Design and Development of Underwater Acoustic Modem for Shallow Waters and Short Range Communication

Table of Contents

Abstract

List of Tables

List of Figures

Nomenclature

CHAPTER 1 - Introduction
1.1 Introduction to Underwater Acoustic Modem
1.2 Classification of the Acoustic/Sonar Communication Systems
1.3 Motivation
1.4Scope of the work
1.5Thesis Outline

CHAPTER 2 – Background Theory
2.1 Introduction
2.2 Classification of Bandwidths for Underwater Acoustic Communication Systems
2.3 Fundamentals of Underwater Acoustic Communication
2.3.1 Sound
2.3.2 Acoustic Pressure
2.3.3 Acoustic Intensity
2.3.4 Speed of Sound
2.4 Underwater Channel Characteristics
2.4.1 Spreading Loss
2.4.2 Absorption Loss
2.4.3 Path Loss
2.4.4 Doppler Effect
2.4.5 Multipath Fading
2.4.6 Ambient Noise
2.5 Bit Error Rate (BER)
2.6 Conclusion

CHAPTER 3-Literature Review
3.1 Introduction
3.2. Literature review on Underwater Acoustic Modems for Short Range Communication
3.3 Summary of Literature Review

CHAPTER 4 –Problem Definition
4.1 Problem Statement
4.2 Project Objectives
4.3 Methods and Methodologies adopted to meet the project objectives

CHAPTER 5 – System Engineering Based Design and Development of Underwater Acoustic Modem
5.1 Introduction
5.2 Need Analysis
5.3 Concept Exploration
5.3.1 Binary Frequency Shift Keying (BFSK)
5.3.2 Binary Phase Shift Keying (BPSK)
5.3.3 Error Protection and Detection
5.4 Concept Definition
5.4.1 Modulation Scheme
5.4.2 Error Protection and Detection
5.5 Advanced Development
5.5.1 System Block Diagram
5.5.2 Software tool used for simulations
5.5.3 Underwater Acoustic Channel Model
5.6 Engineering Design
5.6.1 Block Diagram and Description of the System
5.6.2 Hardware Set up
5.7 Conclusion

CHAPTER 6 – Results and Discussion
6.0 Introduction
6.1 Model 1-Simulations with Additive White Gaussian Noise (AWGN)
6.2 Model 2 – Simulations with AWGN and Path Loss
6.3 Model 3-Simulation with AWGN, Path Loss and 2 Multipath delays
6.4 Model 3-Simulation with AWGN, Path Loss, 2 Multipath delays and shipping noise (Complete underwater acoustic channel)
6.5 Hardware Implementation Results
6.6 Conclusion

CHAPTER 7 – Conclusion and Scope for Future Work
9.1 Summary
9.2 Conclusion
9.3 Recommendation for Future Work

