ABSTRACT

Heavy rain events are often experienced in tropical countries. The operation of high speed satellite transmission in the Ka-band (20/30GHz) will therefore be susceptible to rain attenuation in a tropical country such as Nigeria. This study investigates the effect of rain attenuation in the Space-to-Earth direction for a Nigerian Communication Satellite (NigComSat-1R) located at 42.5 degrees east longitude. A model based on the International Telecommunication Radiowave (ITU-R) rain model is used to estimate and predict the rain attenuation in the satellite’s Ku- and Ka-bands for 20 Nigerian locations namely; Kaduna, Sokoto, Maiduguri, Yola, Gombe, Abuja, Jos, Minna,  Kano, Makurdi, Ikeja, Akure, Enugu, Calabar, Warri, PortHarcourt, Benin, Owerri, Uyo, and Ilorin. These locations were selected based on different rainfall rates and the good representation of the differing physical and climatic details they provide over Nigeria. Daily rainfall data spanning from 2009 to 2013 from over 20 climatic stations situated in the 20 locations of study were collected from the archive unit of the Nigerian Meteorological Agency (NIMET) and analyzed. The data was filtered and processed and rainfall statistics on monthly and annual basis were formed for the 20 locations. This was used along with local rain rate values for rain rate distribution over Nigeria as input into the ITU-R rain model, to calculate the rain attenuation distribution at 0.01 to 1.0 percentages of time unavailability in an average year for a satellite link over Nigeria, while carrying out link performance estimates for the satellite simultaneously. The figures from the calculated values were then plotted, using Microsoft Excel. The results indicate that there is high potential for the Ka-band use in providing video transmission over Nigeria in spite of the high rain intensities with a link availability of 99.8% provided that adequate fade margins are applied to links in places with the highest rainfall rates and highest rain fade calculated values like Calabar and Uyo, if the downlink signal is planned in the horizontal polarized frequency.

TABLE OF CONTENTS

Title Page                                                                                                                                 i

Certification                                                                                                                             ii

Declaration                                                                                                                              iii

Dedication                                                                                                                               iv

Acknowledgements                                                                                                                  v

Contents                                                                                                                                   vi

List of Tables                                                                                                                             x

List of Figures                                                                                                                         xi

List of Symbols                                                                                                                        xiv

List of Abbreviations                                                                                                               xv

Chapter One Introduction

 

Background to the Study Chapter One	Introduction	1
Justification of Study 1.1	to the Study	3
Objectives of the Study 1.2	Justification of the Stu	5
Methodology 1.3 Scope of the Study 1.4	Objectives of the Study	6       6 6
1.7     Arrangement of the Thesis 1.6 Significance of the Study 1.5	vi Significance of the	8    7
Chapter Two Literature Review	CHAP	10

Abstract                                                                                                                              xvii

2.1     Related Historical Works                                                            10

2.2     Effects of Rainfall on Satellite Links                   12

2.3     Characteristics of Rainfall in Tropical Region         14

2.3.1 Classification of Rain Events                                                                     14

2.3.2Rain Drop Sizes and Shapes                                                                     17

2.4 Rain Attenuation Main Parameters Study                                                               17

2.4.1 Specific Attenuation Due to Rain                                                                   17

2.4.2 Effective path Length or Depth of Rain (D_Rain)                                              19

2.4.3 Effective Rain Height (H_Rain)                                                    20

2.4.4 Elevation Angle                         21

2.4.5 Rainfall Rate                                                                                                     22

2.5     Review of Existing Rain Rate Models                                                                       22

2.5.1 Rice-Holmberg Model                                                                23

2.5.2 Moufouma-Martin Model                                                                                 23

2.5.3 Chebil-Rahman Model                                                                                      24

2.5.4 Local Rain Rate Contour Maps Approach by Ojo et al                                    24

2.6  Reviews on Rain Attenuation Prediction Models                                                      25

2.6.1Crane Global Model                                                                                          26

