Report underwater-wireless

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A
Seminar Report
On
UNDERWATER WIRELESS COMMUNICATION
B.TECH 4Th
Semester
Session 2015-2016
SUBMITTED TO: SUBMITTED BY:
Mr. Mohit chhabra POOJA
Ass. Professor Roll no: 11133132
Class: B.Tech (cse)
Section: C 2
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ABSTRACT
While wireless communication technology today has become part of our daily life, the idea of
wireless undersea communications may still seem far-fetched. However, research has been
active for over a decade on designing the methods for wireless information transmission
underwater. Human knowledge and understanding of the world’s oceans, which constitute
the major part of our planet, rests on our ability to collect information from remote
undersea locations.
The major discoveries of the past decades, such as the remains of Titanic, or the hydro-thermal vents
at bottom of deep ocean, were made using cabled submersibles. Although such systems remain
indispensable if high-speed communication link is to exists between the remote end and the surface, it
is natural to wonder what one could accomplish without the burden (and cost) of heavy cables.
Hence the motivation and interest in wireless underwater communications. Together with
sensor technology and vehicular technology, wireless communications will enable new applications
ranging from environmental monitoring to gathering of oceanographic data, marine archaeology, and
search and rescue missions.
i
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CONTENTS
1. Introduction……………………………………………………………………….…..1
2. Communication channel……………………………………………………………....2
3. Wave propagation………………………………………………………………….…3
4. Acoustic modem………………………………………………………………………...4
5. Under water networks………………………………………………………………….5
6. Applications…………………..…………………………………………………….…...6
7. Disadvantages….…………………………………………………………………….....7
8. Conclusion………………………………………………………………………………..8
9. References………………………………………………………………………………...9
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INTRODUCTION
While wireless communication technology today has become part of our daily life, the idea of
wireless undersea communications may still seem far-fetched. However, research has been active for
over a decade on designing the methods for wireless information transmission underwater. Human
knowledge and understanding of the world’s oceans, which constitute the major part of our planet,
rests on our ability to collect information from remote undersea locations.
The major discoveries of the past decades, such as the remains of Titanic, or the hydro-thermal
vents at bottom of deep ocean, were made using cabled submersibles. Although such systems remain
indispensable if high-speed communication link is to exists between the remote end and the surface, it
is natural to wonder what one could accomplish without the burden (and cost) of heavy cables.
Hence the motivation, and interest in wireless underwater communications. Together with sensor
technology and vehicular technology, wireless communications will enable new applications ranging
from environmental monitoring to gathering of oceanographic data, marine archaeology, and search
and rescue missions.
[4]

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COMMUNICATION CHANNEL
The signals that are used to carry digital information through an underwater channel are not
radio signals, as electro-magnetic waves propagate only over extremely short distances. Instead,
acoustic waves are used, which can propagate over long distances. However, an underwater acoustic
channel presents a communication system designer with many difficulties.
The
three distinguishing characteristics of this channel are frequency-dependent propagation loss, severe
multipath, and low speed of sound propagation. None of these characteristics are nearly as pronounced
in land-based radio channels, the fact that makes underwater wireless communication extremely
difficult, and necessitates dedicated system design.
Wave propagation
Fig. 1: Shallow water multipath propagation: in addition to the direct
path, the signal propagates via reflections from the surface and bottom.
Path loss that occurs in an acoustic channel over a distance d is given as A=dka(f)d, where k is the
path loss exponent whose value is usually between 1 and 2, and a(f) is the absorption factor that
depends on the frequency f..
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This dependence severely limits the available bandwidth: for example, at distances on the order
of 100 km, the available bandwidth is only on the order of 1 kHz. At shorter distances, a larger
bandwidth is available, but in practice it is limited by the transducer. Also in contrast to the radio
systems, an acoustic signal is rarely narrowband, i.e., its bandwidth is not negligible with respect to the
center frequency.
Fig. 2: Ensemble of channel impulse
responses (magnitudes).
Within this limited bandwidth, the signal is subject to multipath propagation, which is particularly
pronounced on horizontal channels. In shallow water, multipath occurs due to signal reflection from
the surface and bottom, as illustrated in Figure 1. In deep water, it occurs due to ray bending, i.e. the
tendency of acoustic waves to travel along the axis of lowest sound speed. Figure 2 shows an ensemble
of channel responses obtained in deep water. The multipath spread, measured along the delay axis, is
on the order of 10 ms in this example. The channel response varies in time, and also changes if the
receiver moves. Regardless of its origin, multipath propagation creates signal echoes,
resulting in inter symbol interference in a digital communication system. While in a cellular radio
system multipath spans a few symbol intervals, in an underwater acoustic channel it can spans few
tens, or even hundreds of symbol intervals! To avoid the inter symbol interference, a guard time, of
length at least equal to the multipath spread, must be inserted between successively transmitted
symbols. However, this will reduce the overall symbol rate, which is already limited by the system
bandwidth. To maximize the symbol rate, a receiver must be designed to counteract very long inter
symbol interference.