REFERENCES

List of Tables

Table 5. 1 Standard CRC Polynomials

Table 5. 2 Design Specification

Table 6.1 Performance with AWGN Channel

Table 6. 2 Performance Results for Depth 10 m

Table 6. 3 Performance Results for Depth 30 m

Table 6.6 Performance Results for Depth 50 m

Table 6.7 Performance Results for Depth 10 m

Table 6.8 Performance Results for Depth of 10 m

Table 6. 9 Performance Results for Depth of 30 m

Table 6.10 Performance Results for Depth of 50 m

Table 6.11 Performance Results for Depth of 70 m

Table 6. 12 Final System Performance Specifications

List of Figures

Figure 5. 1. Concept of BFSK Scheme

Figure 5. 2 Performance Curve of BFSK

Figure 5. 3. Concept of BPSK Scheme

Figure 5. 4. Performance Curve of BPSK

Figure 5. 5. Block Diagram of System

Figure 5. 6. Underwater Acoustic Channel

Figure 5. 7. Underwater Acoustic Channel Model

Figure 5. 8. Acoustic Propagation Delay Model

Figure 5. 9. Boric Acid Component Model

Figure 5. 10. Path Loss (Absorption Loss + Spreading Loss) Model

Figure 5. 11. Spreading Loss Model

Figure 5. 12. Ambient Noise Models

Figure 5. 13. Two Multipath Delays

Figure 5. 14. System Model in Simulink

Figure 5. 15. Hardware System Block Diagram

Figure 5. 16. Ultrasonic Transducer

Figure 5. 17. Schematic for IC MAX 232N

Figure 5. 18. Pin Configuration of PIC16F877A

Figure 5. 19. Transmitter Hardware

Figure 5. 20. Receiver Hardware

Figure 5. 21. Complete System Hardware

Figure 6. 1. Model 1 - AWGN

Figure 6. 2. System with AWGN Channel

Figure 6. 3. System with AWGN and Path Loss

Figure 6. 4. Performance with AWGN and Path Loss (Depth 10 m)

Figure 6. 5. Performance with AWGN and Path Loss (Depth 30 m)

Figure 6. 6. Performance with AWGN and Path Loss (Depth 50 m)

Figure 6. 7. System with AWGN + Path Loss + 2 Multipath delays

Figure 6. 8. Performance for Depth of 100 m

Figure 6. 9. Performance for Depth of 50 m

Figure 6. 10. Performance for Depth of 10 m

Figure 6. 11. System with AWGN + Path Loss + Two Multipath delays + Shipping Noise

Figure 6. 12. Performance for Depth of 10 m

Figure 6. 13. Performance for Depth of 30 m

Figure 6. 14. Performance for Depth of 50 m

Figure 6. 15. Performance for Depth of 70 m

Nomenclature

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Acknowledgements

This work is deemed incomplete without acknowledging the various individuals immensely instrumental in ushering a great deal of effort, time and individual guidance.

I would like to thank my academic supervisors Mr. Sreekrishna R. and Mrs. Preetham Shankpal, Assistant professors, Department of EEE, MSRSAS for providing valuable suggestions and encouragement. Their help, support, guidance and confidence boosted my abilities to execute my project successfully.

I would like to express my sincere gratitude to Dr. Hariharan Ramasangu, Professor and HOD, Department of EEE, and Mr. Karthikeyan B.R., Assistant professor, Department of EEE for providing valuable suggestions and encouragement to execute my project successfully.

I would like to extend my sincere thanks to the management of M. S Ramaiah School of Advanced Studies, both teaching and non-teaching staff for the help and support provided during the project. I am extremely grateful to Dr. Govind R. Kadambi, Pro-Vice chancellor, MSRSAS, Dr. H.K. Narahari, Dean, MSRSAS, Dr. S. R. Shankpal, Vice chancellor, MSRSAS whose emphasis for excellence and quality kept me focused on this project and helped to complete it.

I would also like to extend my heartfelt and sincere gratitude to my beloved parents, mother, Mrs. Mamtha Divakar and my father, Mr. G. Divakar for their moral support and encouragement which boosted me towards the completion of this project work. I also would like to show my sincere gratitude and thanks to my sister, Ms. Shruthi Divakar for providing valuable suggestions and moral support in order to complete this project.

Abstract

The existing underwater acoustic modems are designed for deep oceans and long range communication leading to immense consumption of power and high cost. These long range underwater acoustic modems are not suitable choice for deployment in underwater sensor networks, Hence the problem was chosen to design and develop a underwater acoustic modems that operates in shallow waters of depth below 100m and for a short range of below 100 m. Underwater wireless sensor network is contemporary technology that can be applied in the fields of security, surveillance, military, commercial, industrial and environmental. The major drawback is that the traditional underwater acoustic modems cannot be deployed for underwater sensor networks.

This work focusses on the research and development of the underwater acoustic modem for shallow waters and short range communication. The relevant background theory required understand acoustics and for modelling the unique characteristics of the underwater channel is described in detail. Different concepts to model and implement the functionalities of the transmitter and receiver were explored, while converging to the most suitable choice of concepts. The modelled system is simulated for different channel conditions such as depth, range and induced ambient noise. The results were analysed in order to conclude the performance outcome of the system.

