DEVELOPMENT OF UNIVERSAL SOFTWARE RADIO PERIPHERAL
AMPLIFIER FOR UNDERWATER ACOUSTIC PLATFORM USING
SOFTWARE DEFINED RADIO
HAYDAR SABEEH KALASH
A dissertation submitted in partial of the
requirements for the award of the degree of
Master of Engineering (Communication Engineering)
Faculty of Electrical Engineering
Universiti Teknologi Malaysia
JANUARY 2010
III
To everyone who told me not to give up.
IV
ACKNOWLEDGEMENTS
I would like to dedicate my acknowledgement to my supervisor Dr. Sharifah
Hafizah Syed Ariffin, for understanding my character and allowing me to explore
many different exciting research areas before focusing on a dissertation topic. I thank
her for pushing me to hit the ground running and keep the momentum going
throughout my graduate career. Her valuable support and encouragement for me to
complete my research. Her frankness and constructive ideas help me a lot in the
course of my project and without her great experience and vast knowledge, this
thesis could not be completed within the time frame. Her wisdom and advises not
only on my research area, but also life in general and for that, I am very grateful to
her. Her guidance makes me feel close to her and every time she shared her
experienced or gave a stern order.
I am indebted to my many student colleagues for providing a positive and fun
environment in UTM Skudai. I am especially grateful to Adib, Rozaini, Hija,
Hamidreazh, Farid and Mustafa. I would like to thank all my family and friends who
have supported me on my journey through graduate school and have made this
dissertation possible. I would especially like to thank my parents, Prof. Dr. Sabeeh
and Kamila, my sisters, Azhar and Maha, and my Brother, Ammar and Nather, for
always believing in me and providing me with unconditional love and support. With
them around, it is always merry and they help me a lot in channeling my built up
stress in a good way and also stimulating the research environment.
v
ABSTRACT
First and foremost, the development of a modem using the USRP has
applications in oceanographic monitoring and communication. Improved acoustic
connecting would allow more efficient transfer of information between Underwater
Acoustic (UWA) equipment such as autonomous vehicles, piloted vehicles, and
underwater profilers. Therefore it can easily be modified in order to be employed for
the testing of different UWA. This project describes the full system of an underwater
acoustic modem with underwater wireless connection starting with the most critical
component of the system which is the USRP amplifier for the receiver and the
transceiver. In this project we focused on the development of the USRP amplifier.
This amplifier is expected to enhance the signal of the transceiver to Universal
Software Radio Peripheral (USRP) modem and the GNU radio. The platform that we
proposed uses the Software Defined Radio (SDR) as the main controller. This is due
to its flexibility in modulation and able to support coding. Since this is an initial
stage, in this work we only use Gaussian Minimum Shift Keying (GMSK) as the
modulation techniques. The performance of the UWA platform had been tested and
we found that as the frequency increases the attenuation increased as well but with
the USRP amplifier we have managed to decrease it. UWA communication research
will benefit greatly from the adaption of the USRP as an underwater acoustic
modem. The USRP amplifier amplifies the signal that has send by the transceiver
and detected by the receiver will effectively demodulate the signal and analyze the
received data in the USRP modem. GNU radio and USRP SDR has been
successfully implemented. The results demonstrate that the objectives of this
research are archived. It is proved that by implementing GNU radio and USRP SDR
in the new generation of underwater acoustic communication technology, and
improves the utilization of the underwater communication. We conclude that the
proper design of the USRP Amplifier is crucial to obtain high quality performance.
This project has successfully developed a USRP amplifier and the underwater
acoustic communication testbed with expected results.
VI
ABSTRAK
Pembangunan modem menggunakan Universal Software Radio Peripheral
(USRP) mempunyai aplikasi dalam pemantauan Oseanografi dan komunikasi.
Peningkatan akustik komunkasi akan membolehkan memindahkan maklumat antara
Underwater Acoustic (UWA) peralatan seperti kenderaan autonomi, kenderaan diuji,
dan profiler bawah air. Oleh kerana lanya, mudah diubahsuai untuk digunakan bagi
ujian UWA yang berbeza. Projek ini menjelaskan sistem lengkap dari sebuah modem
akustik didalamr air dengan sambungan wayarles bermula dengan komponen yang
paling penting iaytu sistem penguat USRP desain litar untuk penerima dan pemancar.
Dalam projek ini, kami fokus pada pembang anan penguat USRP. Penguat ini
diharapkan dapat meningkatkan isyarat transceiver untuk USRP modem dan radio
GNU. Platform yang telah dicadangkan menggunakan Software Defined Radio
(SDR) sebagai pengendali utama. Ini adalah disebabkan oleh fleksibiliti dalam
modulasi dan mampu menyokong pengekodan. Namun, dalam prosek ini kita hanya
menggunakan Gaussian Minimum Shift Keying (GMSK) sebagai teknik modulasi.
Prestasi platform UWA telah diuji dan didapati bahawa frekuensi meningkat, rosofan
juga meningkat te tapi dengan penguat USRP dapat menangani masalah rosofan ini.
Kajian komunikasi UWA boleh menmanfaat kan dari adaptasi USRP sebagai modem
akustik dalam air. Penguat USRP dapat menguatkan isyarat yang sudah menghantar
dengan transceiver dan dikesan oleh penerima secara berkesan deh demodulasi
isyarat dan menganalisis data yang diterima di modem USRP dalam project ini. GNU
radio dan USRP SDR telah berjaya dilaksanakan. Keputusan kajian menunjukkan
bahawa tujuan dari objektif telah dicapai. Hal ini membuktikan bahawa dengan
menggunakan radio GNU dan USRP SDR pada generasi baru teknologi komunikasi
akustik bawah air, tidak hanya akan meningkatkan penggunaan komunikasi bawah
laut. Kami menyimpulkan bahawa reka bentuk yang Penguat USRP yang sesual
adalah panting untuk mendapatkan prestasi yang berkualiti tinggi. Projek ini telah
berjaya membangunkan Penguat USRP dan testbed komunikasi akustik dalam air
dengan hasil yang dijangka.
VII
TABLE OF CONTENTS
CHAPTER
1
2
TITLE
PAGE
DECLARATION
ii
DEDICATION
iii
ACKNOWLEDGEMENTS
iv
ABSTRACT
v
ABSTRAK
vi
TABLE OF CONTENTS
vii
LIST OF TABLES
x
LIST OF FIGURES
xi
LIST OF SYMBOLS
xiv
LIST OF ABBREVIATIONS
xvi
LIST OFAPPENDIX
xviii
INTRODUCTION
1.1.
Overview
1
1.2.
Problem Statement
3
1.3.
Research Objectives
4
1.4.
Scope of Work
4
1.5.
Organization of the Thesis
5
LITERATURE REVIEW
2.1.
Overview
6
2.2.
Radio Frequency Waves
7
2.2.1
8
Conductivity
VIII
2.3.
Wavelength
9
2.2.3
Air/Water Interface
10
2.2.4
Existing RF Systems
11
Acoustic Waves
12
2.3.1 Absorption Loss
13
2.3.2
Spreading Loss
14
2.3.3
Noise
15
2.3.4
Passive Sonar Equation
17
2.3.5
Multipath
18
2.3.6 Why Acoustic
19
2.4.
Current Acoustic Modems
19
2.5.
Software Defined Radio
22
2.5.1
GNU Radio
25
2.5.2
Universal Software Radio Peripheral
27
2.5.3
GNU Radio and USRP Applications
28
2.6.
2.7.
2.8.
3
2.2.2
GMSK Modulation
31
2.6.1 GMSK basics
31
2.6.2
Generating GMSK modulation
32
2.6.3
Advantages of GMSK modulation
34
Brüel & Kjær Hydrophones
34
2.7.1
Hydrophone Type 8104
35
2.7.2
Hydrophone Type 8105
36
Summary
37
INITIAL EXPERIMENTS
3.1.
Overview
38
3.2.
Hydrophone Experiment
39
3.2.1
Water Tank
40
3.2.2
Sensitivity Measurement and Directivity of
the Produced Headphone in Low Frequency
42
3.3
GNU Radio and USRP Installation
44
3.4
GNU Radio and USRP Test-Run
47
3.5
GNU Radio and USRP Experiment Result
50
IX
3.6
4
Summary
THE UWA PLATFORM AMPLIFER DESIGN
4.1
Overview
58
4.2
Power Amplifier
59
4.3
USRP Amplifier Design
66
4.3.1
68
4.4
The amplifier for UWA platform
Integrated Tests
70
4.4.1
Multipath Measurements
70
4.4.2
Tank Tests
72
4.4.3
Underwater Integrated system for GMSK
Based Acoustic
4.4.4
4.4.3
4.5
5
57
72
General Acoustic Setup and Performance
Evaluation
73
The UWA Platform Test
78
Summary
78
CONCLUSIONS
5.1
Overview
79
5.2
Future Works
80
REFERENCES
82
Appendix A - B
92
X
LIST OF TABLES
TABLE NO.
TITLE
PAGE
2.1
Research Underwater Acoustic Modem Comparison
22
3.1
Electronic characteristics of the produced hydrophone The
42
3.2
The measured sensitivity of the 8105, 8104 and the
produced hydrophone
4.1
43
Parameters Used in GMSK Based Acoustic Performance
Evaluation
74
XI
LIST OF FIGURES
FIGURE NO.
TITLE
PAGE
2.1
Electromagnetic Spectrum
7
2.2
Attenuation vs. Frequency in Fresh Sea Water
9
2.3
RF Wavelength vs. Frequency in Sea Water, Fresh Water
and Air
10
2.4
Air to Water Refraction Loss as a Function of Frequency
11
2.5
Wireless Fibre Systems SeaText Modem
12
2.6
Acoustic Absorption as a function of temperature,
pressure, and pH
14
2.7
Acoustic Spherical and Cylindrical Spreading Loss
15
2.8
The typical sound levels of ocean background noise at
different frequencies
2.9
16
Source Level vs. Transmission Distance for a 40 kHz
carrier an ambient noise of 50 dB re 1 µPa at various levels
of SNR
2.10
17
Ray Trace for a 40 kHz source with a 15 degree beam
angle placed at 10 meters depth in a body of water 11
meters deep with a constant sound speed of 1500 m/s
18
2.11
Software Defined Radio Block Diagram
24
2.12
Basic Structure of GNU Radio Flow Graph
25
2.13
GNU Radio and USRP Structure
26
2.14
USRP in Lab
27
2.15
USRP motherboard
28
2.16
Signal using MSK modulation
32
2.17
Spectral density of MSK and GMSK signals
32
XII
2.18
Generating GMSK using a Gaussian filter and VCO
33
2.19
Block diagram of I-Q modulator used to create GMSK
33
2.20
Hydrophone Type 8104
35
2.21
Hydrophone Type 8105
36
3.1
DS-6121A Iwatsu Digital Storage-scope
40
3.2
The water tank in the Lab
41
3.3
System experiment.
41
3.4
Screenshot of the benchmark_tx.py running on the
Terminal While Transmitting Data
3.5
Screenshot of the benchmark_rx.py running on the
Terminal While Receiving Data
3.6
48
49
Screenshot of the Software-Based Spectrum Analyzer by
using the usrp_fft.py code
50
3.7
Show the experiment in the Lab.
50
3.8
The GNU Radio and USRP Architecture
51
3.9
Configuration of the transmitter.
52
3.10
Packet received within the distance
53
3.11
Attenuation within the distance
54
3.12
Packets received within the bit rate
54
3.13
Packets received within the Transmitting Gain
55
3.14
Packets sent within the Receiving Gain
56
4.1
USRP Amplifier Design
59
4.2
Class A Amplifier Input / Output Characteristic
61
4.3
Class B Amplifier Input / Output Characteristic for one
transistor
4.4
Class AB Amplifier Input / Output Characteristic for one
transistor
4.5
62
63
Block diagram of the power amplifier design making use
of a class A and class B amplifier to achieve linearity and
efficiency
63
4.6
Full system Architecture.
