Underwater Wireless Communication is the wireless communication in which acoustic signals (waves) carry digital information through an underwater channel.
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 hydrothermal
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.
Underwater wireless communication networks (UWCNs) consist of sensors and autonomous underwater vehicles (AUVs) that interact, coordinate and share information with each other to carry out sensing and monitoring functions.
Underwater acoustic communication is a technique of sending and receiving message below water.[1] 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.
Underwater Wireless Communication is the wireless communication in which acoustic signals (waves) carry digital information through an underwater channel.
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 hydrothermal
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.
Underwater wireless communication networks (UWCNs) consist of sensors and autonomous underwater vehicles (AUVs) that interact, coordinate and share information with each other to carry out sensing and monitoring functions.
Underwater acoustic communication is a technique of sending and receiving message below water.[1] 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.
WIMAX stands for Worldwide Interoperability for Microwave Access.WiMAX refers to broadband wireless networks that are based on the IEEE 802.16 standard, which ensures compatibility and interoperability between broadband wireless access equipment.
It is the repeated switching of frequencies during radio transmission, often to minimize the effectiveness of "electronic warfare" - that is, the unauthorized interception or jamming of telecommunications.
CR : smart radio that has the ability to sense the external environment, learn from the history and make intelligent decisions to adjust its transmission parameters according
to the current state of the environment.
Topic on Underwater Communication which includes both underwater wireless and wired communication . A full detailed overview about the topic has been given. Pictures are given to visualize the topic in better way. Covers a major potion like Hydrophones and SONAR. Can be presented as a seminar topic as well .
WIMAX stands for Worldwide Interoperability for Microwave Access.WiMAX refers to broadband wireless networks that are based on the IEEE 802.16 standard, which ensures compatibility and interoperability between broadband wireless access equipment.
It is the repeated switching of frequencies during radio transmission, often to minimize the effectiveness of "electronic warfare" - that is, the unauthorized interception or jamming of telecommunications.
CR : smart radio that has the ability to sense the external environment, learn from the history and make intelligent decisions to adjust its transmission parameters according
to the current state of the environment.
Topic on Underwater Communication which includes both underwater wireless and wired communication . A full detailed overview about the topic has been given. Pictures are given to visualize the topic in better way. Covers a major potion like Hydrophones and SONAR. Can be presented as a seminar topic as well .
A GSM Jammer or cell phone jammer is a device that transmit signal on the same frequency at which the GSM system operates, the jamming success when the mobile phones in the area where the jammer is located are disabled.
Communication jamming devices were first developed and used by military. Where tactical commanders use RF communications to exercise control of their forces, an enemy has interest in those communications. This interest comes from the fundamental area of denying the successful transport of the information from the sender to the receiver.
Nowadays the mobile jammer devices or cell phone jammer software are becoming civilian products rather than electronic warfare devices, since with the increasing number of the mobile phone users the need to disable mobile phones in specific places where the ringing of cell phone would be disruptive has increased. These places include worship places, university lecture rooms, libraries, concert halls, meeting rooms, and other places where silence is appreciated
water softener circuit detail and water monitoring system using Wireless Sensor Network (zigbee configuration) Arduino software coding and real time matlab plotting of arduino data.
In this presentation, Stirling Dynamics discusses the uses and benefits of active controls for aerospace applications. The presentation covers the background of active controls, their current use and Stirling’s view on how active controls could be used in the future to enhance the flight simulation experience and to improve aircraft safety. Stirling has been at the forefront of the active controls since the early 1990s and is well positioned to lead the discussion on the future potential of the technology.
Wdm based fso link optimizing for 180 km using bessel filtereSAT Journals
Abstract Free space optical link is a growing field in communication due to its advantage of wide bandwidth, high security and easy installation. A wavelength division multiplexing (WDM) access network using free space optical (FSO) communication in different weather conditions like haze and rain are discussed in this article and find out the possibility of communication link up to 180 km in clear weather with 2.5 Gbps data rate on the wavelength of 1550 nm and up to 54 km in haze condition using same data rate & wave length. Further the effect of using two different low pass filter (Gaussian and Bessel) at the receiver are discussed and conclude that Bessel filter is better on 2.5 Gbps data rate for WDM based FSO link. Keywords: optical communications, wavelength Division Multiplexing (WDM), free space optics (FSO)
FSO networks under turbulence - Northumbria University 2013 Research ConferenceJoaquin Perez
FSO networks: understanding route diversity under turbulence phenomena towards reliable FSO mesh networks design.
