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.
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 7decibel (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 launchingof 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).
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 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.
This thesis, will fully provide us an opportunity to fully exploitthe 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 amodel of wide acceptability and good result which encompasses our local rainparameters to determine the extent of rain attenuation of these signals.
1.2 Justification of the Study
Radiowave 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 dominantlink impairmentfor a countrylikeNigeria, 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, elevationangle 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 propagationstudies 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)inhisstudy 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 locationsas 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 timeunavailability but failed to carry out link budget calculations for the satellite terminal.
This study therefore focused on the effects of rain on millimetric 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.
1.3Objectives of the Study
This studyaims 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:
- Quantify the possible rain attenuation effects using the NigerianMeteorological Agency (NIMET) rainfall intensity data relating to 19.6GHz (Ka-band) downlink frequency Space-to-Earth direction when it starts operating over Nigeria.
- 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.
- 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.
- 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
This study analyzes the rain attenuation effects on a Ka-band (30/20GHz) satellite system for usagein 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 globalmodel was employed foranalyses and the figures were plotted, using Microsoft Excel for the calculated rain attenuation values.
1.5 Significance of the Study
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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 degreesof 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 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.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 elevationangle 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.
1.6Thesis 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 formedchapter 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
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