RAINDROP SIZE DISTRIBUTION EXTRACTED
FROM RAIN ATTENUATION DATA
HATEM ALMOKHTAR OMAR ABDULKABEER
Universiti Teknologi Malaysia
RAINDROP SIZE DISTRIBUTION EXTRACETD
FROM RAIN ATTENUATION DATA
HATEM ALMOKHTAR OMAR ABDULKABEER
A project report submitted in partial fulfilment of the
requirements for the award of the degree of
Master of Engineering (Electrical - Electronics and Telecommunication)
Faculty of Electrical Engineering
Universiti Teknologi Malaysia
NOVEMBER 2008
iii
To
My beloved parents and brother and sisters for their unwavering
love, sacrifice and inspiration.
iv
ACKNOWLEDGEMENTS
First and foremost, I would like to express my utmost gratitude to my
supervisor, Associate Professor Dr. Jafri bin Din for being a dedicated mentor as well
as for his valuable and constructive suggestions that enabled this project to run
smoothly.
Also, not forgetting my friends and classmates, I convey my full appreciation
for their on-going support and contributions toward this project, whether directly or
indirectly.
Last but not least, I am forever indebted to all my family members for their
constant support throughout the entire duration of this project. Their words of
encouragement never failed to keep me going even through the hardest of times and
it is here that I express my sincerest gratitude to them.
v
ABSTRACT
Attenuation due to rain plays a significant role in the terrestrial and earth-
space communication systems especially at frequencies above 10 GHz. Although
models proposed by ITU-R for the prediction of rain attenuation give a good
estimation about the signal attenuation caused by rain for temperate regions, recent
studies shows that it underestimates the same tropical regions. Tropical regions are
characterized by high intensity rainfall, enhanced frequency of rain occurrence and
the increased presence of large raindrop when compared with temperate climates.
This project will look into the prediction of raindrop size distribution by analyzed
and convert the summarized collected data on rain attenuation. The summarized data
are taken from point to point microwave link measurements at Skudai, Johor Bahru.
The data needs to be taken into consideration on many aspects that will involve in
wave propagation phenomena. An accurate estimation of the drop size distribution
(DSD) is essential in the interval of rainfall parameters from microwave
measurement and in the modeling of microwave propagation through rain.
Information of raindrop size distribution (DSD) is important for prediction of the
microwave signal attenuation due to rain. Drop size distribution is one of major
parameters that effects on microwave signal degradation due to rain, detail
information on (DSD) leads better accuracy on the attenuation prediction due to rain
thus, (DSD) profile is extracted from locally collected rain attenuation data base at
operating frequencies 26 and 38 GHz. The Singapore model of drop size distribution
gives better prediction for rain specific attenuation due to the rain attenuation data
was collected in Johor.
vi
ABSTRAK
Kehilangan data yang dihantar menggunakan teknologi komunikasi wayarles
akibat hujan mempunyai impak yang besar dalam system komunikasi yang
menggunakan spektrum frekuensi di atas 10GHz. Walaupun ITU-R telah
menyediakan cadangan model bagi menganggarkan kesan kehilangan akibat hujan
bagi negara-negara yang mempunyai empat musim, namun model ini tidak sesuai
untuk aplikasi negara-negara tropika. Negara di kawasan tropika mengalami
kekerapan hujan yang lebih tinggi, hujan yang lebih lebat dan titisan air hujan yang
lebih besar berbanding di negara-negara empat musim. Projek ini menyelidik
anggaran distribusi saiz titian air hujan daripada data yang telah dikumpul dan
dianalisis berkenaan kesan hujan pada kehilangan maklumat yang dihantar secara
wayarles. Data-data telah diambil daripada satu laluan komunikasi gelombang mikro
titik-ke-titik yang ukurannya dilakukan di Skudai, Johor Bahru. Maklumat-maklumat
berkenaan fenomena pemancaran gelombang telah diambil kira dalam projek ini.
