Part B | 15.2
360 Part B Autonomous Ocean Vehicles, Subsystems and Control
order of 10 Hz [15.4]. Horizontal transmission is notoriously more difficult due to the multipath propagation,
while vertical channels exhibit less distortion [15.5].
Frequency-dependent attenuation, multipath propagation, and low speed of sound (about 1500 m=s), which
results in a severe Doppler effect, make the underwater
acoustic channel one of the most challenging communication media.
15.1 A Brief History
Among the first operational underwater acoustic systems was the submarine communication system developed in the United States around the end of the Second
World War. This system used analog modulation in
the 811 kHz band (single-sideband analog modulation
AM) [15.6]. Technology has since advanced, pushing
digital modulation/detection techniques into the forefront of modern acoustic communications. In the early
1980s, the advent of digital signal processing sparked
a renowned interest in underwater acoustic communications, leading a group of scientists at the Massachusetts
Institute of Technology and the Woods Hole Oceanographic Institution (WHOI) to propose a system based
on frequency shift keying (FSK) [15.7]. The system
became known as DATS (digital acoustic telemetry
system) and provided a basis for the first generation
of commercial digital acoustic modems [15.8]. Today, coded FSK is used in several acoustic modems,
including the WHOI micro-modem and the Teledyne
(formerly Benthos) telesonar type B modem [15.9].
While FSK relies on simple energy detection (noncoherent detection), and thus offers robustness to channel
impairments, its bandwidth utilization is not efficient.
Motivated by this fact, research in the 1990s focused on
investigating phase shift keying (PSK) and quadrature
amplitude modulation (QAM) for underwater acoustic channels. These modulation methods offer more
bits=sec per Hz of occupied bandwidth, but require a receiver that can track the channel and compensate for
the time-varying multipath and phase distortion (coherent detection). Work carried out at Northeastern
University and WHOI resulted in a channel equalization/synchronization method [15.10], which forms the
basis of a second generation of high-speed acoustic
modems. Through the last decade, these modems have
been used in operations involving both stationary platforms and autonomous underwater vehicles (AUVs),
over vertical and horizontal links at bit rates of about
5 kbps. [15.11] gives an impressive account of a 2009
deployment near the 11 km deep Mariana Trench. A detailed summary of existing technology is given in
Sect. 15.3.
Bit rates in excess of those available with operational modems have been demonstrated as well, but
these results are in the domain of experimental research. At the time of this writing, research is active
on improved, and ever more sophisticated channel estimation and equalization methods for single-carrier
broadband systems, as well as on multicarrier modulation/detection techniques which hold a promise of
reducing the implementation complexity of high-speed
acoustic modems. The success of various communication techniques largely depends on our understanding
of the acoustic communication channel, i. e., our ability
to identify a proper model for signal distortion. We thus
begin our treatment of acoustic communications links
by outlining the channel characteristics in Sect. 15.4,
and then move on to discuss the basic as well as the
emerging concepts of signal processing in Sect. 15.5.
As technology advances, companies around the
world engage more easily in modem development, and
the legacy of US manufacturers such as TeledyneBenthos, WHOI, and Link-Quest are joined by new
ones such as the French Thales and the German EvoLogics. As a result, standardization efforts are becoming necessary to ensure inter-operability between
acoustic modems of different manufacturers [15.12].
While it is ultimately the need for a certain technology
that will dictate the usefulness of its presence on the
market, we must keep in mind that application-driven
technology development is not the only way forward.
More often than not, technology-driven applications
arise – wireless radio industry being the prime example.
In other words, as the acoustic modems’ capabilities
grow, applications that previously were not thought possible may start to emerge.
15.2 Current and Emerging Modem Applications
Modem applications range from ocean observation to
the oil industry and aquaculture, and include gathering
of sensor data from remote instruments for pollution
control, climate recording and prediction of natural dis-
360 Part B Autonomous Ocean Vehicles, Subsystems and Control
order of 10 Hz [15.4]. Horizontal transmission is notoriously more difficult due to the multipath propagation,
while vertical channels exhibit less distortion [15.5].
Frequency-dependent attenuation, multipath propagation, and low speed of sound (about 1500 m=s), which
results in a severe Doppler effect, make the underwater
acoustic channel one of the most challenging communication media.
15.1 A Brief History
Among the first operational underwater acoustic systems was the submarine communication system developed in the United States around the end of the Second
World War. This system used analog modulation in
the 811 kHz band (single-sideband analog modulation
AM) [15.6]. Technology has since advanced, pushing
digital modulation/detection techniques into the forefront of modern acoustic communications. In the early
1980s, the advent of digital signal processing sparked
a renowned interest in underwater acoustic communications, leading a group of scientists at the Massachusetts
Institute of Technology and the Woods Hole Oceanographic Institution (WHOI) to propose a system based
on frequency shift keying (FSK) [15.7]. The system
became known as DATS (digital acoustic telemetry
system) and provided a basis for the first generation
of commercial digital acoustic modems [15.8]. Today, coded FSK is used in several acoustic modems,
including the WHOI micro-modem and the Teledyne
(formerly Benthos) telesonar type B modem [15.9].
While FSK relies on simple energy detection (noncoherent detection), and thus offers robustness to channel
impairments, its bandwidth utilization is not efficient.
Motivated by this fact, research in the 1990s focused on
investigating phase shift keying (PSK) and quadrature
amplitude modulation (QAM) for underwater acoustic channels. These modulation methods offer more
bits=sec per Hz of occupied bandwidth, but require a receiver that can track the channel and compensate for
the time-varying multipath and phase distortion (coherent detection). Work carried out at Northeastern
University and WHOI resulted in a channel equalization/synchronization method [15.10], which forms the
basis of a second generation of high-speed acoustic
modems. Through the last decade, these modems have
been used in operations involving both stationary platforms and autonomous underwater vehicles (AUVs),
over vertical and horizontal links at bit rates of about
5 kbps. [15.11] gives an impressive account of a 2009
deployment near the 11 km deep Mariana Trench. A detailed summary of existing technology is given in
Sect. 15.3.
Bit rates in excess of those available with operational modems have been demonstrated as well, but
these results are in the domain of experimental research. At the time of this writing, research is active
on improved, and ever more sophisticated channel estimation and equalization methods for single-carrier
broadband systems, as well as on multicarrier modulation/detection techniques which hold a promise of
reducing the implementation complexity of high-speed
acoustic modems. The success of various communication techniques largely depends on our understanding
of the acoustic communication channel, i. e., our ability
to identify a proper model for signal distortion. We thus
begin our treatment of acoustic communications links
by outlining the channel characteristics in Sect. 15.4,
and then move on to discuss the basic as well as the
emerging concepts of signal processing in Sect. 15.5.
As technology advances, companies around the
world engage more easily in modem development, and
the legacy of US manufacturers such as TeledyneBenthos, WHOI, and Link-Quest are joined by new
ones such as the French Thales and the German EvoLogics. As a result, standardization efforts are becoming necessary to ensure inter-operability between
acoustic modems of different manufacturers [15.12].
While it is ultimately the need for a certain technology
that will dictate the usefulness of its presence on the
market, we must keep in mind that application-driven
technology development is not the only way forward.
More often than not, technology-driven applications
arise – wireless radio industry being the prime example.
In other words, as the acoustic modems’ capabilities
grow, applications that previously were not thought possible may start to emerge.
15.2 Current and Emerging Modem Applications
Modem applications range from ocean observation to
the oil industry and aquaculture, and include gathering
of sensor data from remote instruments for pollution
control, climate recording and prediction of natural dis-