Acoustic Communication 15.5 Point-to-Point Links:Signal Processing 381
Part B | 15.5
rectly with each other. There is no central authority such
as a base station, although the nodes can form clusters, and the end goal may as well be to transmit all
the data to a common node (a surface sink). The network nodes can either have pre-established routes over
which to send the packets (as would be the case in fixed
bottom-mounted networks), or they can do this in an
ad-hoc manner, learning the routes as they go (which
would be the case in a mobile network). Between these
two extremes lie various hybrid architectures.
15.5.7 Channel Sharing
Channel sharing is the major design issue for bandlimited acoustic networks. Methods for channel sharing can be divided into two types: deterministic and
random. In deterministic access, resources such as frequency bands or time slots are allocated to particular
nodes. This type of multiple access is well suited to
systems in which the nodes transmit continuously, since
the a-priori allocated resources are then well utilized. If
the nodes instead transmit in a bursty manner and not
too often, random access may be the preferred choice.
The nodes now transmit more-or-less at their own will
and in the same frequency band, risking a loss if their
packets collide at the receiver. Medium access control
(MAC) protocols can then be used to orchestrate the
nodes’ behavior and reduce the chances of collision.
Random and deterministic access can also be combined in a reservation-based channel sharing scheme.
Nodes now first contend by transmitting short reservation packets in a random access fashion. The central
authority chooses the winners, and deterministically allocates them fixed resources for the duration of the data
transmission phase.
Deterministic Access
Deterministic access for acoustic networks has been
considered in various forms – frequency, time,
and code-division multiple access (FDMA, TDMA,
CDMA) [15.91]. Typically, FDMA is considered inefficient on account of the fact that the already limited
bandwidth will be wasted if allocated permanently to
a node that does not transmit, while those nodes that
transmit will be doing so at a lower bit rate, thereby
increasing the energy consumption. This reasoning is
justified in today’s applications where the nodes typically do not stream continuously, but only report to
the base station upon request. A polling mechanism is
then put in place via time-division duplexing (TDD).
Although this method may resemble TDMA (and is
often called that) a network based on polling is not
a true TDMA network, as it involves a two-way propagation delay for each transaction, whereas in a true
TDMA network nodes schedule their transmissions so
that they arrive back-to-back to the central node. In an
acoustic setting with variable propagation delays, guard
times are needed to ensure that there is no overlap of
packets at the base station. Acoustic modems that are
currently in use are well conducive to such an implementation [15.92].
An alternative that has been considered for small
multiuser systems is CDMA based on direct-sequence
spread-spectrum (DS-SS) modulation, which simultaneously provides the low probability of detection (LPD)
needed for operation in hostile environments. It may
be worth noting that the usual assumptions involved
in the design of radio DS-SS systems do not hold in
an acoustic channel, because it requires chip-rate (as
opposed to symbol-rate) adaptive processing to take
advantage of the available spreading gain [15.60]. Passive time reversal has also been used for multiuser
detection [15.85].
While today’s applications do not require more
than a single cell, spatial frequency reuse across multiple cells offers the key to large area coverage with
a limited bandwidth. Acoustic propagation, however,
dictates cellular design principles that are more complex than those used in terrestrial networks, and makes
the system capacity heavily dependent on the center
frequency [15.93]. Specifically, moving to a higher frequency region than that dictated by simple SNR maximization (Fig. 15.5) improves the signal-to-interference
ratio and yields a greater capacity.
Random Access
Random access has been used for acoustic networks
in which the nodes transmit infrequently [15.28, 86].
This type of access is also suitable for networks with
many nodes, so long as the their aggregate traffic does
not exceed a certain limit. The simplest form of random access is the Aloha mechanism whereby each
node transmits whenever it wants to, and if it becomes
aware of a collision (via an acknowledgement/negative
acknowledgement (ACK/NAK) feedback from the intended receiver), it re-transmits after a random back-off
time. This simple mechanism can be augmented by
carrier sensing multiple access (CSMA), which imposes an additional condition that a node can transmit
only if it senses the channel to be free. Compared to
radio systems where propagation delay is negligible
compared to the packet duration, this listen-beforetransmitting principle has fewer benefits in acoustic
systems, where the packets propagate slowly, and the
fact that none are overheard does not mean that some
are not present in the channel. Multiple-access collision
avoidance (MACA) introduces a hand-shaking procedure to secure the link using short control packets of
Part B | 15.5
rectly with each other. There is no central authority such
as a base station, although the nodes can form clusters, and the end goal may as well be to transmit all
the data to a common node (a surface sink). The network nodes can either have pre-established routes over
which to send the packets (as would be the case in fixed
bottom-mounted networks), or they can do this in an
ad-hoc manner, learning the routes as they go (which
would be the case in a mobile network). Between these
two extremes lie various hybrid architectures.
