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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2. Osborne, T.L, ”Application of Rain Attenuation Data to 11 Ghz Radio Path

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1977.

3. ITU-R Recommendations, “Propagation in Non-Ionized Media”, Volume PN

Series, 1994

.

4. Ajayi G., O, Feng, S., Radicella, S. M and Reddy, B. M. “Handbook on

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75

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77

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