Part B | 15.3
362 Part B Autonomous Ocean Vehicles, Subsystems and Control
diameter by 0:1 m in length, to 0:1 m in diameter by
0:5 m in length, depending on the frequency of operation (from a few to hundreds of kHz), maximum
range and operating depth.
Power: The power consumption depends on the
range and modulation: traditionally, the total power
consumption is of the order of 0:11 W in receive
mode (depending on the receiver complexity), and
10100 Win continuous transmission mode.
Data rate: (1) underwater acoustic modems designed to operate very reliably in challenging environments, on moving platforms and over distances of several kilometers typically operate under
1000 bps; (2) in less challenging environments (e.g.,
vertical communications in deep waters), acoustic modems can operate at data rates well over
1000 bps.
Horizontal versus vertical transmission: It is critical
to understand that commercial underwater acoustic
modems come in different versions depending on
the type of operation. In particular, acoustic modems
designed for vertical transmission in deep water use
directional transducers and are capable of high data
rates. A traditional mistake is to purchase this type
of acoustic modems for horizontal transmission in
shallow water, especially on moving platforms; in
this case, acoustic modems with omnidirectional
and a lower data rates should be selected.
Shallow water versus deep water communications
(horizontal or slanted): In deep waters, fluctuations
in sound speed (caused by changes in temperature, hydrostatic pressure, and chemical content)
change the direction of propagation of sound (a phenomenon known as sound refraction). This often
results in shadow zones: the sound produced by
a modem will not reach the other modem, even
if both modems are within operating range of one
another. Echoes reflected off the sea surface and
sea bottom may also be an issue at low frequencies, along with the presence of thermoclines or
thermohalines. In shallow waters, the main source
of limitation fades, which results from the many
echoes reflected off the sea surface and sea bottom.
As a result, underwater acoustic modems designed
to operate in shallow waters are less likely to be
in a shadow zone, but the received signal is often
badly distorted and requires specific signal processing techniques (equalization, spread spectrum
signaling, power, error coding) to operate reliably.
Range: for the most part, the maximum operational
range of an underwater acoustic modem depends on
the operating frequency band of the acoustic modem, power, signaling (modulation, error coding,
equalizing), environment, and transducer directivity. At low frequencies (a few kHz), commercial
acoustic modems achieve reliable acoustic communications beyond 10 km horizontally, albeit at a low
data rate (less than 400 bps). As frequency and
bandwidth increases, some acoustic modems can
transmit data at 10,000 bps or more, but the practical range is limited to 1000 m or less.
Operations in dynamic environments: (1) surface activity causes Doppler spread, which limits the data
rate and reliability; (2) platform dynamics cause
Doppler shift, which limits the acoustic modem performance when excessive. Moving platforms usually require the use of omnidirectional transducers to
avoid loss of acoustic communication.
Protocol for point-to-point quality of service (QoS):
reliability, delay, retransmission.
Networking capability: Most acoustic modems are
designed for point-to-point operation using a pointto-point protocol (PPP). Some series are also designed to support multiple access (MA), where two
or more units can be operated within the same environment. In this case, the acoustic modems are
usually operated sequentially and can be taken in
and out of the water with minimal disruption to the
other units. The most sophisticated acoustic modem
now support routing capability between nodes, although this feature is rarely available commercially.
In this case, one or more acoustic modems can operate together to relay messages over longer distances.
Compatibility requirements: Interoperability between underwater acoustic modems produced by
various manufacturers is still work in progress. Protocols and standards exist, but every manufacturer
uses its own proprietary modem signaling and protocol. As a result, mixing and matching acoustic
modems from different manufacturers is not recommended.
15.3.2 Commercially Available Modems
Figure 15.1 provides an overview of the nominal data
rate versus range resulting from a large literature survey. In general, the high rate or high range results are
for deep channels while the cluster of low range, low
rate are for shallow channels. Modems developed by
the research community are represented with diamonds,
while stars denote commercially available systems. The
range-times-rate bound represents an estimate of the
existing performance envelope. While there are exceptions, most reviewed systems are bounded by this
performance limit. Table 15.1 and Fig. 15.2 provide
a few examples of state-of-the-art acoustic modems.
