Characterisation of Worst Month Statistics for Satellite-Earth Links Performance in Tropical Locations
The study presents the results of experimental data showing seasonal variability of rainfall intensities at two locations in Nigeria. The increasing development in satellite technology has brought about several novel mobile satellite services and applications. Consequently, there is a pressing demand for seamless data transfer and accessibility to satellite-earth microwave links in SHF/EHF frequency bands. Mobile platforms available via satellite include e-defence, Tele-banking, Skype, e-learning and so on. Consumers continue to press for ubiquitous coverage, internet traffic by-pass, scalability and improved quality of service of communication systems. However, rain has been the major degradation parameter for the availability of satellite signals especially at frequencies greater than 10 GHz. This paper presents some statistical analysis of rainfall in two tropical locations in Nigeria – Akure (7°17’N, 5°18’E, 358 m) in the Southwest, and Jos Plateau (9°57’N, 8°58’E, 1192 m) in the north central. Rainfall intensities of one-minute integration time were measured for 19 months (June 2013 to December 2014). The degrading effect of rainfall intensities was highlighted in the statistics of the worst month and monthly variations in rain rates along Ku-band signal paths in the study locations. Predicted results showed that rain induced attenuation values above 30 dB occurred during the worst months, while clear sky values are below 2 dB. The worst month statistics obtained were largely different from those proposed by the ITU. Thus, for optimum link budgeting, the modified values of Q and ß should be adapted in these regions. The worst month statistics derived would serve as an essential planning tool for the system link designer for fade analysis and site diversity implementation; and eventually, result in better availability of radio-communication systems in the region. The results will facilitate improved radio-communication planning in the region. More investigation on beacon measurements of rain-induced attenuation is required to corroborate these results. The paper has revealed crucial considerations that affect the quality objectives of telecommunication systems. The results show that AY and AWM can be safely estimated from measured data of one minute integration time, and modelled with ITU-R recommended values. However, it is recommended that the worst-month design criteria of 178 mm/h and 150 mm/h for Akure and Jos, be considered as the actual design goal. Also, the relationship between the worst-month and average year has been given by β and Q1 parameters, as 0.372 and 1.060 for Akure, and 0.207 and 2.042 for Jos. Rain fade levels in worst month in both study locations are about 50% higher than levels in the average year, which suggests the need for alternative methods of mitigating rain fade such as adaptive power control schemes.
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