CHARACTERIZATION OF RAIN-INDUCED DEPOLARIZATION OF MILLIMETRE WAVES IN NIGERIA USING MEASURED CLIMATOLOGICAL DATA

Abstract

Rain - induced depolarization has been a constraint to the effective use of satellite communication systems at milliametre wave band due to distortion of raindrops when dual orthogonal polarizations are employed to double channel capacity without increasing the bandwidth. Rain-induced depolarization has been momentous problem to satellite communication at milliametre wave band especially in the tropical region like Nigeria that often experience high and varying degree of rainfall intensity accompany with large raindrops. In the recent time past depolarization models have been formulated and its effects investigated in the temperate region with meager records of studies of effect of depolarization in the tropical locations, Nigeria inclusive. Hence this study attempt to characterize rain depolarization of millimetre waves in Nigeria using five years climatological data acquired from Troposperic Observatory Data Acquisition Network (TRODAN) of the Centre for Atmospheric Research (CAR) of National Space Research and Development Agency (NASRDA). The characterizations were based on different locations across Nigeria namely: Akure, Akungba, Anyigba, Lagos, Makurdi, Nsukka, Jos, Yola, Port Harcourt, and Sokoto. ITU-R model has been employed in evaluating Cross Polarization Discrimination (XPD) at varying elevation angle, rain rate, polarization tilt angle, frequency and probability of exceedance considering different rain types associated with tropical region. Overall depolarization results are presented in form of contour maps to show the spatio-temporal variation across Nigeria. The study revealed that depolarization between two orthogonal polarizations becomes poor as elevation angle increases and rain rate, frequency, and polarization tilt angle increases. Further result affirmed that Co-Polar Attenuation (CPA) and XPD are inversely related. Comparison of XPD at frequency of 10 GHz and 12 GHz shows that unwanted signals will completely overshadow the co -polarized signal between 0.001% and 0.018% of time signal not available in a year at Jos, Yola, Sokoto and Nsukka. However when the fade level reaches values ranging from 8.93 dB to 11.08 dB at aforementioned frequencies, the crosstalk will be prevalent at the receiver station. Information provided in depolarization maps would serve as good tools for designing communication systems by the satellite communication system designers for the possible level of interference that could be encountered across Nigeria. Finding from this study recommends that mitigating technique or improvement in fade margin is required across Nigeria if link availability and the Quality of Service (QoS) are to be sustained.