long propagation delay which may make the feedback obsolete by the time it arrives. It is thus imperative
to identify those environmental parameters that can
withstand the feedback delay, and focus on the related
transmitter functions.
In light of the large-scale and small-scale channel
variations discussed in Sect. 15.4.4, adaptive mechanisms that have been considered include large-scale
power control and small-scale waveform control. The
first refers to adjusting the total transmitting power
regardless of the modulation/detection method used,
while the latter refers to adjusting the shape of the signal
spectrum, i. e., distribution of the total available power
over a given bandwidth. While the former targets power
savings over extended periods of time (e.g., hours or
days), the latter targets improved signal detection in
a particular communication session (several data packets).
Neither type of adaptivity has been implemented in
a commercial system, although research suggests significant benefits. Specifically, large-scale power control
was shown to have significant benefits both in the sense
of power savings [15.45] and in the sense of interference control in networked systems [15.81].
The work on small-scale adaptation has been versatile, including active time-reversal [15.82], single-mode
excitation [15.83], and adaptive modulation [15.63].
Time reversal (or phase conjugation in the frequency
domain) has in particular been extensively studied as an
adaptive matched-filtering technique for acoustic communications [15.82], both in the sense of receiver adaptivity and transmitter adaptivity. It uses a time-reversed
replica of a received signal waveform to implement
a filter matched to that waveform, and can operate either passively or actively. Passive time-reversal resides
at the receiver side only, where its role is to acquire
a probe signal and use it to perform low-complexity
front-end filtering prior to equalization or interference
cancelation [15.84, 85]. In contrast, active time-reversal
operates at the transmitter side, where its role is to
time reverse the feedback signal and use it as the
basic pulse (basic transmit waveform) that will best
match the channel. By doing so, the transmitter, typically equipped with a large array, focuses its energy not
only in time, but also in space ([15.82] and references
therein). In repeated actions of this type, both ends of
the link can focus their energy.
Adaptive modulation has been considered in the
context of single-carrier MIMO systems [15.61], and,
more recently, in the context of multicarrier systems [15.63], where adaptive power and/or rate control
(adaptive bit loading) can be implemented easily by
adjusting the amplitude and/or the modulation level
of each carrier separately. The performance improvement available from these techniques is contingent on
the quality of the channel state information that is fed
back to the transmitter. Recent results [15.63], which
report on the first experimental demonstration of this
type, suggest the possibility to isolate the more slowly
varying channel parameters (i. e., the predictable propagation path gains) from the more rapidly varying ones
(phases) and use them to design an adaptive modulation
system.
15.5.6 Networks
With the technological advances in acoustic modems,
vehicles, and sensors, the concept of an underwater
network of distributed autonomous nodes [15.14] is
coming closer to realization. Experimental deployments
are well underway, demonstrating the existing hardware capabilities in series of experimental deployments
supported by the US Navy, such as the SeaWeb that
uses telesonar modems [15.28], or deployments that use
WHOI micro-modems [15.86]. Recently, efforts have
begun to standardize acoustic network protocols with
the goal of ensuring inter-operability between different modems. In particular, an international NATO-led
effort is active in establishing JANUS [15.12], a standard that specifies data packet structures, acknowledgement (ACK) mechanisms, bit rates, etc., for use in
given frequency band/transmissison distance configurations. Simultaneously, research is steadily advancing
on the design of multiple-access methods, medium