64
4.7
Block diagram of the USRP amplifier design
65
4.8
Complete Amplifier Linearity
66
XIII
4.9
Receiver Block Diagram
67
4.10
Estimated power coupled in the transmitting frequency
68
4.11
Overall Receiver Gain
69
4.12
Pico PicoScope device
71
4.13
Performance of the underwater integrated system for
GMSK Based Acoustic
73
4.14
Receiver measurement for GMSK
75
4.15
Transmitter measurement for GMSK
76
4.16
Compare the packet received within the distance for the
system with USRP amplifier and without it.
4.17
77
Compare the packet received within the Bit Rate for the
system with USRP amplifier and without it.
77
XIV
LIST OF SYMBOLS
α
-
Attenuation
𝑓
-
Frequency
-
Conductivity
λ
-
Wavelength
eα(f)d
-
Absorptive loss for acoustic wave propagation
d
-
Propagation distance
α(f)
-
Absorption coefficient of frequency f
f1 f2
-
Relaxation frequency
P1, P2,
-
Pressure dependencies
A1, A2
-
Constants
SNR
-
Desired signal to noise ratio
SL
-
Source level
TL
-
Transmission loss
NL
-
Noise level
Q
-
Total signal sent in the system
Zfr(Ω)
-
Electrical Impedance
ρ
-
instantaneous departure of the pressure
x
-
Mean position
h
-
Mean at depth
ω
-
Angular frequency
P0
-
Pressure amplitude
Pfa
-
Low probability of false alarm
Pd
-
Probability of detection
T
-
Time
𝜎
XV
t
-
Sample time
N
-
Number of samples
𝑇𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑛
-
Time Measurement
𝑁200
-
Value of the sample
Ac(τ )
-
Amplitude delay profile
M
-
Effective signal length
τ̄
-
Mean delay
τrms
-
rms delay spread
XVI
LIST OF ABBREVIATIONS
AcTUP
-
AIDS Coalition to Unleash Power
ADC
-
Analog to Digital Converter
B&K
-
Brüel &Kjær
BR
-
Bit Rate
COTS
-
Commercial Of The Shelf
CPM
-
Continuous Phase Modulation
DAC
-
Digital to Analog Convertor
DBPSK
-
Differential Binary Phase Shift Keying
DDC
-
Digital Down Converter
DSP
-
Digital Signal Processor
DQPSK
-
Differential Quaternary Phase-Shift Keying
DUC
-
Digital Up Converter
ELF
-
Extremely low frequency
FE
-
Front End
FFT
-
Fast Fourier Transform
FHS
-
Frequency Hop Sequence
FIR
-
Finite Impulse Response
FSK
-
Frequency-shift keying
FRONT
-
Front-Resolving Observational Network with Telemetry
GMSK
-
Gaussian Minimum Shift Keying
GNU
-
"Genuinely Not Unix" Operating System composed of free
software
GPS
-
Global Positioning System
GSM
-
Global System for Mobile
GSSSM
-
Global Software System for Mobile communications
XVII
GUI
-
Graphical User Interface
IF
-
Intermediate Frequency
IIR
-
Infinite Impulse Response
JTRS
-
Joint Tactical Radio System
LDPC
-
Low-Density Parity-Check
MAC
-
Medium Access Layer
MIMO
-
Multiple-Input, Multiple-Output
MSK
-
Minimum Shift Keying
MW
-
Mega-Watt
NIC
-
Network Interface Card
NS
-
Denotes value
OFDM
-
Orthogonal Frequency-Division Multiplexing
OS
-
Operating System
PC
-
Personal Computer
PR
-
Packet Received
PRR
-
Packet Received Ratio
PSK
-
Phase-Shift Keying
QAM
-
Quadrature Amplitude Modulation
QPSK
-
Quadrature Phase-Shift Keying
RF
-
Radio Frequency
Rx
-
Receiver
SDR
-
Software Defined Radio
SNUSE
-
Sensor Networks for Undersea Seismic Experimentation
SWIG
-
Simplified Wrapper and Interface Generator
TRG
-
Telematic Research Group
Tx
-
Transmitter
USB
-
Universal Serial Bus
USRP
-
Universal Software Radio Peripheral
UWA
-
Underwater acoustic
VCO
-
Voltage-Controlled Oscillator
XVIII
LIST OF APPENDICES
APPENDIX
TITLE
PAGE
A
TC913B Specifications
87
B
LT1113 Specifications
93
CHAPTER 1
INTRODUCTION
1.1
Overview
Underwater Acoustic (UWA) has become widely focused from many
engineers and it has surfaced as a powerful technique for aquatic applications, and it
has attracted more and more attention from the research community recently. UWA
communication is beginning to revolutionize our understanding of the physical world
by providing fine resolution sampling of the surrounding environment. The ability to
have many small devices streaming real-time data physically distributed near the
objects being sensed brings new opportunities to observe and act on the world which
could provide significant benefits to mankind. For example, dense wireless sensor
communication have been used in agriculture to improve the quality, yield and value
of crops, by tracking soil temperatures and informing farmers of fruit maturity and
potential damages from freezing temperatures [1]. They have been deployed in
sensitive habitats to monitor the causes for mortality in endangered species [2].
UWA communications have also been used to detect structural damages on bridges
and other civil structures to inform authorities of needed repair and have been used to
monitor the vibration signatures of industrial equipment in fabrication plants to
predict mechanical failures [3].
Underwater Acoustic (UWA) a kind of measuring and controlling system
consisting of unmanned or autonomous underwater vehicles (UUVs/AUVs) and
sensor nodes that have sensing, communication computing and moving capabilities.
2
UWA, which have the features of distributed space, distributed time and distributed
function, is a typical autonomous and intelligent system which can independently
accomplish specific tasks depending on the changing environment over a given
volume of water.
UWAs are envisioned to enable applications for oceanographic data
collection, pollution monitoring, offshore exploration, disaster prevention, assisted
navigation and tactical surveillance applications [1]. Multiple unmanned or
autonomous underwater vehicles (UUVs/AUVs), equipped with underwater sensors,
will also find application in exploration of natural undersea resources and gathering
of scientific data in collaborative monitoring missions. To make these applications
viable, there is a primary need to enable underwater communications among
underwater devices.
Acoustic communications are the typical physical layer technology in
underwater systems. Wire communications are difficult to deploy and are unsuitable
for moving. In fact, radio waves suffer from such high attenuation, while optical
waves are affected by scattering and high precision in pointing the narrow laser
beams. Acoustic wireless communications enable the UWA [2]. However, the
complexity of underwater environment and Acoustic communications are the
challenges to UWA. Hence, UWA have become a hot research topic.
The unique characteristics of the underwater acoustic communication
channel, such as limited bandwidth capacity, high propagation delays and low
reliability are time-variant, space-variant and frequency-variant[3]. Moreover, the
ocean environment is dynamic and complex. Hence, theory analyses and precise
simulation is difficult for UWA.
A few experimental implementations of underwater acoustic have been
reported in the last few years. The Front-Resolving Observational Network with
Telemetry (FRONT) project relies on acoustic telemetry and ranging advances
pursued by the US Navy referred to as ‘telesonar’ technology [4]. The Seaweb
3
network for FRONT Oceanographic Sensors involves telesonar modems deployed in
conjunction with sensors, gateways, and repeaters, to enable sensor-to-shore data
delivery and shore-to-sensor remote control. Researchers from different fields
gathered at the Monterey Bay Aquarium Research Institute in August 2003 and July
2006 to quantify gains in predictive skills for principal circulation trajectories, i.e., to
study upwelling of cold, nutrient-rich water in the Monterey Bay, and to analyze how
animals adapt to life in the deep sea. However, experiment research on UWA is
elementary and seldom work on developing platform for UWA is reported according
our knowledge.
In this project, we design and develop a physical experiment platform for
UWA. The experiment platform consists of system control of UWA connection,
which can complete point-to-point communication performance tests and end-to-end
connection experiments. This platform serves as the testing and evaluating system of
UWA, which is convenient, flexible and scalable. Based on this experiment platform,
a lot experiments are made and a great deal of data on acoustic communication.
1.2
Problem Statement
Though commercial acoustic modems are available, there are not many to
choose from and their proprietary nature makes customization of these products
expensive or simply infeasible [13]. Researcher’s needs a better option to prototype
their algorithms and further develop the field of underwater acoustic communication.
In particular, the ability to add functionality to a proprietary system is difficult. For
example, as researchers we wish to investigate designs with different parameters
such as carrier frequency, bit rate, and packet size. To parameterize, commercial
modems do not supply the flexibility and commercial modems do not furnish the
flexibility to parameterize [15].
For Underwater Acoustic communication the design of the Modem is very
important. There are many research on underwater modem for AUV and UUV but
however there modem con not support variable modulation. By using SDR platform
4
user can add coding and varied the modulation to increase the throughput of the
transmission.
Since the characteristics of the underwater acoustic channel cannot be
properly modeled with a static configuration, it is important to be able to change the
properties of an acoustic modem at run time. Underwater acoustic system has the
flexibility of software defined radios (SDR) and the advantages of the network layers
of GNU Radio and Linux, ultimately providing and end-to-end network for easy
underwater development from the physical to application layer.
This thesis describes the full system of underwater acoustic modem with
underwater wireless sensor connected to the most critical component of the system
which is the USRP modem, hydrophone, and the USRP amplifier design circuit.
1.3
Research Objectives
The main objective of this research is to develop the circuit amplifier with the
USRP platform test bed to enable the communications and prove the data is transfer.
As a result, the objectives of the proposed research are:
•
Develop and Design amplifier for Acoustic transmitter and receiver.
•
Implement the amplifier on the USRP Platform.
•
Implement the Underwater Acoustic transceiver system.
5
1.4
Scope of Work
The scope of this research is including exploring on how to use GNU Radio
and USRP to construct the acoustic system communication. These includes: learning
the python and C++ language, installing the GNU Radio software on Linux, and
learn how to use it as the software that will control the USRP board in order to
construct the acoustic system communication test bed. Second is the development of
the amplifier circuit of the test bed that can configure its modulation according to the
system communication of the channel. Third is integrate the circuit amplifier with the
USRP platform test of UWA can be configured to improve the communication
between the nodes by sensing the packets are bad or good condition. Next is to
implement the acoustic system underwater network to prove the data is transfer. The
experiment platform consists of system control of UWA connection, which can
complete point-to-point communication performance tests and end-to-end connection
experiments. This platform serves as the testing and evaluating system of UWA
1.7
Organization of the Thesis
The rest of the thesis is organized as follows: Chapter 2 provides the relevant
background for understanding the selection of using acoustic modems instead of RF
modems for underwater communication based on the physics of the underwater
environment, and also describes existing commercial and research underwater
acoustic modems to illustrate the novelty and applicability of our design. Chapter 3
describes all the initial components experiment of the integration System spritely.
Chapter 4 describes the USRP Amplifier design and why we need in our research
and finally the integration system test results in the water tank. The final chapter
concludes the outcomes of the research and proposes a few ideas for future works.
CHAPTER 2
LITERATURE REVIEW
2.1
Overview
Present underwater communication systems involve the transmission of
information in the form of radio frequency (RF) waves, or acoustic waves. Each of
these techniques has advantages and limitations.
As stated in the previous chapter, acoustic waves are widely used in
underwater communication systems due to the relatively low attenuation of sound in
water. Thus, quite a few companies and research groups have developed underwater
acoustic modems for various undersea applications such as; USCmodem,
UCImodem, AquaModem, etc.
This chapter describes and compares existing commercial and recent research
underwater acoustic modems to better illustrate the novelty and applicability of the
USRP modem design. Also explores the effect of the underwater environment on RF
and acoustic waves and describes the rational for using acoustics for the USRP
Modem to meet the requirements of the target applications described
7
2.2
Radio Frequency Waves
Radio frequency waves are electromagnetic waves in the frequency band
below 300GHz. An electromagnetic wave is a wave of energy having a frequency
within the electromagnetic spectrum (Figure 2.1) and propagated as a periodic
disturbance of the electromagnetic field when an electric charge oscillates or
accelerates [16]. Underwater radio frequency communications have been investigated
since the very early days of radio [17], and had received considerable attention during
the 1970s [18], however few underwater RF systems have been developed due to the
highly conducting nature of salt water. This section discusses the effect of
conductivity, wavelength, and air/water interface on RF waves and describes existing
underwater systems that make use of RF waves.
