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 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 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.
1.3 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:
a. 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.
b. 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.
c. 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.
d. 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
studyanalyzes 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
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
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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