Part B | 15.5
378 Part B Autonomous Ocean Vehicles, Subsystems and Control
carriers, and typically involves fewer interfering terms.
Methods based on one-shot linear equalization of a full
block of carriers [15.41, 69], as well as recursive linear
or decision-feedback equalization [15.70], have been
investigated. Further improvements are available from
front-end (pre-FFT) filtering, which extracts the information about the time-varying channel before it has
been lost in the process of FFT demodulation. A receiver that mimics the operation of an optimal, channelmatched filter was proposed in [15.71], while one that
follows the analogy of fractionally-spaced equalization
in the frequency domain was investigated in [15.72].
Virtually all acoustic OFDM systems addressed to
date have focused on coherent detection and the attendant issues of channel estimation and Doppler tracking.
However, a properly designed OFDM system (one in
which there is no ICI) is well suited to differentially
coherent detection as well. Differential encoding can
be performed either across blocks or across carriers. In
the latter case, differential coherence is naturally satisfied with tight carrier packing, which simultaneously
supports bandwidth efficiency. Preliminary experimental results [15.64, 73] have demonstrated the benefits
of differentially coherent OFDM, whose computational
complexity is by far lower than that of any coherent
system, and whose performance can surpass that of coherent detection when channel estimation fails.
15.5.3 Data Link Reliability
Channel coding or forward error coding (FEC) in a crucial feature of underwater acoustic modems. By introducing a redundancy part in the transmitted message,
error correction codes allow for detection and correction of the bit errors caused by noise and inter-symbol
interference. Generally, the ability to correct the errors
is achieved by adding redundancy, which in turns reduces the effective data rate. Therefore, a proper coding
scheme for a communication system is chosen based on
the trade-off between the desired data bit rate and error
rate performance.
Traditional FEC techniques are divided into convolutional codes (usually decoded with the ubiquitous Viterbi algorithm) and block codes (e.g., BoseChaudhuri-Hocquenghem or BCH, Reed–Solomon or
RS) [15.74, 75]. Concatenated codes usually combine
convolutional and block codes to improve the reliability of the acoustic modem even further. More recently,
turbo codes [15.76, 77] and low-density parity-check
(LDPC) codes [15.78] have become part of the field
of telecommunication as they offer near Shannon error
correction capability. Having inherently a low coding
rate and a potentially large decoding latency, turbo
codes have emerged as a good choice in communication
systems where achieving a very low error rate performance is a top priority. A communication link designed
for the transmission of the command-and-control messages to a remotely operated robot or vehicle is such
a system. Since control messages do not contain a large
number of bits that need to be transmitted in a short
time, high bit rates are not required.
Even the most sophisticated error codes cannot
ensure complete link reliability. To guarantee an errorfree communication link (if it is physically possible),
the acoustic modem must include a quality of service
(QoS) feature, typically in the form of an automated
retransmission query (ARQ) process implemented between two acoustic modems [15.54, 79]. This operation
requires a communication protocol that includes acknowledgments and may dramatically reduce the overall data rate. The optimization of the QoS protocol is
a key feature of modern acoustic modems. Acoustic
modem users should be aware that commercially advertised throughputs (data rates) assume a single message
transmission and do not include any retransmission.
15.5.4 Turbo Equalization
Turbo equalization is a very powerful method used to
improve the removal of inter-symbol interference and
additive noise. This technique can be implemented with
single-carrier and multiple-carrier signaling. In particular, extensive research in combining turbo equalization
and OFDM has taken place over the past decade. The
simplest and best explanation is provided in the seminal work published in [15.80]. In short terms, turboequalization jointly optimizes adaptive equalization and
channel decoding in an iterative process. The result
is a dramatic improvement of transmission quality,
even for high spectral efficiency modulations and timevarying fading channels. Modern underwater acoustic
modems make growing use of this technique, which is
powerful but processor intensive.
At the heart of this process is interleaving and error
coding. The binary information at the source is carefully error-coded, modulated, and interleaved before it
is pulse-shaped and transmitted through the water channel. In a traditional equalized receiver, the distorted
signal is equalized, deinterleaved, converted to the binary domain, and the error-coded binary information is
decoded and, hopefully, every binary error is corrected.
The obvious issue with this method is the fact
that some error may remain. The turbo-equalizer goes
two steps further in using this information. First of
all, it modulates and interleaves the error-corrected binary information (which may still contain errors), thus
cleaning the signal of some (or most of the) intersymbol interference. The original signal is equalized
378 Part B Autonomous Ocean Vehicles, Subsystems and Control
carriers, and typically involves fewer interfering terms.