Figure 2.1: Electromagnetic Spectrum
8
2.2.1
Conductivity
Pure water is an insulator, but as found in its natural state, water contains
dissolved salts and other matter, which makes it a partial conductor. The higher
water's conductivity, the greater the attenuation of radio signals that pass through it.
Propagating waves continually cycle energy between the electric and magnetic fields,
hence conduction leads to strong attenuation of electromagnetic propagating waves
[19]. Sea water has a high salt content and thus high conductivity varying from 2
Siemens/meter (S/m) in the cold arctic region to 8 S/m in the Red Sea [20]. Average
conductivity of sea water is considered to be 4 S/m whereas conductivity of fresh
water is typically on the order of a few mS/m [21].
Attenuation of radio waves in water increases both with increase in
conductivity and increase in frequency. It can be calculated from the following
formula [20]:
α= 0.0173 �𝑓𝜎
(2.1)
Where α is attenuation in dB/meter, f is the frequency in Hertz, and σ is the
conductivity in S/m.
Figure 2.2 shows attenuation as a function of frequency for sea water (4 S/m)
and fresh water (0.01 S/m). Attenuation in sea water is very high and to communicate
at any reasonable distance, it is necessary to use very low frequencies. However, the
consequence of using very low frequencies is the need to use larger antennas to
capture the signal of larger wavelength.
9
Figure 2.2: RF Attenuation vs. Frequency in Fresh Sea Water
2.2.2
Wavelength
Wavelength in water is calculated from the following formula [20]:
λ= 1000 �10/(𝑓𝜎)
(2.2)
Where λ is the wavelength in meters, f is the frequency in Hz, and σ is the
conductivity in S/m. Figure 2.3 plots wavelength vs frequency in air, sea water (with
conductivity 4 S/m), and fresh water (with conductivity 0.01 S/m). A signal's
wavelength in air is considerably reduced underwater (especially in salt water)
leading to considerable differences in antenna engineering for terrestrial and
underwater communications.
10
Figure 2.3: RF Wavelength vs. Frequency in Sea Water, Fresh Water and Air
2.2.3
Air/Water Interface
As the attenuation loss in water is high, higher transmission distances may be
achieved by having the signal leave the water near the transmitter, travel via an airpath, (where attenuation loss is low) and re-enter the water near the receiver.
However, as RF waves travel from air to water or water to air, there is a refraction
loss due to the change in the medium. This loss can be calculated via the following
formula [20]:
Refraction Loss (dB) = -20log (7.4586/106) �10/(𝑓𝜎)
(2.3)
Where f is the frequency in Hz, and σ is the conductivity in S/m. Figure 2.4
illustrates refraction loss as a function of frequency for sea water and fresh water. As
frequency increases refraction loss decreases.
11
Figure 2.4: Air to Water Refraction Loss as a Function of Frequency
Similar communications could be carried out underground depending on the
conductivity of the surrounding rock [19, 20].
2.2.4
Existing RF Systems
Because the conductivity of sea water poses severe attenuation to RF signals,
only a few systems using RF underwater have been designed. Extremely low
frequency (ELF) radio signals have been used in military applications. Germany
pioneered radio communications to submarines underwater during World War II,
where their "Goliath," antenna was capable of outputting up to 1 to 2 Mega-Watt
(MW) of power, strong enough to send signals to submarines submerged in the
Indian Ocean [22]. Later, a U.S. and Russian ELF system used 76Hz and 82Hz radio
frequency signals respectively to transmit a one-way `bell ring' to call an individual
submarine to the surface to terrestrial radio for higher bandwidth communication
[23].
Until recently it was deemed impractical to use high frequency waves for
communication purposes. However, with new antenna designs, recent experiments
indicate that radio waves within the frequency range 1-20MHz can propagate over
12
distances up to 100 m, at rates beyond 1 Mbps, using dipole radiation with
transmission powers on the order of 100W [24, 25]. The antennas are very different
from those used for terrestrial communications [22, 24, 25]; instead of having direct
contact with seawater (as terrestrial antennas have direct contact with air), the metal
transmitting and receiving aerials are surrounded by waterproof electrically insulating
materials [24, 25] allowing an electromagnetic signal to be launched from a
transmitter into a body of seawater and picked up by a distant receiver.
Figure 2.5: Wireless Fibre Systems SeaText Modem
The first commercial underwater radio-frequency (RF) modem in the world,
Sea Text (Figure 2.5), was released by Wireless Fiber Systems [26] in September
2006. It can communicate over several tens of meters at a rate of 100bps. Wireless
Fiber Systems released a second RF modem, Sea Tooth, which can support 1-100
Mbps within a 1 meter range [26].
2.3
Acoustic Waves
Acoustic waves are caused from variations of pressure in a medium. Due to
the greater density of water, they travel 4-5 times faster in water than they do in air
(traveling in water at an average of 1500 m/s - the speed of sound subject to the
water's temperature, salinity and pressure), but are about 5 orders of magnitude
slower than electromagnetic waves. They have been widely used in underwater
communication systems due to the relatively low attenuation of sound in water.
However, acoustic waves can be adversely affected by absorption loss, spreading
loss, ambient noise, and severe multipath, which is discussed in this section.
13
2.3.1
Absorption Loss
The absorption of acoustic waves in sea water depends on the temperature,
salinity, and acidity of the sea water as well as the frequency of the sound wave. The
absorptive loss for acoustic wave propagation can be expressed as eα(f)d, where d is
the propagation distance and α(f) is the absorption coefficient of frequency f [27]. For
seawater, the absorption coefficient at frequency f in kHz can be written as the sum of
chemical relaxation processes and absorption from pure water [28]:
(2.4)
Where the first term is the contribution from boric acid with f1 as its
relaxation frequency, the second term is from the contribution of magnesium sulphate
with f2 and its relaxation frequency, and the third term is from the contribution of
pure water. The pressure dependencies are given by P1, P2, and P3 and A1, A2, and A3
are constants. Figure 2.6 shows the variation in total absorption vs. frequency for
different oceans of different temperature, pressure, and pH [29]. Since α(f) increases
with frequency, high frequency waves will be considerably attenuated within a short
distance while low frequency acoustic waves can travel far.
14
Figure 2.6: Acoustic Absorption as a function of temperature, pressure, and pH [29]
2.3.2
Spreading Loss
Spherically through a body of water the energy radiated from an Omni
directional source spreads. Much of the energy is lost, since all the energy is not
directed in a single direction but in all directions. This is called spreading loss. Loss is
frequency independent note that spreading. The power loss caused by spreading is
proportional to the square of the distance, in deep water. In shallow water, sound is
bounded by the surface and the sea or resulting in cylindrical spreading. In this case,
sound power loss increases linearly with the distance from the source. For a practical
underwater setting, the spreading loss falls somewhere between spherical and
cylindrical spreading, with power loss proportional to dβ where β is between 1 (for
cylindrical spreading) and 2 (for spherical spreading) [30]. In logarithmic terms, the
classical equation for spreading loss is 10 log (dβ) [30] (see Figure 2.7).
15
Figure 2.7: Acoustic Spherical and Cylindrical Spreading Loss
2.3.3
Noise
From a myriad of unidentified sources ambient noise is defined as the noise
associated with the background blare emanating. Its distinguishing features are that it
is due to multiple sources, individual sources are not identified, and no one source
dominates the received field [31] Underwater sound is generated by a variety of
natural and man-made sources including breaking waves, rain, marine life, bubbles,
surface-ships, and military sonars. The primary source of ambient noise can be
categorized by the frequency of sound. In the frequency range of 20-500 Hz, ambient
noise is primarily generated by distant shipping, in the range 500-100,000 Hz
ambient noise is mostly due to spray and bubbles associated with breaking waves. At
frequencies above 100 KHz, thermal noise (noise generated by the Brownian motion
of water molecules) dominates.
In 1962 Wenz and colleagues set out to measure background sounds in the
ocean and summarized them in a graph showing typical sound levels at different
frequencies [32]. (Figure 2.8 was adapted from [32] by [31]). The sound levels in this
graph are in dB relative to 1 µPa. Thus, when selecting a suitable frequency band for
communication, besides path loss, noise should be also considered [33, 34].
16
Figure 2.8: The typical sound levels of ocean background noise at different
frequencies [31]
17
2.3.4
Passive Sonar Equation
Given a source level, ambient noise level and equations for absorption and
spreading loss, the passive sonar equation can be use to determine the maximum
transmission distance achievable for a desired signal to noise ratio at the receiver.
The passive sonar equation is given by equation (2.5) as shown below:
SNR(dB) = SL - TL – NL
(2.5)
Where SNR is the desired signal to noise ratio at the receiver, SL is the source
level, TL is the transmission loss due to absorption and spreading, and NL is the noise
level attributed to the ambient noise level of the environment and 10*log10(Signal
Bandwidth). Figure 2.9 shows the relationship between required source level and
range for four different SNR values at the receiver for a 40 kHz carrier with 1 kHz
bandwidth and an ambient noise of 50 dB re 1 µPa (see Figure 2.9).
Figure 2.9: Source Level vs. Transmission Distance for a 40 kHz carrier an ambient
noise of 50 dB re 1 µPa at various levels of SNR
18
2.3.5
Multipath
Underwater, there exist multiple paths from the transmitter to receiver, or
multipath. Two fundamental mechanisms of multipath formation are reaction at the
boundaries (bottom, surface and any objects in the water), and ray bending (as sound
speed is a function of temperature, salinity, and depth, rays of sound always bend
towards regions of lower propagation speed) [35]. Multipath due to reactions of the
surface and bottom is common in shallow waters whereas multipath due to ray
bending is common in deep waters. Understanding of these mechanisms is based on
the theory and models of sound propagation. Ray theory and the theory of normal
modes provide the basis for such propagation modeling.
Figure 2.10: Ray Trace for a 40 kHz source with a 15 degree beam angle placed at
10 meters depth in a body of water 11 meters deep with a constant sound speed of
1500 m/s
Bellhop is a commonly used, highly efficient ray tracing model. The
underwater acoustic propagation modeling software, AcTUP [36], can perform twodimensional Bellhop acoustic ray tracing for a given sound speed profile c(z) or a
given sound speed field c(r , z), in ocean waveguides with at or variable absorbing
boundaries. Output options include ray coordinates, travel time, amplitude, acoustic
pressure or transmission loss. Figure 2.10 shows the Bellhop ray tracing model for a
35 kHz source with a 15 degree beam angle placed at 10 meters in a body of water 11
meters deep with a constant sound speed of 1500 m/s. Multipath can adversely affect
19
communications because a large delay spread (the time difference of arrival of the
first and last path at the receiver) introduces time dispersion of a signal, which causes
severe inter-symbol interference. Typical underwater channels may have a delay
spread around 10ms, but occasionally delay spread can be as large as 50 to 100ms
[37] or as small as 3 ms [38]. The delay spread of a receiver placed at 10 m, 100 m
from the source in Figure 2.10 is only 300 microseconds.
2.3.6
Why Acoustic
The radio wave and acoustic wave fields have their own advantages and
limitations for acting as an underwater wireless communications carrier. Radio waves
can provide high data rates, but are subject to strong attenuation by the conductivity
of sea water, whilst acoustic waves provide long transmission distances but support
relatively low data rates and are subject to multipath. Since
the underwater
applications require low power and transmission distances greater than 100 meters,
acoustics remains the most robust and feasible carrier to date for wireless
communication in these underwater sensor networks. As acoustics have been widely
used in underwater communications and this project will use acoustics for the
modem, the next section is focus in describing and comparing existing commercial
and research underwater acoustic modems.
2.4
Current Acoustic Modems
Research underwater acoustic modems have been designed with the objective
of reducing power consumption or cost or with the objective of testing new
communication algorithms to increase bit rate or better counter the effects of harsh
environments. This section presents an overview of some of the research modems
that have been designed in the past decade.