The modelled system can efficiently operate for a depth of 30m, 50m and 70m for a range up to 50m. The hardware was developed using minimum number of components as a proof of concept for efficient data transmission and reception using acoustic signals. The hardware was tested to operate efficiently in air, however hardware tests for underwater is suggested for future work, which will provide much better performance since acoustics is more suitable for communication in water than air.

CHAPTER 1 - Introduction

In recent year’s communication technologies underwater has become an active area of research due to its important applications in military, oceanographic data collection, disaster prevention and pollution monitoring. Communication is an important process that enables exchange of data transfer between two or more entities or nodes.

1.1 Introduction to Underwater Acoustic Modem

The underwater acoustic modem is responsible for gathering sensor information, processing and communicating with other nodes and a group of these nodes communicating underwater using acoustic links creates a network known as “Underwater Acoustic Sensor Network (UASN). In underwater environment, communication using Radio Frequency (RF) is not possible since such high frequencies are easily absorbed in water vanishing the signals and if lower frequencies are used then it requires a very large antenna and high power consumption, which is practically not a wise choice for underwater communications. Therefore, communication using sonar or acoustics was seen to be reliable alternative that suits best the underwater environment using which data communication can happen for longer distances.

1.2 Classification of the Acoustic/Sonar Communication Systems

Acoustic or Sonar Communication Systems are classified into two types i) Passive Sonar System i.e., it only detects and receives acoustic waves from target bodies and does not transmit any acoustic waves (simplex mode), ii) Active Sonar Systems are the once that can receive as well as transmit acoustic waves and these are classified into two modes i.e. full duplex and half duplex communication systems. In full duplex, two way communication is possible in which the sonar waves are emitted and received between two nodes while in half duplex systems emit and receive sonar waves from its own system. Both modes of active sonar systems as well as passive sonar systems makes use of audible frequency range 50 Hz to 20 KHz and in certain applications it can go as high as 50 KHz. Passive sonar communication systems can detect any underwater entities such as moving submarines, crowds of fisheries and some military spying activities. Active sonar communications is used for various underwater tasks such as underwater object identification, fisheries, undersea explorations and military work.

1.3 Motivation

In recent years, communication underwater has become an active area of research due to the big gap between the advancements of communication technology in territorial while the underwater communications/applications had laid low. In order to implement applications underwater such as undersea explorations, assisted navigation, disaster prevention and surveillance requires setting up of wireless sensor networks underwater using acoustic links and this has not been an easy task due to the unique characteristics of water. Therefore it is necessary to design and develop an acoustic modem that can adapt well to the underwater environment.

This thesis proposes design and model of communication system that involves the design and development of an acoustic modem/transceiver for shallow waters and short range communication, that will become the basis for the underwater wireless sensor networks, since the commercially existing acoustic modems are not designed for short range communication, in order to trigger the idea of small, dense and cheap sensor nets, as these commercial acoustic modems are very expensive, they draw high power, bulky and all designed for long range communication. Therefore deployment of 100’s of such commercial modems will be very expensive and unreliable for oceanographic applications. Hence this lead to the motivation for the design of an acoustic modem that consumes low power and is cost effective in order for making it practically possible for deployment of several such acoustic modems setting up a sensor network that can be used for different underwater applications. The design and development of Underwater Acoustic Modem for underwater applications has been chosen to resolve the issues pertaining to the Commercial off-the-shelf (COTS) underwater acoustic modems that are not suitable for shallow waters and short range communication.

1.4 Scope of the work

This work presents software implementation, that involves the design and development of an acoustic modem for shallow waters, and short range underwater applications. The work focuses on modelling of an underwater acoustic channel and, design and development of a transmitter-receiver pair that can communicate efficiently underwater. The performance of modelled system is analysed for different channel conditions. Different attenuation and noise components are modelled representing the underwater channel medium. The Hardware is developed using ultrasonic transducers as a proof of concept for efficient transmission and reception of the data using acoustics.

