A B S T R A C T

Changes in the weather condition (Rainfall, Snowfall, Harmattan, etc) affect signal transmission. Over the years, there have been issues with rainfall as regard signal transmission, because rainfall causes signal attenuation which breaks the communication link from the transmitting satellite to the receiving station. The satellite signal is transmitted in the Ku band where it is greatly affected by rainfall. This research work investigates the effects of rain attenuation on satellite for Ikeja, Lagos State.Measured rain attenuation values were compared with some other relevant slant path rain prediction models. The Simple Attenuation model (SAM), Bryant and ITU-R models were selected for rain attenuation prediction and the results were compared with measured rain attenuation. Result shows the ITU-R model presents the closest attenuation value for satellite link. The ITU-R model is modified to suit the results.

CHAPTER 1                                             

INTRODUCTION

1.1       Background

In the mid-1970s, attention was focused, during various international conferences or meetings on microwave communications, on the problem of saturation of the frequency bands, particularly in developed countries, and it was found necessary to use higher frequencies. The World AdministrationConference of Radio Communications (WARC) on frequency sharing, which was held in Geneva in 1979, allowed developed as well as developing countries to share frequencies in order to plan their future radio communication networks which were to use wideband microwave [1].

            Unfortunately, as higher frequenciesare used, rainfall and noise induced by atmospheric gases becomes a serious source of attenuation for microwave communications [1].The result of these is evidenced in satellite-earth microwave signal amplitude’s fading (slow or rapid), scintillations (amplitude or/and phase), depolarization, and receiver antenna noise. Attenuation is caused when a rain-cell(s) intersects the propagation path of the radio-waves and deep fades occur when the rain-cell(s) fills a large section of the Fresnel’s ellipsoid between the transmitter and the receiver [2]. Attenuation experienced in tropical areas is caused by considerably higher rainfall rates and larger raindrop size as compared to other parts of the world [3]. It becomes particularly severe at frequencies higher than 10 GHz, especially for small aperture antenna such as Very Small Aperture Terminal (VSAT). Attenuation increases with rain rate and frequency in the 10–40-GHz band in tropical regions and vertical polarization produces less attenuation than horizontal polarization at the 15- (Ku), 21- (Ka), and 38- (Q/V) GHz bands [3]. This phenomenon is even more critical in areas where the rainfall is very high, namely tropical, and more specifically in equatorial regions[1].

In the design of Earth-space links for communication systems, several factors must be put into consideration. These factors include absorption in atmospheric gases; absorption, scattering and depolarization by hydrometeors such as clouds and precipitation. All these effects must be considered as they are the causes of signal impairment for earth-space systems.

For temperate regions, the rain attenuation increases inversely with elevation angle due to the large rain cell size, while for tropical regions, attenuation is directly proportional to elevation angle for the same rain rate. This necessitates the need for modeling the propagation factors for tropical regions [2, 4, 5].

The effect of rainfall is more severe in tropical regions which are characterized by heavy rainfall intensity and presence of large raindrops. Raindrop size distribution changes with geographical location and it can strongly influence rain specific attenuation and consequently, the total rain attenuation.

The rain attenuation analyses are important for the study of the rain fade characteristics, which is a useful piece of information in the link budget estimation for predicting the expected outage as a result of rain attenuation on a microwave link. Long-term rain attenuation data are usually available for the terrestrial links in most tropical regions and so it can be converted and employed for satellite application. This method however subjects application on the earth space link to signal outage which is influenced by the actual earth-space rain path data.

There are two broad classes of rain attenuation predictions on any microwave link: The analytical models which are based on physical laws governing electromagnetic wave propagation and which attempt to reproduce the actual physical behaviour in the attenuation process; and the empirical models which are based on measurement databases from stations in different climatic zones within a given region [6-8].

Utilization of higher frequency bands such as the Ku-band for satellite communication provides a number of important benefits. It relieves congestion in the lower frequencies which are shared with terrestrial links; it exploits the larger bandwidths available at higher frequencies and provides cheaper implementation of spectrum conservation techniques and a more efficient use of the geostationary arc. However, the severity of atmospheric impairments (especially due to rain) on radio wave propagation increases with the increase in frequency. Therefore, extensive knowledge of the propagation phenomena affecting system availability and signal quality in these bands are required.

Although theoretical and experimental studies of rain attenuation can be found in many literatures, the measured rain attenuation data is still insufficient in order to estimate the link within the individual spot beam. Most of the studies carried out in developed countries have employed the use of satellite beacon experiments, whereas tropical regions are being faced with signal outage which calls for the need to employ the most suitable model or rather, develop absolute prediction model for the tropical region.

The effect of rainfall as an influential hydrometeor will be investigated on Ku band signals. Applicable rain attenuation models will be considered for this investigation and the most suitable for this region will be determined. The rain attenuation depends on the frequency, elevation angle, polarisation, temperature, size distribution of raindrops, and on their fall velocity. The frequency dependence of the specific rain attenuation can be obtained from Rec. ITU-R P.838[9].

1.2       Rain characteristics

            The distribution of rain along the radio propagation path is inhomogeneous. The non-uniformity of rainfall in both the horizontal and vertical directions makes the estimation of slant path attenuation complex [10].If rainfall rate is measured only at a single point, it is difficult to know enough about the structure of a rain cell at some distance away from the observation point, the non-homogenous nature of rainfall may lead to incorrect estimates of the specific attenuation. The structure of rain drops can assume various shapes. It is spherical for small size cells, while it is considered oblate spheroidal or oblate distorted for medium and large size rain drops respectively [10]. And therefore the attenuation of the horizontally polarized waves is greater than the attenuation of the vertically polarized waves.Rainfall intensity can be measured by different types of rain gauges located at the surface of the Earth. Rain in the tropics, in most cases, occurs in forms of cells which are a complex mixture of stratiform and convective rains with the convective rain accounting for about 70% or more of the total rain in most cases [1]. The physical significance of rain cells is well established with cell diameter generally decreasing with increasing rain rates. Most models of rain are temperate based and do not take into account this mixture, the transition between them and the convective nature of the rains [1].

1.3       Geographical Dependence of Rain Attenuation

Rain attenuation statistics depend on local climatic conditions. Typical values of rain intensity (and to some extent also drop size distributions) vary significantly worldwide. In this section, ITU-R methods and datasets are used to give an overview of the geographical dependence of the rain attenuation of millimetre waves.

1.4       Objective of the project

• To compare measured rain attenuation values with the prediction model attenuation value and some other relevant prediction models.

1.5       Scope of the project

The scope of this research work shall be limited to the application of a tested mathematical model for converting the rainfall attenuation data on the terrestrial links to be used for satellite application at Ku band.

1.6       Summary of project

In summary this chapter introduces the project; chapter two gives the overall view of the entire research. Chapter three reveals the data analysis and the various methods involved in rain attenuation prediction of earth-station.Chapter four shows the results and discussions of the data analysed for the rain attenuation model. Chapter five is Conclusion and Recommendation for the right attenuation prediction for Lagos at the Ku-band.

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