15.5.7 Channel Sharing
Channel sharing is the major design issue for bandlimited acoustic networks. Methods for channel sharing can be divided into two types: deterministic and
random. In deterministic access, resources such as frequency bands or time slots are allocated to particular
nodes. This type of multiple access is well suited to
systems in which the nodes transmit continuously, since
the a-priori allocated resources are then well utilized. If
the nodes instead transmit in a bursty manner and not
too often, random access may be the preferred choice.
The nodes now transmit more-or-less at their own will
and in the same frequency band, risking a loss if their
packets collide at the receiver. Medium access control
(MAC) protocols can then be used to orchestrate the
nodes’ behavior and reduce the chances of collision.
Random and deterministic access can also be combined in a reservation-based channel sharing scheme.
Nodes now first contend by transmitting short reservation packets in a random access fashion. The central
authority chooses the winners, and deterministically allocates them fixed resources for the duration of the data
transmission phase.
Deterministic Access
Deterministic access for acoustic networks has been
considered in various forms – frequency, time,
and code-division multiple access (FDMA, TDMA,
CDMA) [15.91]. Typically, FDMA is considered inefficient on account of the fact that the already limited
bandwidth will be wasted if allocated permanently to
a node that does not transmit, while those nodes that
transmit will be doing so at a lower bit rate, thereby
increasing the energy consumption. This reasoning is
justified in today’s applications where the nodes typically do not stream continuously, but only report to
the base station upon request. A polling mechanism is
then put in place via time-division duplexing (TDD).
Although this method may resemble TDMA (and is
often called that) a network based on polling is not
a true TDMA network, as it involves a two-way propagation delay for each transaction, whereas in a true
TDMA network nodes schedule their transmissions so
that they arrive back-to-back to the central node. In an
acoustic setting with variable propagation delays, guard
times are needed to ensure that there is no overlap of
packets at the base station. Acoustic modems that are
currently in use are well conducive to such an implementation [15.92].
An alternative that has been considered for small
multiuser systems is CDMA based on direct-sequence
spread-spectrum (DS-SS) modulation, which simultaneously provides the low probability of detection (LPD)
needed for operation in hostile environments. It may
be worth noting that the usual assumptions involved
in the design of radio DS-SS systems do not hold in
an acoustic channel, because it requires chip-rate (as
opposed to symbol-rate) adaptive processing to take
advantage of the available spreading gain [15.60]. Passive time reversal has also been used for multiuser
detection [15.85].
While today’s applications do not require more
than a single cell, spatial frequency reuse across multiple cells offers the key to large area coverage with
a limited bandwidth. Acoustic propagation, however,
dictates cellular design principles that are more complex than those used in terrestrial networks, and makes
the system capacity heavily dependent on the center
frequency [15.93]. Specifically, moving to a higher frequency region than that dictated by simple SNR maximization (Fig. 15.5) improves the signal-to-interference
ratio and yields a greater capacity.
Random Access
Random access has been used for acoustic networks
in which the nodes transmit infrequently [15.28, 86].
This type of access is also suitable for networks with
many nodes, so long as the their aggregate traffic does
not exceed a certain limit. The simplest form of random access is the Aloha mechanism whereby each
node transmits whenever it wants to, and if it becomes
aware of a collision (via an acknowledgement/negative
acknowledgement (ACK/NAK) feedback from the intended receiver), it re-transmits after a random back-off
time. This simple mechanism can be augmented by
carrier sensing multiple access (CSMA), which imposes an additional condition that a node can transmit
only if it senses the channel to be free. Compared to
radio systems where propagation delay is negligible
compared to the packet duration, this listen-beforetransmitting principle has fewer benefits in acoustic
systems, where the packets propagate slowly, and the
fact that none are overheard does not mean that some
are not present in the channel. Multiple-access collision
avoidance (MACA) introduces a hand-shaking procedure to secure the link using short control packets of