2.6.2 The ITU-R Model                                                                            26

2.7  Features of Ka-band

2.7.1 The Ka-band Satellite System and Current Status                                            28                                                                                27

2.8Ka-band Satellite Link Multiple Access Techniques                                         31

2.8.1 Frequency Division Multiple Access (FDMA)                                                  32

2.8.2 Time Division Multiple Access (TDMA)                                                           32

2.9        Digital Modulation Techniques for Satellite Links                                              34

2.9.1 Phase Shift Keying (PSK)                                                                                   34

2.9.2 Binary Phase Shift Keying (BPSK)                                                                     35

2.9.3 Quadrature Amplitude Modulation (QAM)                                                         35

2.10    Forward Error Correcting Scheme (FEC)                                                                      36

• Link Budget 36

2.11.1    Hardware Specifications and Frequency Parameters              37

2.11.2 Effective Isotropic Radiated Power (EIRP)             37

2.11.3 Free Space Loss                                                                                              38

• Height Above Mean Sea Level                                                                       38

2.11.5    Outage Percentages            38

2.116      Slant Range  39

2.11.7 System Interference and Channel Guard Bands     39

2.11.8  System Noise Temperature                                                                              40

2.11.9     Pulse Design and Base Band Channel                                                             40

2.11.10 Energy per Bit to the Spectral Noise Density Ratio (E_b/N_o)                            41

 

  Chapter Three Methodology                                                                                                  4

3.1   Rain Attenuation Modeling                                                                                            42

3.1.2   Principal Sources of Rainfall

3.1.1 Study Area                                             42

3.1.3     Nigerian Climate and Rainfall Distribution                                                          43

3.2    Rain Attenuation Prediction on Satellite                                                                            47

3.2.1    Cumulative Distribution of Rain Rate (mm/hr)                                                     47

3.3 Calculation of Long Term Rain Attenuation Statistics from Point

Rainfall Rate                                                                                                                       50

3.3.1   Prediction of Attenuation Statistics from an Average Year                                    53

3.4     Link Budget Calculations                                                                                                   56

 

        Chapter Four Results and Discussion                                                                                        58                                                                                 

4.1     Results                                                                                                                               58

4.2     Results Discussion                                                                                                            65

4.2.1 Research Findings                                                                                                    67

4.2.2 Contributions to Knowledge                                                                                    68

Chapter Five Conclusion and Recommendations                                                 

Appendix I: Antennas elevation and azimuth angle calculations                                 5.1   Conclusion

References Appendices            5.2   Recommendations

Appendix II: Downlink Rain Attenuation Calculations

Appendix III: Link Budget Calculations

LIST OF TABLES

Appendix IV: NigComSat-1R Characteristics

 

Table 2.1	Regression Coefficient for Estimating Specific Attenuation (gR)		18
Table 3.1	List of the 20 locations used with their respective abbreviations		49
Table 3.2a	Rainfall rate exceeded in mm/hr corresponding to different                   ITU-R climatic zones		53
Table 3.2b	Rain rate measured at 19 Nigerian locations which belongs to P and N ITU-R region during year 2008		53
Table 3.3	NigComSat-1R Ku/Ka Band parameters		56
Table 3.4	Summary of final link budget parameters		57
Table 4.1	Calculated Attenuation due to Rain Exceeded at 0.01% for Ku and Ka Bands NigComSat-1R downlink frequencies.		59
Table 4.2		Calculated Attenuation due to Rain Exceeded at 0.1% for Ku and Ka Bands NigComSat-1R downlink frequencies.		60
Table 4.3		Calculated Attenuation due to Rain Exceeded at 1% for Ku and Ka Bands NigComSat-1R downlink frequencies.		61

Table I.1             Antennas Elevation and Azimuth Angle calculations                                  80

Table II.1             NigComSat-IR Ka-Band Downlink Rain Attenuation Results                     for the 20 stations used in the study

Using the NigComSat-IR at Ku and Ka-bands        Table III.1           Typical link budget calculated values at different frequencies