In last mile extensions of MANs, wireless mesh networks are multi-hop networks being used as backbone networks connecting end-users with the access points connected to the Internet. Wireless mesh networks are an attractive option over optical fibres because of their ease of installation and cost effectiveness of deployment[1]. Moreover, Free Space Optics (FSO) technology is an attractive option for use in mesh networks [2, 3]. However, time-variant influence of the atmosphere in FSO links that introduces one of the main drawbacks [4]. In order to overcome the turbulence induced fading in FSO systems, several techniques have been proposed These include: spatial transmitter/receiver diversity [5] [6]; adaptive beam forming [7]; wavelength diversity [8], multiple-beam communication [9], novel modulation techniques and hybrid RF/optical link scheme. Moreover, topology design and routing are essential tools for FSO mesh networks performance. The turbulence phenomena also influences in the topology and routing design of complex FSO networks, then route diversity techniques will improve the mesh network reliability [14]. For example, route diversity application within mesh optical networks deployed Tokyo provided interesting experiment results in [15]. This presentation will offer an overview of turbulence phenomena on FSO mesh networks from route diversity point of view.
References
[1] I. F. Akyildiz, X. Wang, and W. Wang, "Wireless mesh networks: a survey," Computer Networks, vol. 47, pp. 445-487, 2005.
[2] Z. Hu, P. Verma, and J. J. Sluss, "Improved reliability of free-space optical mesh networks through topology design," J. Opt. Netw., vol. 7, pp. 436-448, 2008.
[3] A. Kashyap, K. Lee, M. Kalantari, S. Khuller, and M. Shayman, "Integrated topology control and routing in wireless optical mesh networks," Computer Networks, vol. 51, pp. 4237-4251, 2007.
[4] Z. Ghassemlooy, W. Popoola, and S. Rajbhandari, Optical Wireless Communications : System and Channel Modelling with MATLAB: CRC Press 2012.
[5] S. M. Navidpour, M. Uysal, and M. Kavehrad, "BER performance of free-space optical transmission with spatial diversity," IEEE Trans. Wireless Commun., vol. 6, pp. 2813-2819, Aug 2007.
[6] H. Moradi, H. H. Refai, and P. G. LoPresti, "Switch-and-stay and switch-and-examine dual diversity for high-speed free-space optics links," IET Optoelectron, vol. 6, pp. 34-42, 2012.
[7] R. K. Tyson, "Bit-error rate for free-space adaptive optics laser communications," J. Opt. Soc. Am. A:, vol. 19, pp. 753-758, Apr 2002.
[8] V. Weerackody and A. R. Hammons, "Wavelength Correlation in Free Space Optical Communication Systems," in Proceedings of IEEE Military Communications Conference 2006, 2006, pp. pp. 1-6.
ESTIMATION OF CHANNEL IN OFDM WIRELESS CHANNEL USING LS AND MMSE TECHNIQUESIAEME Publication
In recent years with the increase in digital data communication, the need for high data rates with less information loss or distortion is being a continuous research area and new techniques are being invented in this area. Large amount of people are using the air interface for proper communication which also have a lot of drawbacks which include multipath fading, Inter symbol interference (ISI), Doppler shift etc..This paper is being presented on basis of channel estimation of wireless mobile OFDM channels using known pilot symbols.
In this i tried to explain about under water communication.
Introduction of underwater communication.
