seminar underwater wireless

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UNDER WATER COMMUNICATION 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. SWARNANDHRA COLLEGE OF ENGINEERING AND TECHNOLOGY

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Page 1: Seminar Underwater Wireless

UNDER WATER COMMUNICATION

ABSTRACTWhile 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.

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1. INTRODUCTION

Wireless Underground Communication Networks (WUCNs) constitute one of the

promising application areas of the recently developed wireless networking techniques. The

WUCNs consist of wireless devices that operate below the ground surface. These devices are

either

(i) Buried completely under dense soil or

(ii) Placed within a bounded open underground space such as underground mines and

road/subway tunnels.

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.

1.1 OBJECTIVE

Our 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.

1.2 ORGANIZATION OF THESIS

This thesis is divided into six chapters. Chapter1 describes the introduction of under

water communication and its need. Chapter2 describes the Wave propagation, Why Sound as

a communication medium in UW-ASN, Traditional approaches for ocean bottom monitoring.

Chapter3 describes under water networks, Centralized network topology, decentralized

network topology and SONAR. Chapter4 gives the technologies used earlier and today.

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Chapter5 explains the advantages, disadvantages and applications. Chapter6 is the final

conclusion and future scope and references.

UNDER WATER ACOUSTIC COMMUNICATION

2.1 INTRODUCTION

Underwater acoustic communication is a technique of sending and receiving message

below water. There are several ways of employing such communication but the most

common is using hydrophones. Under water communication is difficult due to factors like

multi-path propagation, time variations of the channel, small available bandwidth and

strong signal attenuation, especially over long ranges. In underwater communication there are

low data rates compared to terrestrial communication, since underwater communication

uses acoustic waves instead of electromagnetic waves. Electromagnetic transmission is more

difficult in water. Acoustic transmission is better suited to water than air

Speed of sound in water ~ 1500m/sec

Speed of sound in air ~ 340m/sec

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.

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2.2 WAVE PROPAGATION

Fig. 2.1: 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.

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.

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. 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

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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 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.

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.

Fig. 3: Multichannel adaptive decision-feedback

Equalizer (DFE) is used for high-speed underwater acoustic communications. It supports any

linear modulation format, such as M-ary PSK or M-ary QAM

2.3 SOUND AS A COMMUNICATION MEDIUM IN UW-ASN

UW-ASN:: Underwater Acoustic Sensor Network

Radio waves propagate at long distances through conductive sea water only at extra low

frequencies (30-300 Hz), which require large antennae and high transmission power. Optical

waves do not suffer from such high attenuation but are affected by scattering. More

over transmission of optical signals requires high precision in pointing the narrow laser beam.

2.4 TRADITIONAL APPROACHES FOR OCEAN BOTTOM MONITORING

Uses sensors to record data. Deploy underwater sensors to record data during the monitoring

mission, and then recover the instruments. This approach has the following disadvantages:

Real time monitoring is not possible.

No interaction is possible between onshore control systems and the monitoring

instruments.

If failures or misconfiguration occur, it may not be possible to detect them before the

instruments are recovered.

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The amount of data that can be recorded during the monitoring mission by every

sensor is limited by the capacity of the onboard storage devices (memories, hard

disks, etc).

Disadvantages:

Real time monitoring is not possible.

No interaction b/w onshore control systems and the monitoring instruments.

failures or misconfiguration may occur

Limited storage capacity

3. UNDER WATER NETWORKS3.1 CENTRALIZED NETWORK TOPOLOGY

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

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emerging applications include fleets of AUVs deployed on collaborative search missions, and

ad hoc deployable sensor networks for environmental monitoring.

Fig. 3.1: Centralized network topology

3.2 DECENTRALIZED NETWORK TOPOLOGY

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.

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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 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. deep-sea observatory. Fig. deep-sea observatory.

Research 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.

3.3 SONAR

SONAR Sound Navigation And Ranging

Two types of technologies: Active SONAR, Passive SONAR

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Sonar (originally an acronym for Sound Navigation And Ranging) is a technique that

uses sound propagation (usually underwater, as in Submarine navigation) to navigate,

communicate with or detect other vessels.