20
The University of Southern California's Information Sciences Institute
designed a prototype modem for the Sensor Networks for Undersea Seismic
Experimentation (SNUSE) project [39]. Their primary design goal was to provide an
inexpensive, low power modem to operate over 50-500 meters for seismic monitoring
applications. To accomplish their objective, the prototype included an inexpensive
ultra low-power wake up receiver that consumes only 500 microWatts and an
inexpensive off the shelf 8-bit microcontroller and radio frequency integrated circuit
for control and frequency shift keying based communication. They did achieve a
prototype costing less than $100 with a maximum transmit power of 2W, receive
power of 25mW, and idle power of 500uW, but could only perform in-air testing with
the hardware they designed.
Researchers at the University of California Irvine proposed the use of
software acoustic modems running on generic speakers and microphones to establish
acoustic communications for underwater sensor networks with the idea that the use of
generic hardware can greatly reduce the cost of the modem design [40]. Their
frequency shift keying based modem design implemented on the Tmote In- vent
module [41] can achieve a bit rate of 24 bps at a 10 meter range or 48 bps at a 3 meter
range in water.
The AquaModem [42], designed at the University of California Santa
Barbara, was designed for short range (< 1km) eco-sensing applications in a shallow
horizontal underwater channel. The AquaModem uses M-ary direct sequence spread
spectrum signaling, with joint detection and channel estimation performed by
matching pursuits to effectively handle multipath interference. It was implemented on
a TI TMS320C6713 DSP. and made use of custom made $2500 transducers with a
center frequency of 24kHz and a double-sided bandwidth of 7.8 kHz. The modem
was field tested in a shallow water coral reef and achieved ranges up to 440 meters
with a bit rate of 133 bps and an uncoded symbol error rate < 1% [43].
Researchers at Kookmin University, Korea, designed an underwater acoustic
modem that makes use of four small air transducers to communicate to other nodes
facing to the north, east, south, and west respectively and a fifth transducer to
21
communicate to a surface node. The design uses an ATmega128 as its
microcontroller unit which interfaces to an Arm PXA270 processor for medium
access layer (MAC) control. It operates at 30 kHz and is capable of transmitting data
up to 5 kbps at ranges up to 30 meters [44].
Vasilescu et. al developed the AquaNode that is dually networked: optically
for point-to-point transmission at 330 kbps and acoustically for broadcast
communication over ranges of hundreds of meters at 300 bps [45]. The acoustic
modem is are built around an Analog Device Blackfin BF533 fixed point DSP
processor and uses FSK modulation on a 30kHz carrier frequency. The nodes selflocalize and can be used to form static undersea networks. With all the nodes running
at full power, the battery provides 1-2 weeks of continuous operation.
Researchers at the University of Connecticut designed an orthogonal
frequency division multiplexing based modem on a TMS320C6713 DSP with the
goal of providing a higher data rate solution to other research modems [46]. The
modem is capable of transmitting data at a raw data rate 3.1 kbps after rate ½
nonbinary LDPC coding and QPSK modulation. The modem has been tested in water
in a lab test tank, but has not been field tested, thus no range information is available
[47]. Researchers at Northwestern Polytechnical University in China also designed a
DSP based OFDM modem, implementing the design on a ADSP-TS101 and
achieving an uncoded bit rate of 1kbps in a lake (range not specified) [48]. Sozer and
Stojanovic developed a reconfigurable acoustic modem (rModem) [49] designed to
act as a physical layer prototyping platform. This platform includes a high processing
power floating point DSP for the implementation of various physical layer protocols
and an FPGA that enables users to operate at any carrier frequency and bandwidth
within the 1kHz - 100kHz band by setting carrier coefficients, filter coefficients, and
interpolation/decimation rates. The current rModem design interfaces to a daughter
card that can drive a Teledyne AT-408 transducer that operates in the 9-14kHz band.
Because the rModem is intended to be used as a research tool rather than a
commercial product, high processing power and cost is tolerated. The rModem
hardware and software operability was field tested in Woods Hole, MA [50].
22
Numerous other researchers have implemented communication algorithms on
a laptop computer and used commercially bought hardware to connect to the laptop
and conduct in-water experiments. Examples include OFDM based implementations
[51, 52, 53, 54, 55, 56] and multiple-input, multiple-output (MIMO) based
implementations [57, 58, 59]. These designs are useful for examining the capabilities
of different modulation schemes, but are not designed for real-time deployment.
Table 2.1 compares the described research modems in terms of platform, modulation
scheme, bit rate, and range. `NS' denotes values not specified in the literature.
Table 2.1: Research Underwater Acoustic Modem Comparison
Modem
Platform
Mod
USC
MCU
FSK
Bit
Rate
NS
Range
(m)
NS
BER
Comments
Ref
10-5
in air tests only
low-power
wake up circuit
Uses generic
hardware
for increased
bit rate tank
test only
Uses for
UUV/AUV
for high
multipath
for very short
range
dual networked
functionality
low cost
39
(CODED)
UCI
Tmote
FSK
12
5
10%
uConn
DSP
OFDM
6200
NS
NS
rModem
DSP
varied
varied
200
NS
Aqua
Modem
Kookmin
DSP
DSSS
133
440
1%
MCU
NS
5000
30
NS
MCU
FSK
300
400
NS
FPGA
varied
varied
<1Km
4%
Aqua
Node
USRP
2.5
40
41
46
49
42
44
45
68
Software Defined Radio (SDR)
Software defined techniques have been of interest in recent years not only for
terrestrial radios but also underwater acoustics. The term “Software Defined Radio”
(SDR) was introduced by Joseph Mitola from MITRE Corporation in 1991. His first
paper on SDR was published in 1992 at IEEE National Telesystems Conference [5].
Though the concept was first proposed in 1991, software-defined radios have their
origins in the defense sector since the late 1970’s in both the U.S. and Europe. One
23
of the first public software radio initiatives was a U.S. military project named
SpeakEasy [6]. The primary goal of the SpeakEasy project was to use programmable
processing to emulate more than 10 existing military radios, operating in frequency
bands between 2 to 2000 MHz [90]. Second goal was to make the radio device easily
able to incorporate new coding and modulation standards in the future, so that
military communications can keep pace with advances in coding and modulation
techniques.
SPEAKeasy program is then evolved into the Joint Tactical Radio
System (JTRS) in 1999 [5].
SDR forum in calibration with IEEE P1900 working group defined SDR as
“Radio in which some or the entire physical layer functions are software defined”
[68]. In the definition, SDR forum further defined physical layer as “The layer
within the wireless protocol in which processing of RF, IF, or baseband signals
including channel coding occurs. It is the lowest layer of the ISO 7-layer model as
adapted for wireless transmission and reception.” [62]. In SDR, all the signal
manipulations and processing works in radio communication are done in software
instead of hardware. Therefore, in SDR, signal will be processed in digital domain
instead in analog domain as in conventional radio. In SDR, signal digitization work
is done by using the analog to digital converter (ADC). Figure.2.11 shows the
concept of Software Defined Radio. As depicted in this figure, the ADC process is
taking place after the front end (FE) circuit. FE is used to down convert the high
frequency signal to a lower frequency called the intermediate frequency (IF) or if
possible to base band frequency; this is needed due to the limitation of current
commercial of the shelf (COTS) ADC chip speed. The ADC will digitize signal and
pass it to the baseband processor for further processes; demodulation, filtering,
channel coding, and etc. All this baseband processing work is done in software
which contras with the conventional radio where all this processes are done in
hardware.
In general, Software Defined Radio (SDR) is defined as a software based
communication platform which characteristics can be reconfigured and modified to
perform different functions at different times.
24
Hydrophone
Figure 2.11 Software Defined Radio Block Diagram
Although at the beginning, the excitement of SDR was due to its ability to support
multiservice radio, but in the last decade, SDR research has shift to a new direction
which SDR is chosen to be use as the platform for underwater acoustic research. We
have modified one of the digital communication applications that is included with the
GNU Radio distribution. The application was created to connect PCs forming a
system, using Universal Software Radio Peripheral (USRP). The USRP created by
Ettus Research [60], is a radio frontend that is commonly used with GNU Radio.
Although the option of using a sound card provides a low cost solution, the USRP
offers a wider frequency range as well as more dedicated hardware. The USRP has a
total of 4 ADCs and 4 DACs allowing for up to 16 MHz of bandwidth each way,
which is proficient for the underwater acoustic channel.
In this work, a software defined radio (SDR) platform called GNU radio and
universal software defined radio peripheral (USRP) is used as the testbed or platform
for the Underwater Acoustics.
2.5.1
GNU Radio
GNU Radio [61] is a free software toolkit which consists of a huge numbers
of signals processing blocks library (i.e. modulators, filters, amplifiers and etc). This
25
signal processing blocks can be liked together for building and creating the baseband
part of the dedicated radio.
In GNU Radio, all the signal processing block code is written in C++ and it is
compiled with an optimizing, modern, C++ compiler including.
The list and
documents on the available signal processing block can be found in [62]. Most of the
required signal processing block to build a radio is already available for one to use,
however, a new signal processing block also can be created. A good tutorial in
creating a new signal processing block can be found in [62].
In order to build the dedicated radio using GNU Radio, several signal
processing blocks has to be tie together. The most basic structure of the constructed
radio is as shown in Figure 2.12. It consists of three parts; signal source, signal
processing and signal sink. Signal source is where the signal is generated. There is
several signal source block available in GNU Radio for instance USRP, sound card
(microphone), file, signal source (signal generator) and etc. Signal Processing is
where signal source is manipulated before it is loaded into signal sink block. Most of
the base band processing for signal manipulation is already available in GNU Radio
for instance FIR filters, IIR filters, FFT, Multipliers, and etc. Signal sink is where the
signal is translate into the form that user desired. Examples of signal sink are sound
card (speaker), USRP, data in integer form, graph and etc.
Signal Source
Signal
Processing
Signal Sink
Figure 2.12: Basic Structure of GNU Radio Flow Graph
In GNU Radio, python is used as the glue to tie the signal processing blocks
together. Python is an interpreted language which give advantage to user as user does
not required compiling the written code. This will speed up the development process.
Signal processing blocks which is connected together is called flow graph. Tutorial
on how to the flow graph can be found in [62]. Simplified wrapper and interface
generator (SWIG) is used as the interface compiler which allows the integration
26
between C++ and Python language. Figure 2.13 shows the structure of GNU Radio
and USRP SDR.
SWIG (C++ and Python interface)
C++ signal processing blocks
(GNU Radio signal processing blocks)
Software
Python Flow Graph
(Signal processing blocks that tied together)
USRP
(Motherboard)
Hardware
USB interface
Figure 2.13 GNU Radio and USRP Structure
GNU Radio has a tremendous forum base community support [63]. There are
hundreds of expert engineers and academicians with decades of experience in
communication, signal processing, and related fields who will try to help in the
forum. Basically a good question will be answered in less than a day. GNU radio
also has a huge number of users including commercial, academic, government,
research institutions and other organizations and individual. List of these users can
be found in [61].
2.5.2
Universal Software Radio Peripheral
The baseband signal of the radio which is generated by GNU Radio needs
SDR hardware for it to be linked to the real world. The recommended SDR hardware
27
to be used with GNU Radio is the universal software radio peripheral (USRP) [66].
USRP is developed by Ettus research lab, Mountain View, California. Its function is
to change the analog value of the spectrum to the digital domain and to change the
digital domain signal to analog value. USRP is connected to the device which
running GNU Radio or any other software defined radio software via USB2.0 port.
Figure 2.14 shows the USRP connection which is available in telematic research
group (TRG) lab.
USRP
Connector
USB Cable
Figure 2.14: USRP in Lab
USRP consist the motherboard. Figure 2.15 shows the USRP motherboard. It
consists of four 12-bit Analog to Digital Converter (ADC) with sampling rate of
64MS/s, four 14-bit Digital to Analog Converter (DAC) with speed of 128MS/s, two
Digital up Converter (DUC) to up convert the baseband signal to 128MS/s before
translating them to the selected output frequency, a programmable USB 2.0 controller
for communication between USRP and GNU Radio and an FPGA for implementing
four Digital Down Converter (DDC) and high rate signal processing. The whole
design of USRP including motherboard is open source and can be found in [66].
