Analysis of rain cell size distribution for application in site diversity

Abstract

Communication systems operating at higher frequency bands suffer from severe attenuation due to rain, which is highly variable in time and space. However, the temporal and spatial inhomogeneity of rain fields can be exploited to improve the availability of the communication link. Knowledge of rain cell size distribution is relevant for the modeling of earth-space propagation in radio communication. To determine the spatial structure of rain cells, long term rain rate time series can be processed by applying the synthetic storm technique assuming some known value of storm translation speed. The underlying hypothesis is that rain patterns move along a line with a constant speed and that advection is the predominant mechanism to account for the spatial variability of rain-rate. The hypothesis holds when a statistical description of rain structure is required, rather than the exact space distribution of rain. Furthermore, as rainfall patterns move over a rain gauge, it is possible to estimate the horizontal extent of rain cells from the duration of various rain rate thresholds as recorded by the rain gauge if a mean advection velocity of rain cells is assumed. In this paper, a comparison is made between cell advection speed, Doppler speed measurements recorded in Chilbolton, England and local wind speed measurements at Brize Norton, some 63 km from the radar location. The distribution of rain cell diameters is determined by applying the rain cell translation velocity. In an alternative approach, the PPI (Plan Position Indicator) radar scans of different rain events were analysed to determine the size distribution of various rain-rate thresholds. The rain cell size distributions obtained using the two methods are compared, and the implications of these results on the configuration of site diversity in earth-space communication systems are discussed.