1.5 Thesis Outline

Chapter 2 describes the background theory related to acoustics and underwater channel. It describes the different characteristics of underwater channel with relevant equations used for modelling it.

Chapter 3 carries out the survey on Underwater Acoustic Communication System by referring journals, white papers, books, patents and related documents.

Chapter 4 defines the problem statement and project objectives followed by methods and methodology adopted to meet the objectives.

Chapter 5 describes the design, modelling and development of an Underwater Acoustic Modem that meets the requirement of shallow waters and short range communication, by invoking the Systems Engineering principles and practices.

Chapter 6 discusses and analyses simulation and hardware implementation results.

Chapter 7 summarizes the research results, concludes the outcome of the project and suggests the scope for future work

CHAPTER 2 – Background Theory

2.1 Introduction

This chapter describes the fundamental theory for acoustics and the physical characteristics of the underwater channel in detail. It describes the important theory required to know for designing of underwater acoustic modems and for modelling the underwater channel.

2.2 Classification of Bandwidths for Underwater Acoustic Communication Systems

Underwater acoustic communication systems can be classified into two types i.e. Long range and Short range-shallow water systems. The long range systems can operate from 20 to 2000km range in deep ocean waters having a bandwidth limit of 500Hz to 10KHz while the short range systems operates over several tens of meters with a bandwidth limit of 10 to 100KHz and shallow water usually refers to a water depth lower than 100m (Akylildiz, Pompili and Melodia, 2005).

Table 2. 1 Bandwidth Availability

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Underwater acoustic communication links can be classified such as very long, long, medium, short and very short. The Table2.1 shows the available bandwidths for different ranges for underwater acoustic channels. Apart from these they are also classified as vertical links in order to communicate with the nodes present in the vertical plane and horizontal links in order to communicate with the neighbouring nodes present on the same plane. Sine this work focuses on the design of an underwater acoustic modem for short range communication therefore the system will be designed to operate between the bandwidth of 20 to 50 KHz.

2.3 Fundamentals of Underwater Acoustic Communication

Similar to the exchange of data in electrical area, they can also happen in acoustics which can be described by important physical quantities and definitions as follows;

2.3.1 Sound

It is the vibrations of any object can transmit its motion or energy to the surrounding physical medium. This phenomenon results in propagation of vibrations in which the particles vibrates in the same direction as the direction of propagation forming longitudinal waves.

2.3.2 Acoustic Pressure

The Sound Pressure is the force (N) of sound on a surface area (m2) perpendicular to the direction of the sound. The SI-units for the Sound Pressure are N/m2 or Pa. Sound is usually measured with microphones responding proportionally to the sound pressure (p). The power in a sound wave goes as the square of the pressure. It is given by,

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Where, = fluid density

C = velocity of sound wave propagation

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V = particle velocity

Acoustic pressure (P) is analogues to the potential difference in electrical circuits and is the called the specific impedance. Another equation that is analogous to ohm’s law is the acoustic impedance given by,

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Z = Acoustic Impedance that is a function of frequency

P = acoustic pressure

U = Acoustic volume flow

2.3.3 Acoustic Intensity

It’s denoted by ‘I’ with a unit as w/m2, i.e. the energy per second that crosses the unit area. A plane wave equation is given by,

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The equation is viewed as the acoustic power density produced by a source. Generally a reference intensity ‘Ir’ is defined for each medium. In the case of underwater reference intensity that is produced by a plane wave with root mean square pressure of 1upa.

2.3.4 Speed of Sound

The speed of sound depends on the temperature, salinity and pressure (depth) of the water under the sea surface.