Problem due to Multipath Propagation
Techniques used for underwater communication
1. Single Carrier Systems
2. MCM Techniques
3. Space-Time Modulation Techniques
Applications
Limitations
Conclusion
Development of an FHMA-based Underwater Acoustic Communications System for Mu...Waqas Tariq
This paper describes the design of an underwater acoustic communications system for multiple underwater vehicles, based on frequency-hopping multiple-access (FHMA) and tamed spread-spectrum communications. The system makes used of the tamed spread-spectrum method, frequency hopping, 4FSK, and a rake receiver. In order to make the system more practical, the underwater channel and the effect of the number of users on the bit error ratio (BER) are also taken into account. Since the necessary proving experiments are not easily conducted in the ocean, a platform is developed that uses the sound card of a computer, combined with a sound box and microphone, to transduce energy for acoustic communications. Simulated and experimental results indicate that this system could provide reliable underwater communications between multiple underwater vehicles.
Realization of ofdm based underwater acoustic communicationeSAT Journals
Abstract Nowadays underwater communication plays a vital role in applications from commercial extends to military purposes. Present underwater communication systems involve the transmission of information in the form of sound, electromagnetic (EM), or optical waves. All these techniques has their own benefits and limitations. Acoustic communication is the most versatile and widely used technique in underwater environments because of its low attenuation compared with others. Acoustic waves are more applicable for thermally stable, deep water settings. But acoustic waves in shallow water can be adversely affected by temperature gradients, surface ambient noise, and multipath propagation due to reflection and refraction. The much slower speed of acoustic propagation in water, about 1500 m/s (meters per second), compared with that of electromagnetic and optical waves, and is another limiting factor for efficient communication and networking. Nevertheless, the currently favorable technology for underwater communication is upon acoustics. In this paper, we are planning to design a simple underwater acoustic system. We first discuss about the problems of underwater communication. Then we are designing a data transmission system in underwater and its analysis is done in the next step. Keywords: Underwater acoustic communication, Orthogonal frequency division multiplexing, Differential phase shift keying
Error Rate Performance of Interleaved Coded OFDM For Undersea Acoustic LinksCSCJournals
Studies on undersea acoustic communication links, set up through highly complex and inhomogeneous underwater channel using various orders of QAM and PSK based OFDM techniques, have been reported in open literature. However, their bit error rate performances still need to be improved. Coding, when combined with OFDM, helps to detect and correct errors without having the overhead of too many retransmissions, as the bandwidth is a scarce resource in undersea scenario. The technique of interleaving, which is frequently employed in digital communication and storage systems to enhance the performance of the coding schemes, can be used to improve the error rate performance of the coded OFDM. The error rate performances of interleaved convolutional and BCH coded OFDMs for undersea acoustic links for binary phase shift keying and its differential variant have been studied in this paper. It is found that at high SNR, the process of interleaving and coding offers significant improvement in the error rate performance. It is also worth mentioning the fact that interleaving improves the performance of both convolutional and BCH coded OFDM systems.
Under water acoustic (uw a) communication architecture and the key notions of...eSAT Publishing House
IJRET : International Journal of Research in Engineering and Technology is an international peer reviewed, online journal published by eSAT Publishing House for the enhancement of research in various disciplines of Engineering and Technology. The aim and scope of the journal is to provide an academic medium and an important reference for the advancement and dissemination of research results that support high-level learning, teaching and research in the fields of Engineering and Technology. We bring together Scientists, Academician, Field Engineers, Scholars and Students of related fields of Engineering and Technology
Under water acoustic (uw a) communication architecture and the key notions of...eSAT Journals
Abstract The study of the communication architecture for the underwater is very important due the wide applications based on these sensing devices. The more observed examples of applications are climate change monitoring, study of marine life, pollution control, and military purposes. It is observed the radio frequency (RF) electromagnetic waves [1], Optical electromagnetic waves, underwater Optical communication [2] waves all have been used for the underwater sensor networking. But these have shown with its slight disadvantages for the underwater related works. So this can be overcome by the usage of the acoustic communication as a means of transmission technology for the underwater networked system. It is also observed that the underwater acoustic signals are suffering from the some transmission loss, high propagation delay, limited bandwidth. So to eliminate these observed factors with respect to the underwater sensor networks the communication architecture has been developed to provide the point to point, having low data rate and high bandwidth signal and delay tolerant applications. Therefore this article has been written to give the details of how the communication architecture can be designed and for the underwater network and the key factors which relates to the propagation of underwater acoustic signals. Keywords: UW-A, Doppler Spread, Attenuation, Variance
What is the main difference between the First and the Second generati.pdfjovankarenhookeott88
Two small insulating spheres with radius 9.00×102m are separated by a large center-to-center
distance of 0.485 m . One sphere is negatively charged, with net charge -1.90 C , and the other
sphere is positively charged, with net charge 4.10 C . The charge is uniformly distributed within
the volume of each sphere.