28
Altera Cyclone FPGA
(EP1C12)
Cypress FX2 USB controller
(CY7C68013A)
Analog Device ADC/DAC chip
(AD9862BTS)
USB 2.0
Slot
Daughterboard Slot
(4 Slots in Total)
Figure 2.15 USRP motherboard
2.5.3
GNU Radio and USRP Applications
Software defined radio has received a lot of attention most notably in the
research community. The ability to use software to modulate and manipulate the
received and transmitted signals allows for rapid development without the need or
cost of specialized hardware. GNU Radio [67], one of the most popular SDR
frameworks, is comprised of a flow graph and signal processing blocks. The signal
processing blocks are written in C++ and act as the “heavy lifters” whereas the flow
graph is setup in Python in order to move data from one block to the next. In this way
many modulation schemes can be created using standard C++ blocks (already
29
included in GNU Radio) and connecting them together in a flow graph. There is a
large community of users who have contributed to this open source project, both
signal processing blocks as well as various applications. There are many
contributions have led to a large library of modulation schemes including GMSK,
PSK, QAM, CPM, OFDM, and more. The GMSK was the modulation which has
been used for this research and it will be explained in the next section.
Besides GNU Radio package itself, there are also a lot of applications
developed on top of GNU Radio and USRP SDR by the third parties group for
instance GSM, Bluetooth, GPS, IEEE802.11, IEEE802.15.4 and etc
•
GSM based projects: Some examples of GNU Radio and USRP GSM based
projects are the Open BTS project which aim to provide cellular service for
the price of 1/10 from the current charge, the A5 project [70] which aim to
crack the GSM encryption and decode the GSM communication in a
reasonable amount of time, the Global Software System for Mobile
communications (GSSSM) project which can demodulate and decode GSM
live packet and display in Wireshark [69] and etc.
•
Bluetooth base project: Dominic Spill shows that Bluetooth packet can be
decoded using GNU Radio and USRP. This first open source Bluetooth
sniffer can be used to monitor the exchange packet between two Bluetooth
devices without known. However, this sniffer does not yet support channel
hopping due to the USRP limitation. Detail of the project can be found in
[71] including the source code.
In [73] Ali Tabassam demonstrate the
possibilities of using GNU Radio and USRP to acquire the Bluetooth master
address and its clock without capturing and decoding the frequency hop
sequence (FHS) packet.
•
GPS based projects: Gregory Heckler develop the GPS L1 C/A receiver on
top of GNU Radio and USRP. This project is known as the GPS-SDR project
which is hosted at [74]. A complete source code and forum based support can
30
be found in the website.
A complete GPS references for developer is
available in GNU Radio website [72].
•
IEEE802.11 based projects: The BBN project is one of the dominant GNU
Radio and USRP IEEE802.11 project [75]. The developed radio is able to
decode low rate on air IEEE802.11b packet from the network interface cards
(NICs) reliably at 1Mbps and partially at 2Mbps. P. Fuxjäger develops the
IEEE802.11p transmitter. This project is using MATLAB to derive the radio
from the standard document and the individual MATLAB block is then ported
to GNU Radio one by one.
•
IEEE802.15.4 based projects: Thomas Schmid develop the GNU Radio OQPSK modulation code based on the CMOS IEEE802.15.4 RF IC
architecture [77].
This modulation is then used to support GNU Radio
IEEE802.14.5 radio which can communicate with Berkeley TelosB and
MicaZ mote. The source code of the project is available in [76]. Thomas
Schmid code is then used as the base of IEEE802.15.4 SDR radio in this
master project.
Besides the available standard radio ported on top of GNU Radio and USRP, a
number of researcher also utilize the underwater acoustic research.
•
Software-Defined Underwater Acoustic Networking Platform: Dustin
Torres, Jonathan Friedman, Thomas Schmid, Mani B. Srivastava proposed
Universal Software Radio Peripheral (USRP) underwater modem and GNU
Radio to act as a solution for research in implementing underwater modems
and related signal processing to provide an end-to-end networking approach
for underwater acoustic development. [78].
•
A Flexible MAC/PHY Multihop Testbed: Ketan Mandke, Soon-Hyeok
Choi, Gibeom Kim, Robert Grant used in particular, the Universal Software
Radio Peripheral (USRP) board to implement the RF front-end of Hydra,
31
PHY is implemented in C++ using the GNU Radio framework, and MAC is
implemented in C++ using the Click modular router framework [79]
2.6
GMSK Modulation
Gaussian Minimum Shift Keying (GMSK), or to give it its full title Gaussian
filtered Minimum Shift Keying, GMSK is a form of modulation used in a variety of
digital radio communications systems. Digital modulation while still applying the
spectrum efficiently it has advantages of being able to carry. One of the problems
with other word forms of phase change over keying is that the sidebands extend
outwards from the main carrier and one of the problems with other descriptors of
phase change over keying is that the sidebands extend outwards from the main
carrier. [81].
2.6.1
GMSK basics
GMSK modulation is based on Minimum Shift Keying (MSK), which is itself
a form of phase shift keying (PSK). One of the problems with standard forms of PSK
is that sidebands extend out from the carrier. To overcome this, MSK and its
derivative GMSK can be used [81].
MSK and also GMSK modulation are what is known as a continuous phase
scheme. Here there are no phase discontinuities because the frequency changes occur
at the carrier zero crossing points. This arises as a result of the unique factor of MSK
that the frequency difference between the logical one and logical zero states is always
equal to half the data rate (see figure 2.16). This can be expressed in terms of the
modulation index, and it is always equal to 0.5 [81, 80].
32
Figure 2.16: Signal using MSK modulation
A plot of the spectrum of an MSK signal shows sidebands extending well
beyond a bandwidth equal to the data rate (see figure 2.17). This can be reduced by
passing the modulating signal through a low pass filter prior to applying it to the
carrier. The requirements for the filter are that it should have a sharp cut-off, narrow
bandwidth and its impulse response should show no overshoot. The ideal filter is
known as a Gaussian filter which has a Gaussian shaped response to an impulse and
no ringing. In this way the basic MSK signal is converted to GMSK modulation [80]
Figure 2.17: Spectral density of MSK and GMSK signals
2.6.2
Generating GMSK modulation
There are two main ways in which GMSK modulation can be generated. The
most obvious way is to filter the modulating signal using a Gaussian filter and then
apply this to a frequency modulator where the modulation index is set to 0.5. (see
figure 2.18) This method is very simple and straightforward but it has the drawback
33
that the modulation index must exactly equal 0.5. In practice this analogue method is
not suitable because component tolerances drift and cannot be set exactly [80].
Acoustic
Figure 2.18: Generating GMSK using a Gaussian filter and VCO
A second method is more widely used. Here what is known as a quadrature
modulator is used. The term quadrature means that the phase of a signal is in
quadrature or 90 degrees to another one. The quadrature modulator uses one signal
that is said to be in-phase and another that is in quadrature to this. In view of the inphase and quadrature elements this type of modulator is often said to be an I-Q
modulator (see figure 2.19). Using this type of modulator the modulation index can
be maintained at exactly 0.5 without the need for any settings or adjustments. This
makes it much easier to use, and capable of providing the required level of
performance without the need for adjustments. For demodulation the technique can
be used in reverse [80].
Figure 2.19: Block diagram of I-Q modulator used to create GMSK
34
2.6.3
Advantages of GMSK modulation
There are several advantages to the use of GMSK modulation for a radio
communications system. One is obviously the improved spectral efficiency when
compared to other phase shift keyed modes [81].
A further advantage of GMSK is that it can be amplified by a non-linear
amplifier and remain undistorted this is because there are no elements of the signal
that are carried as amplitude variations. This advantage is of particular importance
when using small portable transmitters, such as those required by cellular technology.
Non-linear amplifiers are more efficient in terms of the DC power input from the
power rails that they convert into a radio frequency signal. This means that the power
consumption for a given output is much less, and this results in lower levels of
battery consumption; a very important factor for cell phones [81, 80].
A further advantage of GMSK modulation again arises from the fact that none
of the information is carried as amplitude variations. This means that is immune to
amplitude variations and therefore more resilient to noise, than some other forms of
modulation, because most noise is mainly amplitude based [80].
GMSK based radio system. Theoretically, any type of radio signal can be
generated using the GNU Radio and the USRP. However, for prove of concept
purposes, in this project we choose to transmit a GMSK radio signal with the bit rate
of 20kb/s
2.7
Brüel & Kjær Hydrophones
The selected acoustic sensor is produced from Brüel & Kjær hydrophones
which are individually calibrated waterborne-sound transducers which have a flat
frequency response and are omni-directional over a wide frequency range. Their
35
construction is such that they are absolutely waterproof and have high corrosion
resistance. All Brüel &Kjær hydrophones are manufactured using lead-free nitrile
butadiene rubber. B&K hydrophone type 8105 and 8104 were considered as a
reference and the sensitivity of the system design [82]. In the next chapter will
described the experiment of B&K hydrophone on the water tank.
2.7.1
Hydrophone Type 8104
A wide-range standard measuring transducer for making absolute sound
measurements over the frequency range 0.1Hz to 120 kHz with a receiving sensitivity
of −205 dB re 1V/μPa (100 atm.; 1000 m (3250 ft.) ocean depth). It can also be used
as a sound transmitter (projector) which makes it ideal for calibration purposes by the
reciprocity, calibrated-projector and comparison methods. The main features of Type
8104 are shown in Figure 2.20 [82].
Figure 2.20: Hydrophone Type 8104
36
2.7.2
Hydrophone Type 8105
A small, spherical transducer for making absolute sound measurements over
the frequency range 0.1Hz to 160 kHz with a receiving sensitivity −205
of
dB re
1V/μPa. It is rugged, being capable of withstanding pressures of up to 107 Pa (100
atm.; 1000 m (3250 ft.) ocean depth). This hydrophone has excellent characteristics:
at 100 kHz, it is omnidirectional over 360° in the x-y (radial) plane and 270° in the xz (axial) plane. Type 8105 is illustrated in Figure 2.21[82].
Figure 2.21: Hydrophone Type 8105
37
2.8
Summary
This chapter described the effect of the underwater environment on RF, and
acoustic waves. All two physical wave fields have their own advantages and
limitations for acting as an underwater wireless communications carrier; radio waves
can provide high data rates, but are subject to strong attenuation by the conductivity
of sea water, subject to attenuation by the turbidity of sea water, acoustic waves
provide long transmission distances but support relatively low data rates and are
subject to multipath. Acoustics remains the most robust and feasible carrier to date
for wireless communication in these underwater sensor networks. As acoustics have
been widely used in underwater communications and we have selected acoustics for
our research.
This chapter also described and compared existing commercial and recent research underwater acoustic modems. The modem described in this thesis differs from
the commercial and research modems described above The SDR platform chosen for
this research is the GNU Radio and USRP SDR. This platform is chosen due to the
rich of its library and large community who give and share their experience in
utilizing this platform as their SDR platform. However, to install the GNU Radio and
to begin using it is quite hard. The learning curve is quite high. Therefore the next
chapter provides step-by-step to install GNU Radio software and how to run the GNU
Radio and USRP for the first time.
CHAPTER 3
INITIAL EXPERIMENTS
3.1
Overview
This chapter first describes the Hydrophone sensitivity measurement which
has an important role in analyzing the output data in underwater acoustic applications
such as environmental noise measurements or seismic exploration. Therefore,
calibration of hydrophones in different frequency bands is one of the most important
and substantial goals of an underwater acoustic system. Different methods have been
introduced for measuring hydrophone sensitivity in a water tank in frequencies
between 2 to 20 kHz in which the tank dimensions are proportional to the acoustic
wavelength of the transmitter.
The second part of this chapter will cover the GNU Radio and USRP
Installation step by step, then the test run of the GNU Radio with the USRP. In order
to verify the operation of system design, we tested the components separately and the
in the next chapter we will test the full integrated system.