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Figure 2. 1. Sound Profiles Underwater

Figure 2.1 shows the variation of the speed of sound as a function of depth of the ocean. The speed of sound ranges between 1400 and 1570 m/s. This is roughly 1.5 km/s (just under 1 mile/s) or about 4 times faster that sound travel through air.

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Figure 2. 2. Sound Speed

The speed of sound is affected by the variables of the ocean such as temperature, salinity and depth (pressure). The ocean pressure refers to the weight of the underlying water and not the pressure associated with the sound wave, which is much smaller. In the Figure 2.2, it is seen that the temperature changes a large amount, decreasing from 20 degrees Celsius (°C) near the surface in mid-latitudes to 2 degrees Celsius (°C) near the bottom of the ocean. Salinity changes by only a small amount, approximately 34 to 35 Practical Salinity Units. Pressure increases by a large amount, from 0 at the surface to 500 atmospheres (atm) at the bottom. Therefore the speed of sound in water increases with increase in water temperature, increasing salinity and increase in depth (pressure).

2.4 Underwater Channel Characteristics

The chemical and physical characteristic of underwater, delays and affects the propagation of sound due to common phenomenon such as spreading and absorption, this phenomenon’s is responsible for attenuating the acoustic signals underwater. Another major phenomenon attenuating acoustic signal in shallow waters is the multipath fading that causes inter symbol interference at the receiver end and the ambient noise.

2.4.1 Spreading Loss

It does not represent a loss of energy, but refers to the fact that propagation of the acoustic pulse is such that the energy is simply spread over a progressively larger surface area thus reducing its density and it is given by

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Where, ‘r’ is the range in meters and ‘k’ is the spreading factor. If there’s a medium in which signal transmission occurs unbounded and spherical spread, then k = 2, i.e. source intensity decreases with the square of the distance ‘r’. In case of bounded spreading, k = 1 (for cylindrical spreading). Spreading loss has a logarithmic relationship with the range ‘r’ and its impact on the acoustic signals underwater is very significant for very short range up to 50m ,hence for shorter ranges, spreading loss plays a larger part compared to the absorption loss.

2.4.2 Absorption Loss

It is the loss that occurs in the form of heat due to viscosity and ionic relaxation of boric acid and magnesium sulphate (MgSo4) molecules, as the sound wave propagates outwards underwater. The effect of viscosity is significant at higher frequencies above 100 KHz, whereas the ionic relaxation effects of MgSO4 affect the frequency range from 10 KHz to 100 KHz. The boric acid affects the lower frequencies up to a few KHz. In general, the absorption coefficient α increases with increasing frequency and decreases as depth increases. Numerous factors and properties of sea water such as acoustic frequency, pressure (depth), temperature, salinity and acidity play a major role in characterizing the absorption co-efficient and is given as

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Where reperesents the frequency in KHz of the sound wave being transmitted, over the channel. A1 is the boric acid component in the seawater and is given by,

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Where pH is the acidity of the seawater. The speed of sound propagation (c) is given by,

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Where T is the temperature in water (oC), s is the salinity in mg/l and d is the depth in meters.

P1 is the pressure of the boric acid in the sea water, its normalised value for shallow waters is given as 1 Pa. The relation frequency (f1) of the boric acid is given as,

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The MgSO4 component (A2) present in the water is given as,

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Whereas the depth pressure (P2) in Pa, for the MgSO4 is given as,

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A3 is the pure water component given as,

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Pure water depth pressure (P3) in Pa is given as,

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2.4.3 Path Loss

The total path loss is the combined effect of the absorption loss and spreading loss. For very short range communication below 50 m, the contribution of absorption term is less significant than the spreading term (Burrowes. G and Khan. J, 2011).

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Figure 2. 3. Absorption Loss and Spreading Loss (Burrowes, G and Khan, J, 2011)

According to the model in Figure 2.3, the value of k has the most significant impact on the path loss at shorter ranges. Spherical spreading loss is the decrease in the source level when there are no boundaries such as water surface or sea floor, thus known as unbounded spreading. In reality, the acoustic energy cannot propagate in all directions due to boundaries such as the water surface and sea floor, thus leading to cylindrical spreading. Cylindrical spreading is when the acoustic energy encounters the water surface and sea floor and is trapped within these boundaries radiating horizontally away from the source.

