Time diversity modelling and implementation for broadcast satellite systems at V-band

• Main Author: Udofia, Kufre
• Published: University of South Wales 2011
• Subjects: 621.38411
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.558346

Deployment of commercial satellite networks at higher frequency bands such as the Ka and V -band has become essential to meet an increasing demand for high data rate multimedia services. Satellite systems operating at high frequency bands offer large bandwidth and are able to provide better quality of service for broadcasting. However, ability to render these services can be severely impaired by rain induced attenuation. Hence, this thesis presents time diversity (TD) as a highly promising link restoration scheme that makes for efficient use of higher frequency bands without imposing unrealistically high fixed propagation margins on the satellite link. Accurate knowledge of rain attenuation distribution is necessary for a proper design of a TD model to operate with required service availability. Two years of beacon measurements of long term attenuation data have been analysed to characterise rain attenuation at a selected location such as Sparsholt, based on seasonal and annual distributions. It is observed that cumulative distributions of rain attenuation increase in seasonal order of summer, autumn, winter and spring. The summer months are characterised by highest rain attenuation, perhaps owing to a prevalence of convective rainfall which is usually intense and short-lived. Hence, the seasonal variation can be exploited to improve the link availability. Furthermore, analyses have revealed significant variations in TD performance at 20, 40 and 50 GHz. Similar to rain attenuation, the summer months yielded the highest TD gain. The seasonal analyses also led to the observation that cumulative distributions of TD gain decrease along the trend of gain magnitudes, following the seasonal order of summer, autumn, winter and spring. The seasonal behaviour of TD gain can be exploited to estimate the optimum gain applicable for all seasons to improve the link availability. The analyses also led to derivations of models to estimate optimum delay as a function of the rain event depth and event duration respectively. Models have demonstrated the variability between optimum TD gains and the TD gains achieved using other values of delay and fade depth. Based on reviews of existing TD models, a novel model to statistically predict TD was derived using 2 years of ITALSAT beacon measurements. The new model was validated by comparing the TD gain predicted at 50 GHz from measured TD gain at 40 GHz with the TD gain at 50 GHz predicted using existing empirical models, example the Fukuchi et al and Matricciani models. The performance measure provided average root mean squared errors of 13%, 36% and 35% respectively. Also, comparison of the TD model with the actual measurements yielded a performance measure of <5% for link frequencies 20, 40 and 50 GHz. Statistical performance of a designed V-band (50/40 GHz) link is evaluated through link power budget analysis. A measure of the overall link performance based on the overall carrier to noise ratio was observed. The analysis was based on Satellite beacon measurements and 4 diversity delays (10s, 30s, 60s, and 180s) using two diversity combining methods, namely the selection combining (SC) and maximal ratio combining (MRC). The MRC method provided the best joint overall carrier to noise ratio. Finally, a near real-time implementation of TD at V-band was simulated, to demonstrate the efficacy of the technique in fade mitigation. Time diversity was applied to impaired signal frames, that is, signals with overall carrier to noise ratio lower than the 9.6 dB threshold due to rain attenuation. At the receiver the original and delayed channels were combined using SC and MRC. TD implemented using MRC showed significant reductions in outage seconds, leading to BER as low as 4.82x 10-7. Furthermore, it was found that SC provided little benefit, whereas MRC yielded a substantial reduction in outage seconds, sometimes in excess of 50% depending on event dynamics. Hence, from these simulations, TD is revealed as being an efficient fade mitigation technique.