LIST OF FIGURES

Figure 1.1           Map of Nigeria Showing the 20 locations used in the study                              9

Figure 2.2           Rain height and different rain layers

Figure 2.1  Hydrometeor effects over satellite path

Figure 2.3          Slant rang calculation (Stengel, 2012)                                                                 39

 

Figure 3.1a        Five years average rainfall for Maiduguri (2009 to 2013)                                   45

 

Figure 3.1b        Five years average rainfall for Jos (2009 to 2013)                                               45

 

Figure 3.1c        Five years average rainfall for Ikeja (2009 to 2013)                                            46

 

Figure 3.1d       Five years average rainfall for Calabar (2009 to 2013)                                        46

 

Figure 3.2         Schematic presentation of an Earth-space path giving the Parameters to             50

be input into the ITU-R Prediction Process

Figure4.1.a Predicted rain Attenuation in Nigeria for 0.01% unavailability of an average year for horizontal and vertical polarization for links                                                      to NigComSat-1R at Ku-band (12.6GHz).

Figure 4.1.b Predicted rain Attenuation in Nigeria for 0.01% unavailability

of an average year for horizontal and vertical polarization for links       to NigComSat-1Rat Ka-band(19.6GHz).

Figure 4.2.aPredicted rain Attenuation in Nigeria for 0.1% unavailability

of an average year for horizontal and vertical polarization                                                                    for links to NigComSat-1R at Ku-band (12.6GHz).

Figure 4.2.b      Predicted rain Attenuation in Nigeria for 0.1% unavailability

of an average year for horizontal and vertical polarization              for links to NigComSat-1R at Ka-band (19.6GHz).                                             Figure 4.3.aPredicted Rain Attenuation in Nigeria for 1% unavailability

of an average year for horizontal and vertical polarization              for links to NigComSat-1R at Ku-band (12.6GHz).                                                  64

 

Figure4.3.bPredicted Rain Attenuation in Nigeria for 1% unavailability

of an average year for horizontal and vertical polarization              for links to NigComSat-1R at Ka-band (19.6GHz).

at 20 locations in Nigeria              

LIST OF SYMBOLS

 

A                       attenuation of radiowave propagating through free-space(dB)

A_0.01                                        attenuation at 0.01% of time

A_ppredicted attenuation

D                                    diameter of rain cell (km)

dB                                      decibels

f                                        frequency (GHz)

k                                       regression coefficient for specific attenuation

N (D)                               represents the particle size distribution in mm-¹ m-³

L (G)                  ground length

L_E                        effective length in Km

L_S                       slant path length in Km

L_Gground Length

h_R                      rain height in Km

h_s                       height above mean sea level in Km

r_0.01                                     path reduction factor

l  the wavelength

h              antenna efficiency

N  number of interval

p      time percentage, percentage M            mean annual accumulation

R      rainfall rate (mm/hr)

R_0.01rainfall rate (mm/hr) for 0.01% of the time

α            regression exponential for specific attenuation

b                                  exponential attenuation

gspecific attenuation

g_Rspecific attenuation due to rain (dB/km)

θ                          elevation angle

F_z                                             azimuth angle

polarization tilt angle
                           

LIST OF ABBREVIATIONS

BER                         Bit Error Rate

dBi                          Decibels in relative to Isotropic Reference Antenna

DTH                       Direct-to-Home

E_b/N_o                Energy per bit to noise spectral density ratio

EIRP                    Effective Isotropic Radiated Power

FEC           Forward Error Correction

FMT                Fade Mitigation Technique

FSS                        Fixed Satellite Service

FTA                  Free-to-Air

G/T                  Antenna Gain to Noise Temperature ratio (Figure of Merit)

GHz                Gigahertz

HDTV        High-Definition Television

HTS            High Throughput satellite

HPA            High power amplifiers

ICT               Information and Communication Technology

IP                  Internet Protocol

ITU               International Telecommunication Union

ITU-R              International Telecommunication Union Radio-Communication Sector