2.4.4 Doppler Effect

The motion in the plane in which the acoustic waves is travelling towards or away from the receiver results in a shift in the carrier frequency. If Transmitter (Tx) and Receiver (Rx) are approaching, the frequency is higher, while if they are moving apart the frequency is lower. The causes for the Doppler Effect are as follows; i. Apparent shifts in frequencies of transmitted signal due to the motion of Tx/Rx or both, this shift depends on the relative velocity of the Tx/Rx.

ii. Rapid fluctuations in the receiving conditions due to small movements of the receiver.

Note that the Power Spectral Density (PDF) is the direction of the waves reaching the receiver is uniformly distributed between 0 and 2π. The Doppler shift of the received signal is given by,

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Where ‘fc’ is the original signal frequency and is the relative velocity between the Tx and Rx or nodes.

2.4.5 Multipath Fading

Fading basically means distorting a signal over a certain propagation medium. It happens mainly due to multipath propagation also known as multipath induced fading. In multipath fading, signal will reach the receiver not only via the direct path but also as a result of reflections from other objects underwater. In underwater multipath is governed by two objects

i. Sound reflection at the surface, bottom and any objects present underwater.
ii. Sound refraction in water.

This can lead to inter-symbol interference (ISI). In this case, one may need to use appropriate error protection scheme to minimize the effect of ISI.

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Figure 2. 4. Multipath Fading

The Figure 2.4 shows the reflections caused as the transmitted waves bounces either at the surface or bottom of the sea and reaches the receiver along multiple paths. This effect most commonly occurs in shallow water environments. Sound refraction in water is a consequence of sound speed variation with depth, therefore it is mostly evident in deep waters.

2.4.6 Ambient Noise

For underwater acoustic modem (UAM) operating at a frequency range of interest (10 KHz to 100 KHz), the ambient noise power spectral density (psd) decreases with increasing frequency. Ambient noise is basically caused due to external factors that interfere with the signal transmission. The four different types of ambient noise are described as follows (Simanjuntak L, 2004); - Turbulence Noise It’s the noise generated by the rapid movement of water layers over each other’s at various speeds. The turbulence noise is significant at the bottom of the sea. Only low frequencies of (f < 10 Hz) are affected by turbulence noise. The formula for turbulence noise psd in dB re uPa is given as;

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- Wave Noise

Wave noise can be defined as the motion caused on the sea surface driven by different wind speeds. This has a major influence on frequency range of 100 Hz to 10 KHz. The empirical formula for the wave noise in psd, dB re uPa is given as;

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- Shipping Noise

Shipping noise is caused by shipping activities carried out on the sea ocean surface, and has an impact on interfering with the transmitted signals in shallow waters. The shipping noise will vary depending on the level of boating activities in the oceanic area. The shipping noise is mainly dominant in the frequency range of 10 KHz to 100 KHz. The psd of shipping noise in dB re uPa is given as;

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- Thermal Noise

It’s caused due to the molecular bombarding of the receiver. This noise dominates in the frequency region above of 100 KHz. Thermal noise in dB re uPa per Hz as;

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2.5 Bit Error Rate (BER)

As the name implies, BER is defined as the rate at which errors occur in a transmission system. BER is one of the key parameters to access the full end to end performance of a system. The definition of BER can be expressed by a simple formula i.e.

If the medium between the transmitter and receiver is good and has high SNR, then the BER will be low, the main reason that causes BER is the presence of noise in the data channel.

2.6 Conclusion

The Bandwidth (BW) availability underwater is severely limited to KHz. While designing any systems for underwater applications, different characteristics of the underwater channel needs to be considered such as spreading loss, absorption loss, multipath fading, propagation delay and ambient noises. The reliability of the communication systems are tested based on the signal strength required to achieve a particular BER.