Part A
What is the magnitude E of the electric field midway between the spheres?
Take the permittivity of free space to be 0 = 8.85×1012 C2/(Nm2) .
Part B
What is the direction of the electric field midway between the spheres?
What is the direction of the electric field midway between the spheres?E = N/C
Solution
Sepoaration of the spheres r = 0.485 m, charge on first sphere q = -1.9* 10^-6 C
charge on second sphere q \' = 4.10 * 10^-6 C.
the magnitude E of the electric field midway between the spheres is
E = E \' + E \"
= Kq / ( r/ 2)^ 2 + Kq \' / ( r/ 2)^ 2
= 4K(q+q \') / ( r)^ 2
= 4*8.99*10^9 *[1.9* 10^-6 + 4.10 * 10^-6] / [ 0.485]^2
= 9.17e+5 N / C
b,
field lines are starting at positive charge and ending at negative charge.
so, the field is pointed toward the negative sphere..
Hybrid optimization of pumped hydro system and solar- Engr. Abdul-Azeez.pdffxintegritypublishin
Advancements in technology unveil a myriad of electrical and electronic breakthroughs geared towards efficiently harnessing limited resources to meet human energy demands. The optimization of hybrid solar PV panels and pumped hydro energy supply systems plays a pivotal role in utilizing natural resources effectively. This initiative not only benefits humanity but also fosters environmental sustainability. The study investigated the design optimization of these hybrid systems, focusing on understanding solar radiation patterns, identifying geographical influences on solar radiation, formulating a mathematical model for system optimization, and determining the optimal configuration of PV panels and pumped hydro storage. Through a comparative analysis approach and eight weeks of data collection, the study addressed key research questions related to solar radiation patterns and optimal system design. The findings highlighted regions with heightened solar radiation levels, showcasing substantial potential for power generation and emphasizing the system's efficiency. Optimizing system design significantly boosted power generation, promoted renewable energy utilization, and enhanced energy storage capacity. The study underscored the benefits of optimizing hybrid solar PV panels and pumped hydro energy supply systems for sustainable energy usage. Optimizing the design of solar PV panels and pumped hydro energy supply systems as examined across diverse climatic conditions in a developing country, not only enhances power generation but also improves the integration of renewable energy sources and boosts energy storage capacities, particularly beneficial for less economically prosperous regions. Additionally, the study provides valuable insights for advancing energy research in economically viable areas. Recommendations included conducting site-specific assessments, utilizing advanced modeling tools, implementing regular maintenance protocols, and enhancing communication among system components.
Industrial Training at Shahjalal Fertilizer Company Limited (SFCL)MdTanvirMahtab2
This presentation is about the working procedure of Shahjalal Fertilizer Company Limited (SFCL). A Govt. owned Company of Bangladesh Chemical Industries Corporation under Ministry of Industries.
Explore the innovative world of trenchless pipe repair with our comprehensive guide, "The Benefits and Techniques of Trenchless Pipe Repair." This document delves into the modern methods of repairing underground pipes without the need for extensive excavation, highlighting the numerous advantages and the latest techniques used in the industry.
Learn about the cost savings, reduced environmental impact, and minimal disruption associated with trenchless technology. Discover detailed explanations of popular techniques such as pipe bursting, cured-in-place pipe (CIPP) lining, and directional drilling. Understand how these methods can be applied to various types of infrastructure, from residential plumbing to large-scale municipal systems.
Ideal for homeowners, contractors, engineers, and anyone interested in modern plumbing solutions, this guide provides valuable insights into why trenchless pipe repair is becoming the preferred choice for pipe rehabilitation. Stay informed about the latest advancements and best practices in the field.