Ku       Frequency band between 12 and 18GHz

Ka                    Frequency band between 20 and 30GHz

NASA                   National Aeronautics and Space Administration Mbps                     Megabits per second

NASRDA              National Space Research and Development Agency

NIGCOMSAT-1First Nigerian Communications satellite

NIGCOMSAT-1R          Nigerian Communications satellite-Replacement

NIGCOMSAT Ltd         Nigerian Communications satellite Limited

NIMET                           Nigerian Meteorological Agency

PSK                                Phase Shift Keying

QOS                                Quality of Service

QPSK                              Quadrature Phase Shift Keying

RF                                   Radio Frequency

SATCOM                       Satellite Communication

TRMM                           Tropical Rain Measuring Mission

VSAT                   Very Small Aperture Terminal

 

CHAPTER ONE

INTRODUCTION

1.1   Background to the study

Today, there are a variety of roles played by satellites, among them are for forecasting of weather, Global Positioning Systems, in data gathering, earth observation, and, the most important ones being for communication purposes, navigation systems, and surveillance systems, and so on. Communication via satellite is applied in three main areas: fixed satellite, mobile satellite and broadcast satellite services. Current advancements in satellite technology have led to the emergence of new applications for satellite that include IP-based communications which support digital video services (Giambene, 2007).

In the past, satellite communications took place in frequency bands like L (1/2 GHz), S (2/4 GHz) and C (4/6 GHz). As mentioned above, more and more advanced satellite applications have led to the congestion of the lower frequency bands, and utilization of higher frequency bands has become a necessity so as to support advanced services like video streaming, data communications and voice services, which form the bulk of today’s communication needs. The current efforts are targeted towards the exploitation of the Ku band (12/14 GHz), the Ka band (20/30 GHz) and the V band (40/50 GHz) for better satellite service delivery. Thus, a full knowledge of the merits offered by these higher bands is necessary for service providers to fully tap into them. The higher bands offer the following benefits; larger bandwidth, frequency reuse, and better spectrum availability. At these frequencies however, the presence of rain causes degradation of signals. This problem has become more critical in a tropical country such as Nigeria, which experiences high intensities of rainfall most of the time in a year unpredictably. As a result, signals even in the Ku-band frequency may sometimes be attenuated up to 7 decibel (dB) during raining periods in certain areas of the country with high mean monthly rainfall accumulation.  Due to this, video services may likely suffer a complete signal blackout during rainfalls in spite of uplink power controls (Abdulrahman et al., 2011). December 2011 saw the launching of another satellite by the country code-named the Nigerian Communications Replacement Satellite (NigComSat-1R) geo-stationed at 42.5degrees east with a 99.9% reliability, as a replacement for the Nigerian Communications Satellite (Nigcomsat-1), which was de-orbited on November 10, 2008 due to solar array deployment assembly problem.  It consists of 40 transponders on L, C, Ku, and Ka bands. The improved Ka-band with large spectrum availability and high frequency re-use potential was to enable it to provide broadband and broadcast services at lower costs to Nigerians in the near future. (Ibiyemi, 2011; Ahmed-Rufai, 2012). Against this backdrop, the recent motivation by the Nigerian Communications Satellite (NigComSat), to partner with Satellite Communications specialists, Newtec of Belgium to enable it launch its own Ka-band (30/20GHz) satellite solution in their latest coverage expansion program, is the key reason behind this work. This platform will enable optimal and cost effective voice, data, and video, internet, broadcast and application service solutions over Nigeria via the NigComSat-1R (NigComSat Ltd, 2015).

frequencies above 10GHz (Abdulrahman et al., 2011). It is therefore important to include fade margin when designing the satellite link budget and carry out analysis also, so as to make accurate predictions of rain attenuation effects in order to know whether a satisfactory service can be provided at the required reception point or area. These analysis can only be statistically or experimentally determined from rainfall rates,obtained from long term measurements (at least 3-years), using a standardized model (Ezehet al., 2014).The rain fade margin on the other hand, is a component of the link margin and it is calculated based on the expected rain atenuation over 1 year.However, rain attenuation is one of the most crucial factors to be considered in the link budget estimation for microwave satellite communication systems, operating at

This thesis, will fully provide us an opportunity to fully exploit the estimation and prediction of the rain induced attenuation in order to establish the availability level of such a satellite located at 42.5 degrees east longitude, which will operate at a higher frequency band, using a model of wide acceptability and good result which encompasses our local rain parameters to determine the extent of rain attenuation of these signals.