The speed of sound underwater varies with depth and also depends on the environment. Its nominal
value is only 1500 m/s, and this fact has a twofold implication on the communication system design.
First, it implies long signal delay, which severely reduces the efficiency of any communication
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protocol that is based on receiver feedback, or hand-shaking between the transmitter and receiver. The
resulting latency is similar to that of a space communication system, although there it is a consequence
of long distances traveled. Secondly, low speed of sound results in severe Doppler distortion in a
mobile acoustic system. Namely, if the relative velocity between the transmitter and receiver is ±v,
then a signal of frequency fc will be observed at the receiver as having frequency fc (1±v/c). At the
same time, a waveform of duration T will be observed at the receiver as having duration T(1±v/c).
Hence, Doppler shifting and spreading occur. For the velocity v on the order of few m/s, the factor v/c,
which determines the severity of the Doppler distortion, can be several orders of magnitude greater
than the one observed in a land-mobile radio system! To avoid this distortion, a noncoherent
modulation/detection must be employed. Coherent modulation/detection offers a far better utilization
of bandwidth, but the receiver must be designed to deal with extreme Doppler distortion.
Summarizing the channel characteristics, one comes to the conclusion that an underwater acoustic link
combines in itself the worst aspects of radio channels: poor quality of a land-mobile link, and high
latency of a space link. In addition, current technology offers limited transducer bandwidth (typically a
few kHz, or few tens of kHz in a wideband system), half-duplex operation, and limited power supply
of battery-operated instruments.
.
Fig. 3: Multichannel adaptive decision-feedback
equIlizer (DFE) is used for high-speed
under water acoustic communications. It supports
any linear modulation format, such as M-aryl PSK or M-aryl QAM
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ACOUSTIC MODEM
Acoustic modem technology today offers two types of modulation/detection: frequency shift
keying (FSK) with noncoherent detection and phase-shift keying (PSK) with coherent detection. FSK
has traditionally been used for robust acoustic communications at low bit rates (typically on the order
of 100 bps). To achieve bandwidth efficiency, i.e. to transmit at a bit rate greater than the available
bandwidth, the information must be encoded into the phase or the amplitude of the signal, as it is done
in PSK or quadrature amplitude modulation (QAM).
For example, in a 4-PSK system, the information bits (0 and 1) are mapped into one of four
possible symbols, ±1±j.
The symbol stream modulates the carrier, and the so-obtained signal is transmitted over the
channel. To detect this type of signal on a multipath-distorted acoustic channel, a receiver must
employ an equalizer whose task is to unravel the inter symbol interference. Since the channel response
is not a-priori known (moreover, it is time-varying) the equalizer must “learn” the channel in order to
invert its effect. A block diagram of an adaptive decision-feedback equalizer (DFE) is shown in Figure
3. In this configuration, multiple input signals, obtained from spatially diverse receiving hydrophones,
can be used to enhance the system performance.
The receiver parameters are optimized to minimize the mean squared error in the detected data
stream. After the initial training period, during which a known symbol sequence is transmitted, the
equalizer is adjusted adaptively, using the output symbol decisions. An integrated Doppler tracking
algorithm enables the equalizer to operate in a mobile scenario.
This receiver structure has been used on various types of acoustic channels. Current
achievements include transmission at bit rates on the order of one kbps over long ranges (10-100
nautical miles) and several tens of kbps over short ranges (few km) as the highest rates reported to
date. On a more unusual note, successful operation was also demonstrated over a basin scale (3000
km) at 10 bps, as well as over a short vertical channel at a bit rate in excess of 100 kbps.
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The multichannel DFE forms the basis of a high-speed acoustic modem implemented at the
Woods Hole Oceanographic Institution.
The modem, shown in Figure 4, is implemented in a fixed-point DSP, with a floating-point co-
processor for high-rate mode of operation. When active, it consumes about 3 W in receiving mode,
and 10-50 W to transmit.
The board measures 1.75 _ 5 in, and accommodates four input channels. The modem has
successfully been deployed in a number of trials, including autonomous underwater vehicle (AUV)
communications at 5 kbps.
Fig. 4: The WHOI micromodem
has dual mode of operation: low
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UNDERWATER NETWORKS
With advances in acoustic modem technology, sensor technology and vehicular technology, ocean
engineering today is moving towards integration of these components into autonomous underwater
networks. While current applications include supervisory control of individual AUVs, and telemetry of
oceanographic data from bottom-mounted instruments, the vision of future is that of a “digital ocean”
in which integrated networks of instruments, sensors, robots and vehicles will operate together in a
variety of underwater environments. Examples of emerging applications include fleets of AUVs
deployed on collaborative search missions, and ad hoc deployable sensor networks for environmental
monitoring.
Fig. 5: Centralized network topology
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Fig. 6: Decentralized network topology.