Anggaran tepat berkenaan distribusi saiz titisan air hujan, atau DSD sangat penting
dalam menyediakan satu model berkenaan karakteristik pemancaran gelombang
dalam keadaan hujan. Oleh sebab DSD merupakan parameter yang mempengaruhi
pemancaran gelombang mikro, justeru anggaran yang dibuat melalui pengukuran
pengumpulan, dan penganalisaan data-data ini mampu membantu dalam membuat
penggangaran tepat kesan kehilangan data akibat hujan. Profil DSD telah diambil
dari satu sesi pengukuran yang dilakukan secara local pada frekuensi beroperasi 26
dan 38 GHz. Model DSD Singapura memberi anggaran kesan kehilangan data
melalui komunikasi wayarles akibat hujan yang lebih baik.
vii
TABLE OF CONTENTS
CHAPTER
TITLE
PAGE
DECLARATIONS ii
DEDICATIONS iii
ACKNOWLEDGEMENTS iv
ABSTRACT v
ABSTRAK vi
TABLE OF CONTENTS vii
LIST OF TABLES x
LIST OF FIGURES xi
LIST OF SYMBOLS xii
LIST OF ABBREVIATIONS xv
LIST OF APPENDICES xvi
1 INTRODUCTION 1
1.1 Introduction 1
1.2 Problem Statement 3
1.3 Objective 4
1.4 Scope of work 4
1.5 Thesis Outline 5
2
RAINFALL CHARACTERISTIC AND RAIN
ATTENUATION 6
2.1 Introduction 6
2.2 Malaysian Climate 6
2.2.1 Rainfall Regions in Peninsular Malaysia 7
2.2.2 Rainfall Region on Sabah and Sarawak 9
viii
2.2.3 Rainfall Characteristic 10
2.3 Types of Rain 10
2.4 Development of Rain Attenuation Studies 11
2.5 Attenuation in Atmosphere 13
2.6 Rain Attenuation 15
2.6.1 Specific Attenuation 16
2.6.1.1 ITU-R Model for Specific
Attenuation
19
2.7 Effective Path Length 21
2.8 Terminal Velocity 22
2.9 Refractive Index 22
2.9 Temperature 23
3 DROP SIZE DISTRIBUTION MEASUREMENT 24
3.1 Introduction 24
3.2 A Brief Review of Tipping Bucket Rain Gauge, And
Joss- Waldvogel, Parsivel And Poss Disdrometers
26
3.2.1 Joss-Waldvogel disdrometer (JW) 26
3.2.2 Parsivel disdrometer 29
3.2.3 Precipitation Occurrence Sensor System
(POSS)
32
3.2.4 Tipping bucket gauges 34
3.3 Drop Size Distribution Models 35
3.4 Literature Review on DSD 37
4 DROP SIZE DISTRIBUTION EQUATIONS 44
4.1 Introduction 44
4.2 Rain Attenuation 44
4.3 Rain Specific Attenuation 45
4.4 Drop Size Distribution 45
4.5 Summery
47
ix
5 THE METHODOLOGY AND RESULTS 48
5.1 Introduction 48
5.2 The overall Project Methodology 49
5.3 ITU-R Study group Three Data Bank 50
5.4 Rainfall rate data 51
5.4.1 Rainfall Rate Data at UTM-Skudai site 52
5.4.2 Data Collection 58
5.4.3 Designing the Algorithm Flow 59
5.4.4 The Program Development for extracting Drop
Size Distribution
60
5.4.4.1 Loading the data into MATLAB 60
5.7.3.1.1 Sub-Program in MATLAB 61
5.4.4.2 Conversion of Voltage to dBm 62
5.4.4.2.1 Sub-Program in MATLAB 62
5.4.4.2.1 Analyzing the Database 63
5.4.4.3 Extracting Drop Size Distribution 64
5.5 Summary 70
6
CONCLUSION AND RECOMMENDATIONS
71
6.1 Introduction 71
6.2 Conclusion
6.3 Future of work and recommendations 72
REFERENCES
74
Appendices A – F 76
x
LIST OF TABLES
TABLE NO TITLE
PAGE
3.1 Log Normal coefficients for Malaysia, Singapore, Nigeria and
Brazil
39
3.2 Annual rain distributions 41
5.1 Rainfall measurement specifications at UTM-Skudai 53
5.2 The average one year cumulative distribution at UTM-Skudai 55
5.3 Proposed and measured values for the parameters a and b for 26
and 38 GHz with horizontal Polarization
56
5.4 The Investigated 38 GHz Links Connected to Measuring Nodes
Located in WCC, UTM, Malaysia
59
5.5 Comparison between extracted DSD and other models at 38 GHz 66
5.6 Correlations between extracted DSD and other models at 38 GHz 67