1.2   Justification of the Study

Radio wave propagating between terrestrial links and earth-space links are adversely affected by rain. The problems become more acute for systems operating at frequencies (Ku, Ka bands) above 10 GHz. Nigeria is located in the tropics unlike the temperate environments such as Europe and North America. The effects of the troposphere on microwave signals will therefore be most severe in the tropics because of high frequency of occurrence of rainfall.

The Ka-band frequency for satellite link which have been introduced in temperate regions is now been considered for use also in many tropical and sub-tropical regions due to high demand in the usage of bandwidth and spectrum congestion. (Walter et al., 2002). A critical look at the orderly use of the electromagnetic frequency spectrum for satellite communications, as well as other telecommunications applications shows that there is currently heavy congestion at the lower frequency spectrum and rain induced attenuation, which leads to propagation impairment on microwave signals at 10GHz frequency and above, has now become the main drawback in the design and deployment of wireless networks that are highly reliable and optimal in performance. (ITU, 2002). At this juncture in Nigeria, the Ka-band (20/30GHz) frequency from her own satellite NigComSat-1R which was launched in December 2011 is set to be fully put into use, due to its bandwidth capability and high frequency re-use potential. (NigComSat Ltd, 2015).However, past and recent studies has shown that rain induced attenuation has always be the dominant link impairment for a country like Nigeria, because it has both tropical and equatorial climates (Badronet al., 2011).So, for efficient utilization, there is the need to determine the relationship between this attenuation effect and the bandwidth at various rain rates, frequency, elevation angle of propagation, communication path and its polarized tilt angle of reception at various locations of interests.

Available literatures have established that there is little information on propagation studies on earth space link as regards using a satellite to provide communication services at Ka-band in this region. Where there is information, there is none that cover several locations (Omotosho, 2008).

Umeh (2010)in his study presented the calculated rain attenuation values of microwave signals for Akure, Ondo state, Nigeria using the ITU-Rmodel at 0.01% of time unavailability He then recommended that further research be extended to other Nigerian locations as well as other percentages of time.

Osahenvemwen and Omorogiuwa (2013)in their paper, again highlighted the effects of rain on satellite communication networks in Warri, Delta state Nigeria. They obtained rainfall data from the Nigerian Meteorological Agency (NIMET) for a period of one year and thereafter predicted the rain attenuation for only that location based on the ITU-R prediction model for Ku- and Ka-bands at various percentages of time unavailability but failed to carry out link budget calculations for the satellite terminal.

This study therefore focused on the effects of rain on millimeter waves at frequencies of 10 – 30 GHz (Ka-band), where the presence of rain degrades the performance of communication systems. It will thus presents theoretical rain attenuation results distributed at 0.01 to 1.0 percentages of time unavailability in an average year by choosing 20 locations across the country were daily rainfall was consistent based on available rainfall data, from the Nigeria meteorological Agency (NIMET) for a period of 5 years, using the ITU-R model. The study will further evaluate the performance of the satellite’s link by estimating the downlink budget of the satellite system, which will be needed to fulfill the required availability objectives.

1.3 Objectives of the Study

This study aims to study the effects of rain attenuation for a Ka-band satellite communications system, so as to analyze the feasibility of the usage of the NigComSat-1R’s Ka-band satellite solution in 20 locations for broadband and broadcast services over Nigeria.