CHAPTER 3-Literature Review

3.1 Introduction

This chapter describes the previous and recent existing designs of acoustic modems for underwater wireless sensor networks and also gives a broad understanding related to this project work based on the literatures, conferences papers, books, journals and publishers. It also serves as a reference based on which the thesis work can be carried out.

3.2. Literature review on Underwater Acoustic Modems for Short Range Communication

The work of Bridget Benson et al focuses on the development of a short range underwater acoustic modem. Commercially available omnidirectional waterproof ultrasonic transducers are very expensive. In this work, a piezoelectric homemade transducer is used. The modulation scheme used is FSK due to its simplicity. The system supports data rates up to 200 bps with a BER of 10-2 for a low SNR of 10 dB The overall cost of the system is 600$. The transmission distance of the system is greater than 350m. Hardware platform used is FPGA (Benson et al, 2010).

The research work of Akylildiz, Pompili and Melodia describes the state of art of underwater sensor networks along with some research challenges for deployment of underwater acoustic sensor networks (UASN). It describes the different applications of the underwater acoustic sensor networks such Some of the major challenges in the design of UWASN’s have been mentioned such as Bandwidth (BW) limitation, high bit error rates followed by shadow zones, limitation in battery power and fouling corrosion. Different UWASN architectures or topologies such as two dimensional static and three dimensional underwater sensor networks have been described. This paper also describes some of the main differences between the territorial sensor networks and UWASN’s with respect cost, deployment, power and memory. An internal architecture of an underwater sensor node that consists of a sensor, sensor interface circuitry, microcontroller, power supply and acoustic modem has been described in detail and they have mentioned a way to make the system waterproof by using PVC housing. Some major challenges faced by the designers with respect to deployment of a low cost, low scale UWASN’s has been listed and briefed. This research gives a good insight related to several design issues to consider for developing an acoustic modem (Akylildiz, Pompili and Melodia, 2005).

In the work of Wills, Yi and Heidemann, the primary aim was to design and develop an inexpensive modem that is affordable for purchase and for deployment of many sensor nodes. The target price of this system is said to be 100$ and the modem is mainly designed for short-range communication of range 50m-500m.The digital hardware platform used is a simple 8-bit Atmel Atmega 128L microcontrollers and this design makes each of the sensor node development cost cheaper. The modulation scheme used for data transmission here is FSK. A BER of 10-5 was seen when the transmitter and receiver were kept close to each other. (Wills, Wei Ye and Heidemann, 2006).

The CORAL communication subsystem is designed for one way communication. According to Pandya et al, the acoustic transducer used is an US Navy design whose operating specifications were found experimentally rather than in a component datasheet. The hardware platform used is a microcontroller that can be interfaced to any sensor for any specific application. The system was tested for shallow underwater with only two transducers placed in the water while the other circuitry was kept outside. The optimal operating frequency of the transducer underwater was found to be 1.7 KHz. The transmission efficiency was tested for a distance of 20cm and was seen to be same as observed for tests conducted for very short distance of 1.2cm. The SNR under water observed was 250 that is maximum for a frequency of 3.8 KHz. In this system, a signal voltage of 5V is being used to demonstrate low power system. However several environmental issues such as harmonics, acoustic reflections and medium noise limited the performance of the interface circuitry posing a challenge, therefore much more work is needed to solve these issues for the practical deployment of the CORAL modems (Pandya et al, 2005).