39
3.2
Hydrophone Experiment
Electroacoustic Transducer is a device that converts electrical energy to
acoustic energy and vice versa. A hydrophone is a transducer which is used as a
receiver and is capable of converting underwater sound waves into electrical signals.
To achieve high sensitivity and wide frequency band, it is important to choose the
material characteristics and physical dimensions wisely. The electrical amplifier of
the hydrophone should also have a low noise level. Typically, hydrophones have a
small size, because:
1. An omni-directional hydrophone in a specific frequency range should have
smaller dimensions compared to the acoustic wavelength in water in the
highest frequency of the mentioned frequency range.
2. To achieve a flat frequency response in a frequency range, the first natural
frequency of the hydrophone should be higher than the frequency range.
Piezoceramic sensors of different material, shapes and dimensions are used in
hydrophone production [83].
Hydrophone sensitivity measurement has an important role in analyzing the
output data in underwater acoustic applications such as environmental noise
measurements or seismic exploration. Therefore, calibration of hydrophones in
different frequency bands is one of the most important and substantial goals of an
underwater acoustic laboratory. Different methods have been introduced for
measuring hydrophone sensitivity in a water tank in frequencies between 2 to 20 kHz
in which the tank dimensions are proportional to the acoustic wavelength of the
transmitter. As a result, these methods are not applicable in frequency bands bellow 1
kHz. Considering the nature of the wave propagation in small, laboratory
environments, creating an environment which ensures the repeatability of the tests is
a substantial acoustic problem. The problem becomes more complex in low
frequencies because of the formation of standing waves. In fact, the first aim of
calibration is to create a uniform pressure field in which the measurement results are
independent of the hydrophone location [84].
40
The measurements were accomplished using a Signal analyzer Type 2035
with Measuring Amplifier Type 2535. In the experiment that will be carried out,
there are three hydrophones will be used that are type 8104 with accuracy of 460*103
Pc/Pa will be used as the transmitting hydrophone. Type 8105 accuracy is 368*103
Pc/Pa and again type 8105 but with accuracy of 347*103 Pc/Pa as receivers.
First the sensitivity of a standard 8105 hydrophone with the diameter of
22mm was measured, then the sensitivity of a standard 8104 hydrophone with the
diameter of 21mm was measured using the standard 8105 as a reference and later the
test was repeated for the produced spherical hydrophone (which has the diameter of
24mm) [85]. The hydrophone output signal was measured using a DS-6121A Iwatsu
Digital Storage-scope (see figure 3.1).
Figure 3.1: DS-6121A Iwatsu Digital Storage-scope
3.2.1
Water Tank
The size of the water tank used for experiment is important for transmitting
and receiving a signal. The relationship between the dimension of the water tank and
the frequency used for experiment should be known. A tank that is bigger in size
would have greater pressure compared to a tank that is smaller and shallower. This
experiment was conducted using a water tank with dimension of 80cm x 125cm x
65cm and the measurement point was marked on the tank, like shown in figure 3.2.
In figure 3.3 shows the full system experiment. Two laptops have been used in
experiment and each of them was connected to sprit USRP Modem. The Modems
was connected to the hydrophones transducer.
41
Figure 3.2: The water tank in the Lab
Laptop
USRP Modem
Tx 8105
Rx 8104
Transducer
Figure 3.3: System experiment.
The size of the tank is important for the positioning of the transmitter and also the
receiving hydrophones, the measuring technique and the Q factor which the total
signal sent in the system. The water tank is a limited medium, therefore we need to
use gating and pulse technique that will create a free space medium like condition.
42
This experiment was conducted using Low frequency of 1 kHz but it fulfills the
specification as a maximum signal receiver for calibration
3.2.2
Sensitivity Measurement and Directivity of the Produced Hydrophone in
Low Frequency
To measure the electronic characteristics of the produced hydrophone, an
Impedance Analyzer was deployed. The electrical impedance, capacitance, dielectric
loss, and the first resonance frequency of the produced hydrophone were measured in
1 kHz. Table 3.1 shows some electronic characteristics of the produced hydrophone
with 10 meters B&K cable[10].
Table3.1: Electronic characteristics of the produced hydrophone
Zfr(Ω)
55.6
fr(kHz)
81
f=1kHz
D(%)
C(nf)
Z(kΩ)
1.7
15.9
10
The dimensions of the tested hydrophones satisfy the equation
h>>x>>a
(3.1)
Where h is mean at depth and x is mean position and a sphere with radius a.
Therefore, the perturbation can be neglected and the pressure amplitude on
the hydrophone surface can be calculated using equation (3.2)
(3.2)
Where g is the acceleration of gravity and ω angular frequency.
43
The measured sensitivities of the hydrophones and the accuracy of the
sensitivity measurements are given in table 3.2.
Table 3.2: The measured sensitivity of the 8105, 8104 and the produced hydrophone
Frequency
Hydrophone Sensitivity(dB re 1V/µPa)
(kHz)
The absolute accuracy of
the sensitivity
measurements (kHz)
8105
8104
Produced
8104
Produced
(Reference
Hydrophone
Hydrophone
Hydrophone)
(P0)
(P0)
50
-202.0465
-202.8
-197.8
0.23
0.06
60
-201.5199
-202.75
-197.58
0.25
0.12
70
-201.3512
-202.6
-197.47
0.4
0.5
80
-200.5538
-201.95
-197.05
0.4
0.42
90
-200.5538
-201.95
-196.98
0.49
0.38
100
-200.1083
-201.6
-196.49
0.23
0.06
110
-199.8938
-201.43
-196.3
0.25
0.12
The element was coated layer by layer. The resulted coat can endure
pressures up to 20 bars. Receiving sensitivity of the produced hydrophone is uniform
in X-Y plane in all angles. By increasing the frequency, the vertical polar pattern
loses its omni-directionality.
The B&K hydrophone type 8105 was considered as a reference and the
sensitivity of the B&K hydrophone type 8104 and the produced hydrophone was
measured with the accuracy of 0.5 dB. Using this method, the sensitivity of the
hydrophones which are deployed in different underwater acoustic applications can be
measured. Considering the complexity of low frequency hydrophone calibration, the
method has satisfactory accuracy compared to other low frequency calibration
methods [11].
44
3.3
GNU Radio and USRP Installation
GNU Radio supports eight different operating systems (OS): Fedora, Debian,
SuSE, Ubuntu, Madriva, Mac OS X, NetBSD and Windows. However, due to many
dependency packages needed by GNU Radio, which makes the installation
bothersome, the most suitable OS to be used are Linux Fedora and Ubuntu as
recommended in [90].
It is quite difficult to install GNU Radio, furthermore, if one is not familiar
with Linux OS. Therefore, this section will give a step by step guide for the GNU
Radio installation process and how to verify that the installation is successful. It is
recommended to log in the Fedora and Ubuntu by using the root instead of others
user type to avoid the difficulty on installing and modifying any of GNU Radio file
along the research. The GNU Radio can be installed by using the stable version or
by using the trunk version. Both are available in [64]. The latest stable version of
GNU radio also can be downloaded by using the command below which should be
pasted on the terminal:
$ svn co http://gnuradio.org/svn/gnuradio/branches/releases/3.2 gnuradio
While the latest version of GNU Radio from the trunk can be downloaded by
entering the following command on the terminal:
$ git clone git:://gnuradio.org/gnuradio.git
All the dependencies or packages pre-installed software must be installed first
before installing the GNU Radio. Missing any one of them will lead into program
difficulties. There was a lot of pre-installed software packages needed for GNU
Radio as listed and explain in [89][88], but all of them can be done by using these
three commands which are found in [66]:
45
$ yum groupinstall "Engineering and Scientific" "Development Tools"
$ yum install fftw-devel cppunit-devel wxPython-devel libusb-devel guile
boost-devel alsa-lib-devel numpy gsl-devel python-devel pygsl pythoncheetah python-lxml
$ yum install sdcc
The first two commands are the basic requirements for GNU Radio while the
third one which is the small device C compiler (SDCC) is for building the firmware
on the USRP. After the dependencies have been installed, a few packages have to be
imported to the PATH using these commands:
$ export PATH=/usr/libexec/sdcc:$PATH
$ export PYTHONPATH=/usr/local/lib/python2.5/site-packages
However, if these two commands are imported only by using the terminal,
then it has to be imported by each of the terminal used. To avoid this problem, these
two commands have to be placed inside the bashrc file which is located inside the
user folder; the root. Take note that this file is hidden inside this folder. Another file
that needs to be modified is the bash-profandile file which is also hidden inside the
same folder. Open this file and add the following command:
LD_LIBRARY_PATH=$LD_LIBRARY_PATH:$HOME/local/lib
PYTHONPATH=/usr/local/lib/python2.5/site-packages
EXPORT PATH
export LD_LIBRARY_PATH
export PYTHONPATH
GNU Radio of latter version 3.1.3 needs to have at least a version of 1.35
boost library and for version 1.35 and above, boost library cannot be installed by
using the yum command but it have to be downloaded and built manually. The
library can be downloaded from boost.sourceforge.net and the link to download the
package for Fedora is at boost_1_36_0.tar.gz. This research uses boost version 1.36.
46
The downloaded boost file is then unpacked and by using the terminal, the current
directory is changed to the previous unpacked boost directory:
$ cd boost_1_36_0
to configure and build the boost library, the following commands are used:
$ BOOST_PREFIX=/opt/boost_1_36_0
$ ./configure --prefix=$BOOST_PREFIX –with
libraries=thread,date_time,program_options
$ make && make install
Finally, boost library has to be exported to the library path so that GNU radio
would be able to find it. The following command is used to export the installed
boost library:
$ export LD_LIBRARY_PATH=$BOOST_PREFIX/lib
After all the pre-works are completed, now GNU Radio is ready to be
installed. First, change the directory of the terminal to the GNU Radio package
directory by using the change directory (cd) command. Then, build and install GNU
Radio by using the following command:
$ ./bootstrap
$ ./configure
$ make && make check
$ make install
The command ./bootstrap is not required if GNU radio from the stable release
version is used.
47
3.4
GNU Radio and USRP Test-Run
After the installation is completed, the GNU Radio has to be tested to make
sure it is installed successfully and is working properly. There are several python
example files that can be run to verify the GNU radio installation and most of them
are placed inside the GNU Radio example folder; ../gnuradio/gnuradioexample/python/.
The most popular test code to try is the dial_tone.py python file located inside
the audio folder, ../audio. This test code is explained in detail in [90] and in [64].
This python file can be run using the terminal by entering the following command:
$ ./dial_tone.py
This example will generates two sine waves and send the output to the sound
card: one on the left channel and one on the right. This wave will sound like an
American version of dial tone and it should come out from the speakers of the PC or
laptop if there was no error with the installation of the GNU Radio.
The dial_tone code does not make any interaction with the USRP and it is
meant to test the GNU Radio independently. In order to make sure that GNU Radio
is working properly with the USRP, the code benchmark_tx.py and benchmark_rx.py
in the digital folder, ../digital, are used. benchmark_tx.py will transmit the data by
using the digital modulation technique trough the USRP. The default modulation is
GMSK and it can be changed to CPM, D8PSK, 8-QAM, DBPSK, DQPSK and
GMSK easily by using the --modulation command option. The following command
is used to run this python file with GMSK modulation:
$ ./benchmark_tx.py –f 75k --modulation gmsk
The –f command option is to specify the frequency of the transmission and in
this case is 75kHz. The benchmark_rx.py is the receiver code which will receive the
48
digitized signal from the USRP and decode that signal back to the original data by
using the same parameter as benchmark_tx.py used. The following command is used
to run this receiver python file:
$ ./benchmark_rx.py –f 75k --modulation gmsk
Note that the parameters of the receiver have to be exactly the same as the
transmitter to make sure that the decoding of the signal is a success.
Figure 3.4 and Figure 3.5 show the screenshot of the transmitted and received
data from benchmark_tx.py and benchmark_rx.py files.