Automobile Management System Project Report.pdfKamal Acharya
The proposed project is developed to manage the automobile in the automobile dealer company. The main module in this project is login, automobile management, customer management, sales, complaints and reports. The first module is the login. The automobile showroom owner should login to the project for usage. The username and password are verified and if it is correct, next form opens. If the username and password are not correct, it shows the error message.
When a customer search for a automobile, if the automobile is available, they will be taken to a page that shows the details of the automobile including automobile name, automobile ID, quantity, price etc. “Automobile Management System” is useful for maintaining automobiles, customers effectively and hence helps for establishing good relation between customer and automobile organization. It contains various customized modules for effectively maintaining automobiles and stock information accurately and safely.
When the automobile is sold to the customer, stock will be reduced automatically. When a new purchase is made, stock will be increased automatically. While selecting automobiles for sale, the proposed software will automatically check for total number of available stock of that particular item, if the total stock of that particular item is less than 5, software will notify the user to purchase the particular item.
Also when the user tries to sale items which are not in stock, the system will prompt the user that the stock is not enough. Customers of this system can search for a automobile; can purchase a automobile easily by selecting fast. On the other hand the stock of automobiles can be maintained perfectly by the automobile shop manager overcoming the drawbacks of existing system.
COLLEGE BUS MANAGEMENT SYSTEM PROJECT REPORT.pdfKamal Acharya
The College Bus Management system is completely developed by Visual Basic .NET Version. The application is connect with most secured database language MS SQL Server. The application is develop by using best combination of front-end and back-end languages. The application is totally design like flat user interface. This flat user interface is more attractive user interface in 2017. The application is gives more important to the system functionality. The application is to manage the student’s details, driver’s details, bus details, bus route details, bus fees details and more. The application has only one unit for admin. The admin can manage the entire application. The admin can login into the application by using username and password of the admin. The application is develop for big and small colleges. It is more user friendly for non-computer person. Even they can easily learn how to manage the application within hours. The application is more secure by the admin. The system will give an effective output for the VB.Net and SQL Server given as input to the system. The compiled java program given as input to the system, after scanning the program will generate different reports. The application generates the report for users. The admin can view and download the report of the data. The application deliver the excel format reports. Because, excel formatted reports is very easy to understand the income and expense of the college bus. This application is mainly develop for windows operating system users. In 2017, 73% of people enterprises are using windows operating system. So the application will easily install for all the windows operating system users. The application-developed size is very low. The application consumes very low space in disk. Therefore, the user can allocate very minimum local disk space for this application.
Water scarcity is the lack of fresh water resources to meet the standard water demand. There are two type of water scarcity. One is physical. The other is economic water scarcity.
CFD Simulation of By-pass Flow in a HRSG module by R&R Consult.pptxR&R Consult
CFD analysis is incredibly effective at solving mysteries and improving the performance of complex systems!
Here's a great example: At a large natural gas-fired power plant, where they use waste heat to generate steam and energy, they were puzzled that their boiler wasn't producing as much steam as expected.
R&R and Tetra Engineering Group Inc. were asked to solve the issue with reduced steam production.
An inspection had shown that a significant amount of hot flue gas was bypassing the boiler tubes, where the heat was supposed to be transferred.
R&R Consult conducted a CFD analysis, which revealed that 6.3% of the flue gas was bypassing the boiler tubes without transferring heat. The analysis also showed that the flue gas was instead being directed along the sides of the boiler and between the modules that were supposed to capture the heat. This was the cause of the reduced performance.
Based on our results, Tetra Engineering installed covering plates to reduce the bypass flow. This improved the boiler's performance and increased electricity production.
It is always satisfying when we can help solve complex challenges like this. Do your systems also need a check-up or optimization? Give us a call!
Work done in cooperation with James Malloy and David Moelling from Tetra Engineering.
More examples of our work https://www.r-r-consult.dk/en/cases-en/
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1. 1113313
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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
[1]
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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
[2]
3. 1113313
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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
[3]
4. 1113313
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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..
[5]
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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
[6]
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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)
[16]