In the work of Jurdak et al, a software modem has been designed and developed that is coupled with generic microphones and speakers. This system eliminates the need for hardware requirements needed to carry out the modulation and demodulation schemes while the software itself takes care of it reducing cost and making it available for deployment of underwater acoustic sensor networks. The system makes use of software designed 8 frequency FSK modulation and demodulation schemes. A capacity of 24 bps can be transmitted underwater without any errors while it can support data rates up to 48 bps. The transmitter used is a Tmote Invent speaker’s whose SNR measured indicated that this speakers supported a error free bit rate of at least 24 bps up to a distance of 13m. The FSK software modem was designed for 8 frequencies that showed high signal quality i.e. 1000,1200,1300,1500,1600,1700,1800 and 2000Hz. The minimum SNR noticed for all these frequencies within a range of 13m are 6.95dB providing data rates up to 24 bps that was error free channel capacity. The paper also describes some of the fundamentals of underwater acoustics parameters and issues to consider while designing an acoustic communication system. In short, this system consists of underwater network of sensor nodes that communicates with each other’s through acoustic links eventually delivering the data to the water surface node connected to a laptop placed on a surface buoy. This laptop uses broadband radio connection to internet to transmit the received data to the station where that is analysed and interpreted using tools (Jurdak et al, 2007).

In the work of Num and Sunshin An, they developed a low power based acoustic modem that basically operates with 3.3V power supply and has a capability of digital data communication. The modulation scheme used in this work is amplitude shift keying (ASK).The system tested to show a data rate of 100 bps, the communication distance of the modem is approximately 3m, however the exact range of the acoustic modem could not be found due to the lack of test facilities. The system has made use of piezo-transducers i.e. Sounder/ projector/speaker at the transmitter and hydrophones at the receiver end, however some problems that needs to be considered for this modem for future work is directional property, reflection and refraction. In addition, this acoustic modem will become the basis for the underwater wireless sensor networks (Num and Sunshin, 2007).

The work of Jeon and Park mainly focuses on the design and implementation of an acoustic modem for underwater sensor networks that has good performance and efficiency. This modem was tested experimentally in water tank, a pond and offshore ocean for various distances and data rates and the bit error rate was observed. The modem used an Atmega128 microcontroller and waterproof piezo-transducer that operates at a frequency of 30 KHz. Two modems were tested in water tank for a depth of 0.7m and for communication distances of 0.2m, 0.5m and 1m for which a maximum of 4kbps data rate was achieved with no errors. The modem was then tested in the pond which is a length of 40m and width of 60m for depths of 1 to 5m, a maximum data rate of 5Kbps was achieved for a distance of 1m and a data rate of 1 Kbps at 30m. The modem was also tested offshore located on the southern sea of Korea and it could achieve a maximum data rate of 1Kbps for a transmit distance of 20m. The system uses a 14.8 volt Li-on battery to supply power to the modem circuitry and the implemented hardware (MCU). No quantifiable details or plots of the bit error rate and SNR have been reported. The performance of the modem was best seen in the pond therefore according to Jun-Ho-Jeon and Sung-Joon-Park, it was concluded that the modem operates better in open space such as pond with small multipath than a limited water tanks. Therefore in overall, the system supports data rates of up to 5 Kbps and can operate up to a range of less than 30m (Jun-Ho-Jeon and Sung-Joon-Park, 2010).

3.3 Summary of Literature Review

From the section 3.1, it is noticed that the maximum SNR was achieved is 250 but for a very short range of 20cm underwater based on the work of Pandya et al. The maximum data rate is achieved is 200bps for a range over 300m based on the work of Benson et al, but the trade off in this modem is the high power consumption at the receiver end and low SNR. The SNR achieved is 6.95dB for data rate of 24 bps for a range of 13m based on the work by Jurdak et al. Therefore based on the literature review it was identified that the performance parameters such as SNR, range, BER and power consumption plays an important role during the design and analysis of an underwater acoustic modem. Based on the literature review it is identified that lower order modulation schemes provides robust communication, and the hydrophones/projectors adds up to the high cost of the system, therefore an alternate ultrasonic transducer needs to be used that is low cost and offers short range communication. The System needs to be simulated for the underwater channel, therefore it is necessary to model the underwater acoustic channel to simulate the modelled system and analyse its performance.

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