5.7 Comparison between extracted DSD and other models at 26 GHz 68
5.8 Correlations between extracted DSD and other models at 26 GHz 69
xi
LIST OF FIGURES
FIGURE NO TITLE
PAGE
2.1 Rainfall regions in Malaysia peninsular 8
2.2 Volume of spherical, uniformly distributed raindrops 12
2.3 Attenuation due to water vapor and oxygen at sea level 15
2.4 Calculated drop shapes of 13 water drops for equivolumetric
sphere radius of 0.25 – 3.25 mm
17
2.5 Specific attenuation at different rain rates and frequencies 19
3.1 Joss-Waldvogel Disdrometer 27
3.2 The size of the impacting drop is retrieved from measured
impact velocity through an empirical relationship between fall
velocity-D (straight line).
28
3.3 Parsivel disdrometer 30
3.4 Two operating Parsivel measured mean velocities 31
3.5 POSS disdrometer 33
3.6 Sample volume for the different disdrometers 34
3.7 Tipping bucket rain gauge 35
3.8 Comparison models of the raindrop size distribution for
Malaysia, Singapore, Nigeria and Brazil
38
3.9 Specific rain attenuation at Guwahati, Calcutta, Dehradun,
compared with the ITU-R model at different rain rates
40
3.10 Specific rain attenuations exceeded for different percentages
of time over a year at Guwahati, Calcutta, and Dehradun.
41
xii
5.1 Screen shoot of the user graphical interface window of
DBSG5.exe program
51
5.2 The rainfall and rain attention data conducted at UTM
companied using computer program
53
5.3 Several years measurement of rainfall rate at UTM-Skudai 54
5.4 Measured Rain Attenuation for 38 and 26 GHz from July 1
1998 to June 30 1999 at UTM Skudai
55
5.5 Measured and predicted specific rain attenuation for 26 GHz 57
5.6 Measured and predicted specific rain attenuation for 38 GHz 57
5.7 Ericsson Microwave Link at 38 GHz in Wireless
Communication Centre (WCC)
59
5.8 Example of Data Collected 61
5.9 Graph of dBm versus Voltage, V 62
5.10 Graph of Attenuation Threshold (dB) versus Time (hour) 63
5.11 The extracted DSD at 38GHz and other DSD models 65
5.12 Comparison between the extracted DSD and other models 66
5.13 The extracted DSD at 26GHz and other DSD models 68
5.14 Comparison between the extracted DSD and other models 69
xiii
LIST OF SYMBOLS
A - Attenuation
a, b - Regression coefficients
A0.01 - Attenuation at 0.01% of time
a1 - excess attenuation (dB) due to water vapor
a2 - excess attenuation (dB) due to mist and fog
a3 - excess attenuation (dB) due to oxygen
a4 - sum of absorption losses (dB) due to other gases
a5 - excess attenuation due to rainfall
Am - Measured attenuation
Ap - Predicted attenuation
AreaP - Drop cross section area (m2)
CP - Number of drops in the pth size
dB - Decibel unite
d - Separation (km) between the two sites
DP - Mid size of the pth channel (mm)
f - Frequency
GHz - Giga Hertz
k, α - Regression coefficients
km - Kilometer
Leff - Effective Path length
L - path length
m - Meter
mm - Millimeter
- Number density of raindrops with equivalent diameter D in the
interval
xiv
Nt - Total number of drops per cubic meter per millimeter
P - Probability
P (%) - Percentage in time of the year
- Total extinction cross section
R(p) - Rain rate at percentage in time of the year
r(P) - Horizontal reduction factor of percentage in time of the year
r0.01 - Horizontal reduction factor for 0.01% time of the year
R0.01 - Rain rate at 0.01% of time of the year
- The terminal velocity of drops in m/s
ΔDp - Width of the pth channel (mm)
Δt - Sampling time (s).
