Modeling of raindrop size distribution and critical diameters for rainfall attenuation over microwave links in Southern Africa.

Abstract

The inability of service providers to constantly meet the design target of 99.99 % availability of the line-of-sight (LOS) microwave links has caused concern among both the operators and consumers. The non-availability of the links is predominantly due to propagation impairments along the propagation link. These propagation effects include cloud, snow, fog, gas attenuation, rain and atmospheric scintillation. Various studies have shown high vulnerability of radio communication systems operating at microwave (3-30 GHz) and millimeter wave (30-300 GHz) to rainfall attenuation especially in the tropical regions characterized by heavy rainfall and relatively large rain drops when compared to the temperate regions. In order to understand the effects of attenuation due to rain on communication systems in any locality (region), a good knowledge of the raindrop size distribution (DSD) and the rainfall rate estimates is necessary for accurate prediction and estimation of the rainfall attenuation. For this study, experimental raindrop size measurements gathered over a period of three years, using the Joss-Waldvogel RD-80 disdrometer installed at the roof top of the Electrical, Electronic and Computer Engineering building, University of Kwa-Zulu Natal, Durban, a subtropical location in South Africa, is analysed. Disdrometer measurements, sampled at one-minute rate over a period of nine months from Butare, an equatorial site in Rwanda, is also analysed for the purpose of comparison. The estimated R0.01 values for Durban and Butare are employed for the purpose of analysis. Based on the statistical analysis of the measured data samples, DSD parameters are proposed from the negative exponential, modified gamma, Weibull and the lognormal models. The DSD models are compared to models from other countries within and outside the region. The Mie scattering approximation at temperature of 20oC for spherical raindrop shape is adopted for the estimation of the scattering functions. The study further investigates the influence of critical raindrop diameters on the specific rain attenuation for the annual, seasonal and various rainfall regimes in southern Africa. This is achieved analytically by integrating the total rainfall attenuation over all the raindrop sizes and observing the differential change in the attenuation over a given range of drop size diameters. The peak diameter at which the specific rainfall attenuation is maxima is determined for different rainfall regimes. Finally, the cross-polarisation discrimination (XPD) due to rain over Durban is computed at two elevation angles. The results of this study will be helpful for the proper design and allocation of adequate fade margins to achieve the expected quality of service (QoS) in a radio communication system operating in the Southern Africa region.