Rain Attenuation Modelling and Prediction for Optical Wireless Communication Systems in Durban, South Africa.

Abstract

The continuous demand for more reliable wireless communication systems with extremely high data rates has accelerated various aspects of research topics to be able to meet future needs. One of the most crucial topics in the field of communication is free-space optics (also known as optical wireless communication). It is well-known that the performance of any optical wireless communication system is strongly influenced by the atmospheric conditions in a given environment. In foggy, rainy, and clear weather conditions, optical signals are known to be attenuated due to scattering. The received signal is diminished in the presence of snow, rain, or even haze. Rain and clear weather conditions will be the focus of this research as there is hardly snow or haze in South Africa, especially Durban since it is a subtropical region. In this research work, rain attenuation modelling and prediction will be done using an empirical method based on the relationship between the observed attenuation distribution and the related observed rain intensity distribution at a 30 second integration period. A disdrometer is used to obtain the rain intensity, and a power meter is used to log the received signal power level every 30 seconds to evaluate the influence of rain on the signal transmitted. The International Telecommunication Union (ITU-R) recommends targeting for 99.99 % system availability; as a result, the rainfall rate (R0.01) in the research region must be estimated for 0.01 percent of the time. The rain intensity and raindrop size distribution (DSD) modelling is then performed from the empirical method, obtaining R0.01 for Durban for all months throughout the experiment period. Using the disdrometer diameter ranges, the spherical droplet assumption is used to estimate the scattering parameters for frequencies between 2 GHz and 1000 GHz. The relationship between the received signal level and the intensity of rain for a particular weather condition at a specific time is then obtained. Transceivers with a fixed length of 7 meters between them, due to shortage of material such as the fiber cables to link the transceivers to the computer for data monitoring and logging, and for accurate alignment, were used to conduct these experiments. This relationship is compared against the French model at a wavelength of 850 nm. The main results obtained from this work reveal that there are extremely high attenuation values compared to the French model, which thus calls for further investigation to provide the optimum model that can accurately predict these effects for reliable optical wireless communications in Durban, South Africa.