θ - The path elevation angle
γR - Specific attenuation
xv
LIST OF ABBREVIATIONS
AGCV - Automatic Gain Control Voltage
C - Centigrade
dB - Decibel
dBm - Decibel mili watt
DBSG5 - ITU-R Study Group Three Data Bank
GHz - Giga Hertz
IEEE - Institute of Electrical and Electronic Engineering
ITU-R - International Telecommunication Union Radio-Broadcasting
mm/h - milimetre per hour
MMS - Malaysia Metrological Services
NASA - National American Space Agency
S - second
TRMM - Tropical Rainfall Measuring Mission
UTM - Universiti Teknologi Malaysia
xvi
LIST OF APPENDICES
APPENDIX TITLE PAGE
A
The ITU-R specific attenuation parameters (ITU-R, P. 838-3)
78
B
The experimental specific attenuation parameters UTM-Skudai. Feb 2001-Feb2002
79
C
Comparison of event rain totals using different distrometers
80
D
Composite DSD for Distrometers 81
E
Scattering Coefficient result for frequency at 26
GHz, temperature at 27° C, Ray refractive index,
Oblate Spheroid Raindrop Shapes, Horizontal
Polarization
82
F
Scattering Coefficient result for frequency at 38
GHz, temperature at 27° C, Ray refractive index,
Oblate Spheroid Raindrop Shapes, Horizontal
Polarization
88
CHAPTER 1
INTRODUCTION
1.1 Introduction
The rapid development in communication systems has brought saturation to
the most desirable frequency band (1 to 10GHz). This fact has led to the utilization
of higher frequencies extending the radio frequency spectrum into the millimeter
wavelength region. Rain is dominating source of attenuation at higher frequencies.
Its intensity and distribution greatly effects transmission quality and limits system’s
availability. Rain effects on microwave systems are more critical in tropical and
equatorial zones, where rain fall is higher than in temperate zones. In temperate
regions the rain effects becomes significant above 10GHz, while in tropical climates
in general and in equatorial climate particularly, since the rain drops are larger than
in temperate climates. When designing microwave links operating above such
frequencies, the major problem in link design is to determine the excess attenuation
due to rainfall.
Malaysia has tropical climate and due to its maritime exposure it experiences
heavy rainfall with mean annual ranging between 2400mm to 3200mm and about
150-200 rainy days per year [1]. It is subject to Northeast monsoon from October to
2
March and Southwest monsoon from April to September. Rain is classified into two
categories; convective and stratiform or widespread. Convective rain is cellular in
nature and covers a limited area. Small cells of heavy rainfall are often embedded in
larger regions. Light and medium intensity rainfall on the other hand is usually
widespread in character, is for a longer duration and covers a wider area. Malaysian
climate is mostly subject to convective rains and heavy downpour rain is the most
seriously damaging to radio transmission at higher frequencies.
