Combined tropospheric attenuation along satellite path at SHF and EHF bands in subtropical region

Abstract

The traffic flow of information across the globe is crucial in today’s communication systems, where about 88% population are connected via several smart devices, hence resulting into constraints on the limited available radio resources. Due to the limitations of terrestrial connectivity affecting communication systems, in terms of geographical coverage area and system capacity, which have become serious issues globally. Therefore, there is a need for communication industries to embrace the use of satellite systems. Satellite services have many advantages some of which includes availability, wide coverage area and the ability to accommodate most of the limitations of the terrestrial systems. However, Earth-to-satellite systems, especially those operating at higher frequencies above 7 GHz, usually suffer from degradation due to hydrometeors which are mainly produced in the troposphere. Hydrometeors include rainfall, hail, gases, clouds and snow among others; of which rainfall is the principal factor which contributes highest impairment along the propagation paths, simply termed as rain attenuation. Moreover, the scenario in the tropical and subtropical regions become more pronounced due to the degree of occurrence of precipitation when compared to the temperate region. Other significant factor that usually affects the propagation of signals is attenuation by scattering and absorption due to rain, water vapour, cloudiness and other gases in the atmosphere. Thus, in order to estimate accurate rain attenuation of a location, there is a need for accurate measurements of rain attenuation components such as rain height, rain rate, altitude, slant-path length, among others; of which rain height plays a significant role in the case of satellite links. However, the attenuation due to other tropospheric components cannot be negligible at higher frequencies over any location in order to proffer solution or cater for impairments that may arise as a result of any atmospheric perturbation in a satellite communications system. The significance of rain height in estimating rain attenuation along the satellite path, is ii crucial and this important component has been extensively dealt with in the temperate region, partially in tropical region with no record in subtropical regions. This study, therefore, focuses on the measurement of rain height to assess the degree of attenuation due to precipitation over several locations across South Africa, a subtropical region. In spite of the extensive works that have been carried out on prediction of rain attenuation based on the recommended rain height by the International Telecommunication Union-Regulation over some of the studied locations, the contribution of local rain height data for rain attenuation prediction will enable better results which are the focus of this study. Hence, this thesis presents 5-year rain height measurements based on zero-degree isotherm height (ZDIH) obtained from the Tropical Rainfall Measuring Mission-Precipitation Radar (TRMM-PR) over a subtropical region-South Africa. The component of this work encompasses rain height cumulative distribution, percentage of exceedances, development of the contour maps of rain heights for South Africa, modeling of rain height, tropospheric attenuation prediction due to gas, cloudiness, scintillation, application of rain height for rain attenuation prediction, estimation of total attenuation and prediction of quality of service based on signal to noise ratio. Findings from this work show that the ZDIH distribution is location dependent. Rain heights value ranges from about 4.305 km from the southern region to 5.105 km in the northern region of South Africa. The parameters of the ZDIH distribution models developed with the use of maximum likelihood estimation technique show a wider variation over some selected locations observed. Finally, attenuation due to rain, gas, cloudiness and scintillation were estimated. In addition, the total attenuation and the quality of service based on the propagation signals at SHF and EHF over some selected stations were evaluated and presented in this work.