Figure 3.4: Screenshot of the benchmark_tx.py running on the Terminal While
Transmitting Data
49
Figure 3.5: Screenshot of the benchmark_rx.py running on the Terminal While
Receiving Data
Besides the benchmark_rx.py, there is another useful python example file that
can be used to verify the functionality of the GNU Radio and USRP receiver which
is usrp_fft.py. usrp_fft.py code will generate a software spectrum analyzer which
utilize the wxPython library for its graphical user interface (GUI). usrp_fft.py file is
placed inside the utilities folder in the GNU Radio package; ../gnuradio/grutlits/src/python/. The following command is entered on the terminal to run the
usrp_fft file:
$./ usrp_fft.py
Figure 3.6 shows the screenshot of the spectrum analyzer generated from the
usrp_fft.py code. Note that the transmitted benchmark_tx.py signal in Figure 3.4 can
be observed in the usrp_fft.py spectrum analyzer.
50
Figure 3.6: Screenshot of the Software-Based Spectrum Analyzer by using the
usrp_fft.py code
3.5
GNU Radio and USRP Experiment Result
Using the same water tank in section 3.2 of the hydrophone experiment for
the initial testing of the GNU Radio and the USRP Modem with the hydrophone. The
tank is 65cm long, 125 cm wide. The average depth is 30 cm. The experiment
consists of two USRP Modem, two laptops and tow transducers. The communication
between the laptops and the USRP Modems are provided with serial ports. 3.7 shows
the experiment in the Lab. In figure 3.8 shows the GNU Radio and USRP
Architecture of the initial experiment.
Transmitter
USRP Modem
Receiver
Figure 3.7: The experiment in the Lab.
51
Figure 3.8: The GNU Radio and USRP Architecture
We conducted experiments in the water tank using a configuration shown in
Figure 3.9. Note that all the experiments in this section will have the same setup
configurations. The carrier frequencies which have been selected for this experiment
are 25 kHz, 35 kHz, 45 kHz and 75 kHz referring to [78]. The modulation format
was GMSK. Each communication blocks included 200 training bits and 300 data
bits. The length of preamble was 100 symbols (packets) and each symbol is equal to
1Mb. The bit rate was 20, 30, 40, 50, 60 and 70 kb/s.
52
Figure 3.9: Configuration of the transmitter.
In this experiment we will show the result of measurement testing of the
hydrophone with USRP Modem, considering the distance, bit rate, time, transmitter
gain and receiver gain. All these considerations are per packets received (PR).
In figure 3.10 we measure the packet received within the distance of the
water tank. We measured in four point of the length of the water tank with the all
four low frequency which we had selected to the research and to achieve the high
Packet receives. The bit rate was fixed in this test as well transmitted gain, received
gain, amplitude and depth. The configurations of the experiment setup are as follows
(BR 20 kb/s, Tx gain 0 dB, Rx gain 20 dB, Amp 378999 and depth 25cm).
53
100
No. of Packets Received
90
80
70
60
75kHz
50
45kHz
40
35kHz
30
25kHz
20
10
0
30
60
90
120
Distance (cm)
Figure 3.10: Packet received within the distance
As a result from figure 3.10, we can see that each of the selected frequencies
that the packet receives increased whenever we increase the distance and frequency.
This is because we transmitted with high speed data. However, if you sent the date
with low speed that allow more samples per symbol to be sent out to the USRP and
received at the transducer. Although it might be expected that a controlled
environment would produce better results than a lab scenario this is not always the
same case. 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.
54
100
90
Attenuation (dB)
80
70
60
75kHz
50
45kHz
40
35kHz
30
25kHz
20
10
0
30
60
90
120
Distance (cm)
Figure 3.11: Attenuation within the distance
In figure 3.11 showing the second experiment the attenuation versus distance.
The measurement shows that the attenuation is increase with higher frequency and
the four curves agree quite well at intermediate distances, whereas there are
deviations at both ends of the range.
No. of Packets Received
100
90
80
70
60
50
75kHz
40
45kHz
30
35kHz
20
25kHz
10
0
20
30
40
50
60
Bit Rate kb/s
Figure 3.12: Packets received within the bit rate
70
55
In figure 3.12 showing different bit rates on the same low frequencies. And
the transmitted gain, received gain and depth are remain the same in this figure but
we have selected the longest distance in the water tank which is 120 cm to have more
symbols referring to the last graph. The measurement shows that the bite rate 20 kb/s
is the most suitable in all the low frequencies.
No. of Packet Received
100
90
80
70
60
50
75kHz
40
45kHz
30
35kHz
20
25kHz
10
0
-20
-15
-10
-5
0
Tramsmitter Gain dB
Figure 3.13: Packets received within the Transmitting Gain
From figure 3.13 and 3.14 measured the packets sent and received on
different Gain for both transmitter and receiver USRP modem. These measurements
prove that the suitable gain for the USRP transmitter is 0dB with 20dB gain for the
receiver USRP. Figure 3.13 the graph show the gain limit of the USRP in the
transmitter which start from 20 to 0dB. Figure 3.14 shows the gain limit of the USRP
in receiver which start from 0dB to -20dB.
56
No. of Packets Received
100
90
80
70
60
50
75kHz
40
45kHz
35kHz
30
25kHz
20
10
0
0
5
10
15
20
Receiving Gain dB
Figure 3.14: Packets sent within the Receiving Gain
In these both figures 3.12 and 3.13 study we understand that if we increase
the gain in USRP, then the packet receives will increase as well. Low gain here will
effect on the signal reaching and the packets will be lost.
57
3.7
Summary
The experiment study of GNU Radio and USRP with hydrophones, tell that
whenever we increase the distance the packets received will increase with the
increase of frequency. Again if we increase the gain in USRP, then the packet
receives will increase as well. Low gain here will effect on the signal reaching and
the packets will be lost.
In general of this chapter, the Underwater utilize GNU radio and USRP SDR
has been successfully implemented. The results demonstrate that the objectives of
this research are archived. It is proved that by implementing GNU radio and USRP
SDR in the new generation of underwater acoustic network technology, it will not
only improve the utilization of the underwater network, but it also will improve the
PRR of underwater acoustic network itself.
CHAPTER 4
THE UWA PLATFORM AMPLIFER DESIGN
4.1
Overview
The power amplifier is responsible for amplifying the modulated signal from
the digital hardware platform. It sends the signal to the pre-amplifier circuit (see
figure 4.1) which further amplifies the signal to a power level that matches the actual
distance between the transmitter and receiver. The pre-amplifier amplifies the signal
that is detected by the transducer so that the digital hardware platform can effectively
demodulate the signal and analyze the received data. This chapter describes the
design of the power amplifier, and pre-amplifier of the analog transceiver.
After verifying the correct operation of the system components separately, we
conducted integrated system tests of the complete design with the UWA platform. To
protect the digital electronics, we added a voltage limiter to the output of the preamplifier to clip all signals. The multipath measurements and test results are
described in this chapter.
59
4.2
Power Amplifier Design
When designing the power amplifier we considered the following
requirements:
1. The amplifier should provide a linear, undistorted output over a relatively
wide bandwidth (10 – 100 kHz) to allow for use with a variety of underwater
transducers.
Figure 4.1: USRP Amplifier Design
2. The amplifier must be power efficient (especially for large output power) as a
deployed modem must be powered from batteries
Power amplifiers are classified according to their circuit configurations and
methods of operation into different classes such as A, B, AB, C, D, G and H. These
classes range from entirely linear with low efficiency to entirely non-linear with high
efficiency [91]. An amplifier is said to be linear if it preserves the details of the
signal waveform, that is:
60
Vo (t) = AVi (t)
(4.1)
Where Vo is voltage output , A is constant and Vi is the voltage input.
The amplifier is said to be efficient if it can convert the majority of the dc
power of the supply into the signal power delivered to the load. Efficiency is defined
as:
Efficiency =
signal power delivered to load
DC power supplied to output circuit
(4.2)
To meet our design requirements, we need to design a power amplifier that
was both linear and efficient. Considering the characteristics of the most common
amplifier classes (A, B, and AB), the following material was gathered from [92, 93,
94].
A Class A amplifier consists of a single transistor for its output stage
conducting over the whole of the input cycle such that the output signal is an exact,
scaled-up, replica of the input and thus completely linear. However, because the
amplifier is continuously conducting, Class A amplifiers are not very efficient,
having a theoretical maximum efficiency of 50% (with typical operation at 10-25%
efficiency). A typical Class A input / output characteristic is shown in Figure 4.2.
61
Figure 4.2: Class A Amplifier Input / Output Characteristic
Unlike a Class A amplifier, a Class B amplifier consists of two
complimentary transistors to handle both halves of the waveform. Each amplifying
device conducts for only half the sinusoidal cycle and neither device conducts when
there is no input, thus increasing the efficiency of the amplifier. Class B amplifiers
typically have an efficiency of about 50%, but have some issues with linearity at the
crossover point, due to the time it takes to turn one transistor off and turn the other
transistor on. A typical Class B input / output characteristic for one of its transistors
is shown in Figure 4.3.
62
Figure 4.3: Class B Amplifier Input / Output Characteristic for one transistor
The Class AB amplifier is a compromise between the Class A and Class B
configurations and is currently one of the most common types of power amplifiers in
existence. Like a Class B amplifier, the Class AB amplifier consists of two
complementary transistors, but unlike the Class B amplifier, both devices are allowed
to conduct at the same time, but just a small amount near the crossover point. Thus
each device conducts for more than half a cycle but less than a whole cycle,
overcoming the inherent non-linearity of Class B designs without the inefficiencies
of a Class A design. Efficiencies for Class AB amplifiers are typically 50% with a
theoretical maximum of 72%. A typical Class AB input / output characteristic for
one of its transistors is shown in Figure 4.4.
63
Figure 4.4: Class AB Amplifier Input / Output Characteristic for one transistor
Since we desired a linear amplifier (in the range of 10-100 kHz) with power
efficiency an amplifier architecture that consists of a Class A and a Class B amplifier
that works in parallel is designed to suit our needs (Figure 4.5).
Figure 4.5: Block diagram of the power amplifier design making use of a class A
and class B amplifier to achieve linearity and efficiency
64
This amplifier is a highly linear Class A amplifier that provides a linear
voltage gain of 11 across input voltages and frequencies. The output of the Class A
amplifier is connected to current sense circuitry. The Class B amplifier is inherently
nonlinear, but when working in tandem with the Class A amplifier, it produces a
linear output for input voltages greater than 600 mVpp across frequencies (see Figure
4.8).
Figure 4.7 show the block diagram of the pre-amplifier design which will be
connected between the USRP modem and the transducer like how is shown in figure
4.6.
Figure 4.6: Full system Architecture.
65
Figure 4.7: Block diagram of the USRP amplifier design
66
40
10kHz
30
Gain (dB)
20kHz
30kHz
40kHz
20
50kHz
60kHz
70kHz
10
80kHz
90kHz
100kHz
0
200 mV
400 mV
600 mV
800 mV
Receiver Input Voltage (mV)
Figure 4.8: Complete Amplifier Linearity
4.3
USRP Amplifier Design
When designing the USRP amplifier (see figure 4.6) for the receiver and
transmitter, the following requirements are considered:
•
The USRP amplifier must amplify signals around the transducer's resonance
frequency (75 kHz) and filter out all other frequencies
•
The USRP amplifier must provide high gain to pick up signals as small as a
couple hundred microVolts
•
The design must be easily modifiable to accommodate different transducers
with different resonance frequencies and bandwidths
67
To meet the above design requirements of a highly sensitive, high gain,
narrow band receiver, the architecture consist a 40dB per decade rollover high-pass
filter as shown in Figure 4.9.