In the design of radio communication system, one of the major concerns is to
assess system unavailability also called outage time. Unavailability or outage time is
the amount of time during which system’s performance will be below some threshold
value or that it will not be useable. For designing a reliable system, the amount of
outage time has to be kept below some objective. In microwave systems, outages can
occur either due to equipment failure or it can be propagation outage. In modern
systems, the equipment outage time can be made negligibly small by using standby
equipment and automatic protection switching systems. However, at high
frequencies, attenuation by rain can cause an outage, which can be easily protected.
Therefore the practical way of achieving a reliable radio system at those frequencies
where there is substantial rain attenuation is to design the system in such a way that
the expected amount of rain outage is below some objective [2]. For a reliable
communication system, unavailability time during a year has to be kept at 0.01
percent [3]. This corresponds to availability time of 99.99 percent during a year.
Most of the studies on rain attenuation have been carried out in the temperate
regions of the world. The rain characteristics in the tropics differ appreciably from
those of the temperate regions in the empirical relationships obtained in the later may
not be very suitable for system designing in the former [4]. There is generally a
limited amount of data available on direct attenuation and rain rate measurements in
tropical regions. Consequently, many of recommendations of the ITU-R are based on
data mainly obtained from temperate regions of the world. The increasing use of high
3
microwave frequencies for telecommunication and broadcasting services thus
necessitate the need for rain attenuation studies in the tropics.
Research in radio wave propagation In Malaysia commenced in 1990s at
University Sains Malaysia (USM), [1], [5] and at University Teknoliogi Malaysia
(UTM),[[6], [7]]. One minute rain rate using rain gauges and drop size distribution
using distrometer were measured and new values of regression of coefficients a,b
were proposed by Din [6].
1.2 Problem Statement
The problem statement of this project is stated in the following points:
i- The incapability of the published prediction models to be sensitive of the
available knowledge of rainfall on Malaysia climate.
ii- The lack of drop size distribution studies in Malaysia, especially for
higher frequency band.
iii- There is no previous calculation for prediction of rain drop size
distribution in Malaysia.
4
1.3 Objective
The objective of this project is to determine rainfall drop size distribution
(DSD) extracted from the UTM rain attenuation data base measured at operating
frequencies 26 GHz and 38 GHz.
1.4 Scope of Work
The primary objective of this thesis is to determine rainfall drop size
distribution (DSD) extracted from the UTM rain attenuation data base measured at
operating frequencies 26 and 38 GHz.
Practical approach is adopted to study the rain attenuation through direct
measurement. The rain attenuation data were measured continuously from the
available point-to-point terrestrial microwave link operating at 26 and 38GHz in
UTM-Skudai. Two years rain attenuation data were collected. Tipping bucket type of
rain gauge with 0.5mm sensitivity was used to collect the one-minute integration
time of rain rate. In the analysis, the measured rain attenuation data was analyzed to
evaluate the ITU-R and other prediction models.
Rain does not distribute uniformly along the terrestrial microwave link. The
horizontal reduction factors are taking into account in order to calculate the total rain
attenuation throughout the link. These reduction factors are accounted for in-
homogeneity of rain rate horizontally. Consequently, the horizontal reduction factor
would make the effective path length of the microwave link shorter than the actual
path length. As a result, the information of one-minute integration time of rain rate,
5
regression coefficient factors and reduction factors would provide adequate
information for the researchers in order to predict the rain attenuation effect on the
microwave links accurately.
1.5 Thesis Outline
This thesis organized into five chapters to completely cover the whole
research activities. The following paragraphs describe each of the following chapters.
Chapter 2 provides brief summary of literature review on the rain attenuation
and drop size distribution. It includes the study on specific attenuation and effective
path length.
Chapter 3 gives an overview of the raindrop size distribution measurements.
Chapter 4 presents drop size distribution equations.
Chapter 5 and resents the results, analysis and discussion for the simulation
program, the calculation of the attenuation and drop size distribution.
Chapter 6 Concludes the thesis. The conclusion is given based on the
analysis of results from the previous chapter. Recommendations for future works are
also presented.
74
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75
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77
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