The information is obtained from published technical
specifications. Every acoustic communication system
362 Part B Autonomous Ocean Vehicles, Subsystems and Control
diameter by 0:1 m in length, to 0:1 m in diameter by
0:5 m in length, depending on the frequency of operation (from a few to hundreds of kHz), maximum
range and operating depth.
Power: The power consumption depends on the
range and modulation: traditionally, the total power
consumption is of the order of 0:11 W in receive
mode (depending on the receiver complexity), and
10100 Win continuous transmission mode.
Data rate: (1) underwater acoustic modems designed to operate very reliably in challenging environments, on moving platforms and over distances of several kilometers typically operate under
1000 bps; (2) in less challenging environments (e.g.,
vertical communications in deep waters), acoustic modems can operate at data rates well over
1000 bps.
Horizontal versus vertical transmission: It is critical
to understand that commercial underwater acoustic
modems come in different versions depending on
the type of operation. In particular, acoustic modems
designed for vertical transmission in deep water use
directional transducers and are capable of high data
rates. A traditional mistake is to purchase this type
of acoustic modems for horizontal transmission in
shallow water, especially on moving platforms; in
this case, acoustic modems with omnidirectional
and a lower data rates should be selected.
Shallow water versus deep water communications
(horizontal or slanted): In deep waters, fluctuations
in sound speed (caused by changes in temperature, hydrostatic pressure, and chemical content)
change the direction of propagation of sound (a phenomenon known as sound refraction). This often
results in shadow zones: the sound produced by
a modem will not reach the other modem, even
if both modems are within operating range of one
another. Echoes reflected off the sea surface and
sea bottom may also be an issue at low frequencies, along with the presence of thermoclines or
thermohalines. In shallow waters, the main source
of limitation fades, which results from the many
echoes reflected off the sea surface and sea bottom.
As a result, underwater acoustic modems designed
to operate in shallow waters are less likely to be
in a shadow zone, but the received signal is often
badly distorted and requires specific signal processing techniques (equalization, spread spectrum
signaling, power, error coding) to operate reliably.
Range: for the most part, the maximum operational
range of an underwater acoustic modem depends on
the operating frequency band of the acoustic modem, power, signaling (modulation, error coding,
equalizing), environment, and transducer directivity. At low frequencies (a few kHz), commercial
acoustic modems achieve reliable acoustic communications beyond 10 km horizontally, albeit at a low
data rate (less than 400 bps). As frequency and
bandwidth increases, some acoustic modems can
transmit data at 10,000 bps or more, but the practical range is limited to 1000 m or less.
Operations in dynamic environments: (1) surface activity causes Doppler spread, which limits the data
rate and reliability; (2) platform dynamics cause
Doppler shift, which limits the acoustic modem performance when excessive. Moving platforms usually require the use of omnidirectional transducers to
avoid loss of acoustic communication.
Protocol for point-to-point quality of service (QoS):
reliability, delay, retransmission.
Networking capability: Most acoustic modems are
designed for point-to-point operation using a pointto-point protocol (PPP). Some series are also designed to support multiple access (MA), where two
or more units can be operated within the same environment. In this case, the acoustic modems are
usually operated sequentially and can be taken in
and out of the water with minimal disruption to the
other units. The most sophisticated acoustic modem
now support routing capability between nodes, although this feature is rarely available commercially.
In this case, one or more acoustic modems can operate together to relay messages over longer distances.
Compatibility requirements: Interoperability between underwater acoustic modems produced by
various manufacturers is still work in progress. Protocols and standards exist, but every manufacturer
uses its own proprietary modem signaling and protocol. As a result, mixing and matching acoustic
modems from different manufacturers is not recommended.
15.3.2 Commercially Available Modems
Figure 15.1 provides an overview of the nominal data
rate versus range resulting from a large literature survey. In general, the high rate or high range results are
for deep channels while the cluster of low range, low
rate are for shallow channels. Modems developed by
the research community are represented with diamonds,
while stars denote commercially available systems. The
range-times-rate bound represents an estimate of the
existing performance envelope. While there are exceptions, most reviewed systems are bounded by this
performance limit. Table 15.1 and Fig. 15.2 provide
a few examples of state-of-the-art acoustic modems.
The information is obtained from published technical
specifications. Every acoustic communication system