High Gain voltage
Amplifier and
Input
Output
High-Pass Filter
Figure 4.9: Receiver Block Diagram
As underwater noise is concentrated in low frequencies (see Figure 2.9) the
first stage (a high pass filter) cancels out a majority of unwanted noise. The high pass
filter consists of two cascaded filters, each with a 20dB per decade rollover. Each
filter has a gain of 10 and a cutoff frequency of 16 kHz thus giving a total gain of
100 (40dB). The second stage is a band-pass filter used to further amplify signals in
the transducer's operating band. It consists of tow cascaded biquad filters, each with a
20dBper decade rollover. The current configuration has the center frequency of the
first and third filters set to 75 kHz and the center frequency of the second filters set to
obtain a flatter frequency response in the pass band. Thus the combined pre-amplifier
provides an ~100dB gain around 75 kHz while attenuating low frequencies at a rate
of 20dB per decade (Figure 4.10).
68
Figure 4.10: Estimated power coupled in the transmitting frequency.
4.3.1
The amplifier for UWA platform
For this project, two amplifiers is required the Transmitter amplifier with
20dB gain because we are refer to [78]. We are using only one amplifier which is
TC913B [A] (see Figure 4.5) and we R1 and R2 is determined by.
𝐺=
R1+R2
R1
Where 10= 10+1/1 =11 the 11 is equal to 20.87 dB
(4.3)
69
The second amplifier is the receiver amplifier with gain of 20 dB. The two
amplifiers used in the receiver are LT1113 [B] (see Figure 4.6). The cascaded twostage amplifier composed of a transconductance amplifier followed by a current
buffer. Compared to a single amplifier stage, this combination may have one or more
of the following advantage is it give higher gain or higher bandwidth. The cascade
improves input-output isolation (or reverse transmission) as there is no direct
coupling from the output to input. This eliminates the Miller effect and thus
contributes to a much higher bandwidth [95]. Here also we determined the R1 and
R2 (see Figure 4.11).
Gain (dB)
Receiver Voltage Gain
200
190
180
170
160
150
140
130
120
110
100
90
80
70
60
50
40
30
20
10
0
10kHz
20kHz
30kHz
40kHz
50kHz
60kHz
70kHz
80kHz
0
20
40
60
80
100
90kHz
100kHz
Receiver Input Voltage (mV)
Figure 4.11: Overall Receiver Gain
The power amplifier is linear in the 10-100 kHz band for inputs greater than
60 mVpp. The pre-amplifier provides a flat, high gain for frequencies 25-75 kHz
matching the operating frequency of the transducer and allowing for reception of a
signal as low as 200 microVolts. All components can be easily modified by replacing
a few standard components.
70
4.4
Integrated Tests
After verifying the correct operation of the entire system component
separately, we conducted integrated system tests of the complete system design in a
tank. To protect the digital electronics, we added a voltage limiter to the output of the
pre-amplifier to clip all signals above 1.3Vpp. To characterize the multipath in the
different environments, we sent a 200ms 25 kHz - 75 kHz chirp signal from the
transmitter to receiver to measure the multipath delay spread. The multipath
measurements and test results are described in the following subsections.
4.4.1
Multipath Measurements
As described in Chapter 2, underwater, there exist multiple paths from the
transmitter to receiver, or multipath. Two fundamental mechanisms of multipath
formation are reaction at the boundaries (bottom, surface, and any objects in the
water), and ray bending (where rays of sound bend towards regions of lower
propagation speed). The amount of multipath seen at the receiver depends on the
locations of the transmitter and receiver and the geometric and physical properties of
the environment.
The extent of the multipath at a receiver can be characterized by the multipath
delay spread. Delay spread can be interpreted as the difference between the time of
arrival of the first significant path (typically the line of site component) and the time
of arrival of the last multipath component. Given the amplitude delay profile, Ac(τ ),
with effective signal length, M, the mean delay,
τ̄
, and the rms delay spread, τrms,
are given as [96]:
(4.4)
71
(4.5)
For frequency shift keying, multipath will cause inter symbol interference
when the multipath delay spread is larger than the symbol duration. inter symbol
interference is a form of distortion of a signal in which one symbol interferes with
subsequent symbols. This is an unwanted phenomenon as the previous symbols have
similar effect as noise, thus making the communication less reliable [97]. Therefore,
because the USRP Modem has symbol duration of 5ms, the delay spread of the
channel must be less than 5ms to ensure reliable communication.
To measure the multipath delay spread of the different test environments, we
sent a 200 ms 25 - 75 kHz chirp signal from the transmitter to receiver and used the
Pico PicoScope device (see Figure 4.12) to collect 5 seconds of the received signal
containing the chirp. We then post-processed the received signal, correlating the
transmitted waveform (the 2ms chirp) with the received waveform to form the
amplitude delay profile. We then used equations 4.5 and 4.4 to compute the rms
delay spread.
Figure 4.12: Pico PicoScope device
72
4.4.2
Tank Tests
We conducted an initial full integrated system test in a 80cm x 120cm x 65cm
water tank filled with fresh water with the transducers spaced 50 cm apart. The
experiment setup is the similar to figure 3.3. Five packets consisting of the reference
code followed by 1000 randomized bits were sent from transmitter to receiver using
all power levels. Calculating the SNR as:
(4.6)
At the input to the transceiver was 20dB for all power levels as the signal was
clipped to 1.3Vpp at each level. All tests revealed 0% bit error rate. The rms delay
spread in the tank was computed to be 1.86 ms from the amplitude delay profile.
4.4.3
Underwater Integrated system for GMSK Based Acoustic
This section discusses the GMSK based acoustic system. Theoretically,
any type of acoustic signal can be generated using the GNU Radio and the USRP
SDR.
However, for prove of concept purposes, in this research we choose to
transmit a GMSK radio signal with the bit rate of 20kb/s. First, the performance of
the platform with the chosen physical parameter is measured in term of the packet
received ratio (PRR) versus the distance of the water tank. This is to determine the
distance that the developed platform can cover and the performance is given in figure
4.13.
73
Input GMSK signal
Output GMSK signal
Figure 4.13: Performance of the underwater integrated system for GMSK Based
Acoustic
4.4.4
General Acoustic Setup and Performance Evaluation
Experimental measurement is carried out to evaluate the performance of the
constructed GMSK acoustic testbed.
The detail of physical parameters used to
construct this GMSK acoustic is given in Table 4.1.
74
Table 4.1: Parameters Used in GMSK Based Acoustic Performance Evaluation
Parameter
Transmitter
Receiver
Frequency
75kHz
75kHz
Modulation
GMSK
GMSK
Bitrate
20kb/s
20kb/s
Amplitude
32767 (The Maximum DAC
Value for USRP)
The performance evaluation is done by measuring the GMSK signal when we
connect the USRP with our pre-amplifiers in the transmitter and the receiver. This
experiment is carried out for the receiver and transmitter in the water tank and using
frequency of 75 kHz and the set-up is shown in Figure 4.14 and 4.15.
The Figure 4.14 shows the receiver measurement of the GMSK. The signal
gain is set to 40dB for the signal through the amplifier. The blue signal is the output
from the Hydrophone, and the red signal is the input from the amplifier to the USRP
modem.
The Figure 4.15 shows the transmitter measurement of the GMSK. The signal
gain is set to 20dB for the signal through the amplifier. The red signal is the output
from the USRP, and the blue signal is the input from the amplifier to the hydrophone.
75
Figure 4.14: Receiver measurement for GMSK
76
Figure 4.15: Transmitter measurement for GMSK
77
No. of Packets Recievd
120
35kHz(amplifier)
100
25kHz(amplifier)
80
60
35kHz(without)
40
20
25kHz(without)
0
30
60
90
120
Distance (cm)
Figure 4.16: Compare the packet received within the distance for the system with
USRP amplifier and without it.
No. of Packets Received
120
35kHz(amplifier)
100
25kHz(amplifier)
80
60
40
35kHz(without)
20
0
25kHz(without)
20
-20
30
40
50
60
70
Bit Rate kb/s
Figure 4.17: Compare the packet received within the Bit Rate for the system with
USRP amplifier and without it.
Figure 4.16 shows the deferent in the packet received within the distance for
the system experiment without and with the USRP amplifier. And as result of these
two experiments we find the USRP amplifier better performance to the underwater
acoustic communication system. In figure 4.17 is also compare between the two
experiments of packet received within the Bit Rate for the system communication of
USRP amplifier and without it. Figure 4.17 shows USRP amplifier has better bit rate.
78
4.4.3
The UWA Platform Test
The integrated system test results in terms of distance, multipath delay
spread, bit error rate, and SNR as expected, the system design performed well
(having a bit error rate of <5%) in environments with a multipath spread less than 5
ms. The results also suggest that higher SNR will only improve performance for
environments with low multipath.
4.5
Summary
This chapter described the full design of the power amplifier with USRP
amplifier. The power amplifier is linear in the 10-100 kHz band for inputs greater
than 500 mVpp. The USRP amplifier provides a flat, high gain for frequencies 25-75
kHz matching the operating frequency of the transducer and allowing for reception
of a signal as low as 200 microVolts. All components can be easily modified by
replacing a few standard components.
Finally we described the integrated system tests used to evaluate the
functionality and performance of the complete composites design. These tests prove
that a short-range underwater acoustic USRP Platform can be designed from lowfrequency and high gain to long Distance. The tests indicate that the USRP Modem
can support data rates of 200 bps for long distance.
The next chapter describes some future improvements that could be made to
this research to make it more versatile for a wider variety of sensor network
applications and underwater environments.
CHAPTER 5
CONCLUSION
5.1
Overview
In the late twentieth century, Underwater acoustic communication system is
growing rapidly hence increasing the demand for the acoustic wave. Nowadays,
most of the acoustic wave is fully allocated and it is difficult to accommodate a new
service. Therefore, new regulation, policies and standard have to be defined in order
to support this rapid growth underwater acoustic network technology.
This thesis is pointing out a novel solution to better option to prototype their
algorithms and further develops the field of underwater acoustic communication. In
particular, the ability to add functionality to a proprietary system is difficult. For
example, as researchers we wish to investigate designs with different parameters
such as carrier frequency, bit rate, and packet size. Commercial modems do not
provide the flexibility to parameterize and experiment available in an open,
reconfigurable platform
GNU radio and USRP SDR has been successfully implemented. The results
demonstrate that the objectives of this research are archived. It is proved that by
implementing GNU radio and USRP SDR in the new generation of underwater
acoustic network technology, it will not only improve the utilization of the
underwater communication.
80
This thesis also describes the full system of underwater acoustic modem with
underwater wireless sensor networks connected to the most critical component of the
system the USRP amplifier design circuit.
The USRP amplifier provides a flat, high gain for frequencies 25-75 kHz
matching the operating frequency of the transducer and allowing for reception of a
signal as low as 200 microvolts. All components can be easily modified by replacing
a few standard components.
Finally we described the integrated system tests used to evaluate the
functionality and performance of the complete composites design. These tests prove
that a short-range underwater acoustic USRP Platform can be designed from lowfrequency and high gain to long Distance. The tests indicate that the USRP Modem
can support data rates of 200 bps for long distance.
5.2
Future Work
This thesis described the design and initial testing of a functional lowfrequency underwater acoustic network for modem prototype for underwater sensor
networks. The USRP Modem can support data rates of 200 bps for long ranges up to
~400 meters in environments with less that 5 ms multipath delay spread. This chapter
discusses possible future improvements to this research to make it more versatile for
a wider variety of sensor network applications and underwater environments.
Power Reduction: Reducing power consumption (particularly idle power
consumption) to ensure a longer deployment lifetime on a limited battery supply.
Further improvements could be made to the transceivers to make it more power
efficient and a low power wake up circuit could be added to greatly reduce
listening/idle power.
81
Wider Bandwidth, Higher Bit Rate: The bandwidth of the receiver could
be widened to amplify signals over the 5 kHz range allowing for a modulation
scheme that uses more bandwidth allowing higher data rates. However, the widening
of the receiver bandwidth comes at a cost of reducing gain, thus reducing
transmission range.
Channel Adaptive Modem: As shown in the system test results, the current
design can only perform well in environments with a low multipath delay spread. An
adaptive algorithm could be programmed into the modem to measure the channel
characteristics and apply channel equalization and/or lengthen the symbol period for
channels with high multipath.
82
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APPENDICES
APPENDIX A
TC913B Specifications
93
94
95
96
97
APPENDIX B
LT1113 Specifications
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113