The specific objectives are to:

19. Quantify the possible rain attenuation effects using the Nigerian Meteorological Agency (NIMET) rainfall intensity data relating to 19.6GHz (Ka-band) downlink frequency Space-to-Earth direction when it starts operating over Nigeria.
20. Conduct feasibility study on rain attenuation effects on NigComSat1-R’s Ka-band and hence analyze the feasibility of its usage for broadband and broadcast services over Nigeria.
21. Use meteorological rain data from satellites and ground stations as input data into ITU-R empirical models to compute the transmission losses resulting from rain attenuation for vertically and horizontally polarized signals on NigComSat-1R’s Ka-band in comparison with its Ku-band downlink frequencies.
22. Study the down-link budget of the satellite communications system and hence, carry out estimation on the satellite link, in order to propose adequate fade margins that can be applied to the links in places with high rainfall intensity and highest rain fade calculated values.

1.4 Scope       

1.5 Significance of the Study

This study analyzes the rain attenuation effects on a Ka-band (30/20GHz) satellite system for usage in Nigeria. The study is limited to the frequency range of 10-30GHz for vertically and horizontally polarized downlink radio signals passing through the rain medium. Thus, several locations in Nigeria were therefore selected for the study namely: Calabar, Warri, Benin, Port Harcourt, Uyo, Owerri, Enugu, Ikeja, Akure, Abuja, Minna, Jos, Ilorin, Makurdi,Sokoto, Kano, Kaduna, Maiduguri, Gombe and Yola.As indicated in figure 1.1. These locations were selected in terms of different rainfall rates and the good representation of the differing physical and climatic details they provide over Nigeria. The ITU-R rain attenuation global model was employed for analyses and the figures were plotted, using Microsoft Excel for the calculated rain attenuation values.

The Ka-band frequency for satellite link which have been introduced in temperate regions is now been considered for use also in many tropical and sub-tropical regions due to high demand in the usage of bandwidth and spectrum congestion. Rain induced attenuation, which leads to propagation impairment on microwave signals at 10GHz frequency and above, is the main drawback in the design and deployment of wireless networks that are highly reliable and optimal in performance. This is so because rain causes attenuation of the signal with varying degrees of severity depending on the intensity, raindrop size, rain rate as well as the frequency of transmission. Thus, rain rates at frequencies of operation beyond 10 GHz pose a serious challenge to the optimal performance of radio links and often cause complete signal outages (total unavailability of service). Therefore there is a need to determine accurately the amount of attenuation caused by varying rainfall rates in satellite links prior to the system’s deployment, so that it can be controlled.At this juncture in Nigeria, the Ka-band (20/30GHz) frequency from her own satellite Nig ComSat-1R which was launched in December 2011 is set to be fully put into use, due to its bandwidth capability and high frequency re-use potential.Nigeria has a tropical and equatorial climate, which is characterized by dominant rainfall. So, for efficient utilization, there is the need to determine the relationship between this attenuation effect and the bandwidth at various rain rates, frequency,and elevation angle of propagation, communication path and its polarized tilt angle of reception at various locations of interests. In this thesis, we fully exploit the opportunity to estimate and predict the rain induced attenuation in order to establish the availability level of such a satellite located at 42.5 degrees east longitude, which will operate at a higher frequency band, using a model of wide acceptability and good result which encompasses our local rainparameters to determine the extent of rain attenuation of these signals.

1.6 Thesis Organization

This section gives a brief account of the thesis, outlining the methods of its progress. The thesis is structured into five-fold comprehensive chapters. In chapter 1, the introduction of the study is carried out. Review of related literatures as well as discussions on rain attenuation prediction on satellite links, rain rate modeling, features of Ka-band, itssatellite link multiple access techniques and link budget estimations, all formed chapter 2. Chapter 3 gives the accounts of the methods and the analytical techniques employed to provide solutions to the set objectives. The analysis and discussions of the results are unfolded in chapter 4. Conclusively, chapter 5 is based on the conclusions and recommendations of the research findings.

Figure 1.1: Map of Nigeria highlighting the locations of the 20 Climatic Stations that was used for the study. It also outlines the possible spot-beam locations for frequency re-use

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