1. Two types of technology share the name "sonar":

2. Active sonar is emitting pulses of sounds and listening for echoes. 

3. passive sonar is essentially listening for the sound made by vessels

4. Sonar systems generally use highly directional beams of sound when searching for

targets. In this way they are able to determine direction to the target, as well as the

distance. The echoes heard in active sonar systems can also be very distinct.

Experienced sonar technicians are often able to tell the difference between echoes

produced by a submarine, a rock outcrop, a school of fish, or a whale.

5. Active SONAR uses a sound transmitter and a receiver

• Principle:

Creates “ping”. Listens “echo”

Active sonar uses a sound transmitter and a receiver. When the two are in the same place it is

monostatic operation. When the transmitter and receiver are separated it is bistatic operation.

When more transmitters (or more receivers) are used, again spatially separated, it is

multistate operation. Most sonar’s are used monostatically with the same array often being

used for transmission and reception

Principle of Active SONAR

Active sonar creates a pulse of sound, often called a "ping", and then listens

for reflections using a sonar Projector consisting of a signal generator, power amplifier and

electro-acoustic transducer/array. A beam former is usually employed to concentrate the

acoustic power into a beam, which may be swept to cover the required search angles. To

measure the distance to an object, the time from transmission of a pulse to reception is

measured and converted into a range by knowing the speed of sound. To measure the bearing,

several hydrophones are used, and the set measures the relative arrival time to each or with an

array of hydrophones, by measuring the relative amplitude in beams formed through a

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process called beam forming. (echo) of the pulse. This pulse of sound is generally created

electronically

Passive SONAR Passive sonar listens without transmitting.

Noise limitations

Passive sonar listens without transmitting. It is often employed in military settings,

although it is also used in science applications, e.g., detecting fish for

presence/absence studies in various aquatic environments..

Passive sonar on vehicles is usually severely limited because of noise generated by

the vehicle. For this reason, many submarines operate nuclear reactors that can be

cooled without pumps, using silent convection, or fuel cells or batteries, which can

also run silently.

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4. TECHNOLOGY IN USE

4.1 ACCOUSTIC 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

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of 100 kbps.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 micro modem has dual mode of operation: low

4.2 VECTOR SENSOR

Vector sensor is capable of measuring important non-scalar components of the acoustic field such as

the wave velocity, which cannot be obtained by a single scalar pressure sensor. They have been

mainly used for underwater target localization and SONAR applications

Earlier underwater acoustic communication systems have been relying on scalar sensors only,

which measure the pressure of the acoustic field. Vector sensors measure the scalar and

vector components of the acoustic field in a single point in space; therefore can serve as a

compact multichannel receiver

In general, there are two types of vector sensors: inertial and gradient. Inertial sensors truly

measure the velocity or acceleration by responding to the acoustic medium motion, whereas

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gradient sensors employ a finite-difference approximation to estimate the gradients of the

acoustic field such as velocity and acceleration.

in fig….Vector sensor communications with three channels the pressure channel p,

represented by a straight dashed line, and two pressure-equivalent velocity

channels pz and py, shown by curved dashed lines.

In the example of vector sensor communications shown, there is one transmitter pressure

transducer, shown by a black dot, whereas for reception we use a vector sensor, shown by a

black square, which measures the pressure and the y and zcomponents of the velocity. This is

a 1×3 single-input multiple-output (SIMO) system. With more pressure transmitters, one can

have a multiple-input multiple-output (MIMO) system also.

4.3 TYPICAL SUBMARINE CABLE SYSTEM

How Submarine Cables Work

Modern submarine telecommunications cables rely on a property of pure glass fibers,

whereby light is transmitted by internal reflection. Because the light signal loses strength en

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route, repeaters are installed along the cable to boost the signal New systems rely on optical

amplifiers – glass strands containing the element, erbium. Strands are spliced at intervals

along a cable & then energized by lasers that cause erbium-doped fibers to boost optical

signals

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Fig 3.3 Modern submarine cable

4.4 LAYING AND MAINTAING CABLES

Laying typically involves:

1. Selection of route

2. Assessment of potential impacts of cable lying on environment

3. Full survey of route & its final selection .Design cable to meet environmental

conditions.