Depending on the application, future underwater networks are likely to evolve in two directions:
centralized and decentralized networks. The two types of topologies are illustrated in Figure 5 and
Figure 6. In a centralized network, nodes communicate through a base station that covers one cell.
Larger area is covered by more cells whose base stations are connected over a separate
communications infrastructure.
The base stations can be on the surface and communicate using radio links, as shown in the
figure, or they can be on the bottom, connected by a cable. Alternatively, the base station can be
movable as well. In a decentralized network, nodes communicate via peer-to-peer, multi-hop
transmission of data packets. The packets must be relayed to reach the destination, and there may be a
designated end node to a surface gateway. Nodes may also form clusters for a more efficient
utilization of communication channel.
To accommodate multiple users within a selected network topology, the communication
channel must be shared, i.e. access to the channel must be regulated. Methods for channel sharing are
based on scheduling or on contention. Scheduling, or deterministic multiple-access, includes
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frequency, time and code-division multiple-access (FDMA, TDMA, CDMA) as well as a more
elaborate technique of space-division multiple access (SDMA).
Contention-based channel sharing does not rely on an a-priori division of channel resources;
instead, all the nodes contend for the use of channel, i.e., they are allowed to transmit randomly at will,
in the same frequency band and at the same time, but in doing so they must follow a protocol for
medium-access control (MAC) to ensure that their information packets do not collide. All types of
multiple-access are being considered for the underwater acoustic systems.
Experimental systems today favor either polling, TDMA, or multiple-access collision
avoidance (MACA) based on a hand-shaking contention procedure that requires an exchange of
requests and clearances to send (RTS/CTS). Intelligent collision avoidance appears to be necessary in
an underwater channel, where the simple principle of carrier sensing multiple access (CSMA) is
severely compromised due to the long propagation delay—the fact that the channel is sensed as idle at
some location does not guarantee that a data packet is not already in transmission at a remote location.
one of the major aspects of the evolving underwater networks is the requirement for scalability. A
method for channel sharing is scalable if it is equally applicable to any number of nodes in a network
of given density. For example, a pure TDMA scheme is not scalable, as it rapidly looses efficiency on
an underwater channel due to the increase in maximal propagation delay with the area of coverage. In
order to make this otherwise appealing scheme scalable, it can be used locally, and combined with
another technique for spatial reuse of channel resources. The resulting scheme is both scalable and
efficient; however, it may require a sophisticated dynamic network management.
In contrast, contention-based channel allocation offers simplicity of implementation, but its
efficiency is limited by the channel latency. Hence, there is no single best approach to the deployment
of an underwater network. Instead, selection of communication algorithms and network protocols is
driven by the particular system requirements and performance/complexity trade-offs.
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Fig. 7:A deep-sea observatory.
today is active on all topics in underwater communication networks: from fundamental capacity
analyses to the design of practical network protocols on all layers of the network architecture
(including medium access and data link control, routing, transport control and application layers) as
well as cross-layer network optimization.
In addition to serving as stand-alone systems, underwater acoustic networks will find application in
more complex, heterogeneous systems for ocean observation. Figure 7 shows the concept of a deep sea
observatory.
At the core of this system is an underwater cable that hosts a multitude of sensors and
instruments, and provides high-speed connection to the surface. A wireless network, integrated into the
overall structure, will provide a mobile extension, thus extending the reach of observation. While we
have focused on acoustic wireless communications, it has to be noted that this will not be the only way
of establishing wireless communication in the future underwater networks.
Optical waves, and in particular those in the blue-green region, offer much higher throughput
(Mbps) albeit over short distances (up to about 100 m). As such, they offer a wireless transmission
capability that complements acoustic communication.
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APPLICATIONS
 Future applications could enhance myriad industries, ranging from the offshore oil industry to
aquaculture to fishing industries, she noted. Additionally, pollution control, climate recording, ocean
monitoring (for prediction of natural disturbances) and detection of objects on the ocean floor are
other areas that could benefit from enhanced underwater communications.
 Environmental monitoring to gathering of oceanographic data
 Marine archaeology
 Search and rescue missions
 Defance .
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DISADVANTAGES
 Battery power is limited and usually batteries cannot be recharged also because solar energy
cannot be exploited .
 The available bandwidth is severely limited.
 Channel characteristics including long and variable propagation delays
 Multipath and fading problems.
 High bit error rate.
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CONCLUSION
In this topic we overviewed the main challenges for efficient communication in under water
acoustic sensor networks. We outlined the peculiarities of the underwater channel with particular
reference to networking solutions the ultimate objective of this topic is to encourage research efforts to
lay down fundamental basics for the development of new advanced communication techniques for
efficient under water communication and networking for enhanced ocean monitoring and exploration
applications
 The aim of this is to build a acoustic communication
 This is not only the way for underwater communication
 By using optical waves which offers higher throughput (Mbps) over short distances (up to
about 100 m)
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REFFERENCES
1. www.redtacton.com
2. www.wikipedia.com
3. www.howstuffworks.com
4. www.google.com
5.
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