Methods based on one-shot linear equalization of a full
block of carriers [15.41, 69], as well as recursive linear
or decision-feedback equalization [15.70], have been
investigated. Further improvements are available from
front-end (pre-FFT) filtering, which extracts the information about the time-varying channel before it has
been lost in the process of FFT demodulation. A receiver that mimics the operation of an optimal, channelmatched filter was proposed in [15.71], while one that
follows the analogy of fractionally-spaced equalization
in the frequency domain was investigated in [15.72].
Virtually all acoustic OFDM systems addressed to
date have focused on coherent detection and the attendant issues of channel estimation and Doppler tracking.
However, a properly designed OFDM system (one in
which there is no ICI) is well suited to differentially
coherent detection as well. Differential encoding can
be performed either across blocks or across carriers. In
the latter case, differential coherence is naturally satisfied with tight carrier packing, which simultaneously
supports bandwidth efficiency. Preliminary experimental results [15.64, 73] have demonstrated the benefits
of differentially coherent OFDM, whose computational
complexity is by far lower than that of any coherent
system, and whose performance can surpass that of coherent detection when channel estimation fails.
15.5.3 Data Link Reliability
Channel coding or forward error coding (FEC) in a crucial feature of underwater acoustic modems. By introducing a redundancy part in the transmitted message,
error correction codes allow for detection and correction of the bit errors caused by noise and inter-symbol
interference. Generally, the ability to correct the errors
is achieved by adding redundancy, which in turns reduces the effective data rate. Therefore, a proper coding
scheme for a communication system is chosen based on
the trade-off between the desired data bit rate and error
rate performance.
Traditional FEC techniques are divided into convolutional codes (usually decoded with the ubiquitous Viterbi algorithm) and block codes (e.g., BoseChaudhuri-Hocquenghem or BCH, Reed–Solomon or
RS) [15.74, 75]. Concatenated codes usually combine
convolutional and block codes to improve the reliability of the acoustic modem even further. More recently,
turbo codes [15.76, 77] and low-density parity-check
(LDPC) codes [15.78] have become part of the field
of telecommunication as they offer near Shannon error
correction capability. Having inherently a low coding
rate and a potentially large decoding latency, turbo
codes have emerged as a good choice in communication
systems where achieving a very low error rate performance is a top priority. A communication link designed
for the transmission of the command-and-control messages to a remotely operated robot or vehicle is such
a system. Since control messages do not contain a large
number of bits that need to be transmitted in a short
time, high bit rates are not required.
Even the most sophisticated error codes cannot
ensure complete link reliability. To guarantee an errorfree communication link (if it is physically possible),
the acoustic modem must include a quality of service
(QoS) feature, typically in the form of an automated
retransmission query (ARQ) process implemented between two acoustic modems [15.54, 79]. This operation
requires a communication protocol that includes acknowledgments and may dramatically reduce the overall data rate. The optimization of the QoS protocol is
a key feature of modern acoustic modems. Acoustic
modem users should be aware that commercially advertised throughputs (data rates) assume a single message
transmission and do not include any retransmission.
15.5.4 Turbo Equalization
Turbo equalization is a very powerful method used to
improve the removal of inter-symbol interference and
additive noise. This technique can be implemented with
single-carrier and multiple-carrier signaling. In particular, extensive research in combining turbo equalization
and OFDM has taken place over the past decade. The
simplest and best explanation is provided in the seminal work published in [15.80]. In short terms, turboequalization jointly optimizes adaptive equalization and
channel decoding in an iterative process. The result
is a dramatic improvement of transmission quality,
even for high spectral efficiency modulations and timevarying fading channels. Modern underwater acoustic
modems make growing use of this technique, which is
powerful but processor intensive.
At the heart of this process is interleaving and error
coding. The binary information at the source is carefully error-coded, modulated, and interleaved before it
is pulse-shaped and transmitted through the water channel. In a traditional equalized receiver, the distorted
signal is equalized, deinterleaved, converted to the binary domain, and the error-coded binary information is
decoded and, hopefully, every binary error is corrected.
The obvious issue with this method is the fact
that some error may remain. The turbo-equalizer goes
two steps further in using this information. First of
all, it modulates and interleaves the error-corrected binary information (which may still contain errors), thus
cleaning the signal of some (or most of the) intersymbol interference. The original signal is equalized