4. Laying of cable Notification of cable position. In some cases, a post-lay survey if

repair or replacement needed, an operational plan may be required along with

requirements outlined above.

Cable Burial

Cables typically buried 1-3.5m under the seabed (can extend to 10m) to protect from fishing

& other activities Burial may extend from shore out to ~2000m water depth, which will

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protect submarine cables from the majority of trawl fisheries Burial may locally disrupt the

seabed along a narrow path &form turbid water, whose extent relates to burial technique,

seabed type & wave/current action . In the absence of cable-based studies, analysis of seabed

disturbance by other activities suggests impacts short-lived (months) where waves/currents

are active, but possibly longer-lived in deeper, less turbulent water

Cable Protection Zones as Sanctuaries

Cable “protection zones” may act as

marine.Sanctuaries to improve biodiversity

& fish stocks. An effective zone must

contain habitats suitable for fish & other

marine life, exist long enough for

ecosystems to develop, be policed to

Prevent illegal Experiment to count fish to

test if cable protection zone acts as a

Marine fishing

4.5 EFFECTS OF NATURAL HAZARDS

1. Katrina as a Category V hurricane, August, 2005. Such events affected

cables by flooding coastal facilities, triggering submarine landslides, &

forming strong, eroding currents/waves Courtesy: NASA.

2. Submarine cables are exposed to a range of natural hazards in all water

depths.

3. In water depths less than ~ 1000m the main hazards are human activities;

natural impacts cause <10% of cable damage.

4. In water depths more than ~1000 m, natural hazards dominate & include:

Submarine earthquakes, fault lines & related landslides break or bury

cables.

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5. Density currents - break or bury.

6. Currents & waves - abrasion, stress & fatigue ~ Tsunami, storm surge &

sea level rise - damage coastal Installations.

5. UNDER WATER COMMUNICATION

5.1challenges

1. Battery Power is limited

2. Limited available bandwidth

3. Channel characteristics

4. UW sensors are prone to failures because of fouling, corrosion, etc

5. Mobility

6. The ocean can be as deep as 10 km

Battery power is limited and usually batteries can not be recharged because solar energy

cannot be exploited. The available bandwidth is severely limited. Channel characteristics,

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including long and variable propagation delays, multi-path and fading problems. High bit

error rates. Underwater sensors are prone to failures because of fouling, corrosion, etc. A

unique feature of underwater networks is that the environment is constantly mobile, naturally

causing the node passive mobility. The ocean can be as deep as 10 km.

5.2 CABLES, SATELLITES ADVATAGES

1. High reliability, capacity & security

2. None of the delays present in satellite traffic

3. Cost-effective on major routes, hence rates cheaper than satellites

4. Submarine cables carry >95% of international voice & data traffic

Advantages of satellites

1. Suitable for disaster prone areas

2. Provides wide coverage for mobile subscribers

3. Suitable for linking isolated regions and small island nations into the international

telecom network.

4. Satellites carry <5% of international voice & data traffic

5.3 Applications of Underwater Communication

1. Seismic monitoring.

2. Pollution monitoring

3. Ocean currents monitoring

4. Equipment monitoring and control

5. Autonomous Underwater Vehicles (AUV)

6. Environmental monitoring to gathering of oceanographic data

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7. Marine archaeology

8. Search and rescue missions

9. Defense

To make these applications viable, there is a need to enable underwater communications

among underwater devices -> Wireless underwater networking. Use sound as the wireless

communication medium. Pollution monitoring: The density or minerality of water is changed

that the presence of pollution. Ocean current monitoring: the of the water is monitored

Disadvantages

1. Battery power is limited and usually batteries cannot be recharged also because solar

energy cannot be exploited.

2. The available bandwidth is severely limited.

3. Channel characteristics including long and variable propagation delays

4. Multipath and fading problems.

5. High bit error rate.

6. CONCLUSION

In this topic we overviewed the main challenges for efficient communication in under

water acoustic sensor networks. We outlined the peculiarities of the under water 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

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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)

Future scope 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.

REFERENCES

To get more information and other stuff if you need to know refer the following links will help you in understanding concepts more clearly.

www.redtacton.com

www.wikipedia.com

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www.google.com

www.howstuffworks.com

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