Accurate Prediction of Scintillation Degradation Applicable to Satellite Communication Systems Design

Abstract

Satellite communication was operated exclusively in C-bandprior to 1970. Since then there has been an explosive growth in the demand for telecommunication services that are either only feasible via satellite or very cost effective by that means. This has prompted a steady growth in the utilization of higher frequencies in the Ku-band and above. The higher frequencies offer various advantages such as increased bandwidth, smaller antennas, and smaller satellite footprints that give higher EIRP permitting greater frequency reuse. The main drawback however is that they are subject to more severe propagation degradation. The small size antennas employed in VSAT and USAT systems significantly reduce the cost of earth station terminals and also eliminate tracking requirements, but they lose the mitigating effect of aperture averaging and hence experience stronger scintillation. The result is random fading and enhancement in the received signal amplitude, which will have a significant impact on the performance of low-margin communication systems operating at high frequencies (> ~10GHz) and low elevation angles, and utilising small antennas. Scintillation effects need to be considered in the design and link-budget calculations of these systems. In this thesis, the results are shown of an extensive measurement analysis of tropospheric scintillation, using the ITALSATsatellite beacon signals at 18.7, 39.6 and 49.5 GHz, recorded at Sparsholt, U.K., at an elevation angle of 29.9°. The analysis was carried out in order to study the effects of scintillation due to tropospheric turbulence and their impact on satellite digital communication systems. The first part of the thesis deals with the preprocessing of raw propagation data and presents various statistical results relating to the stationary aspects of the scintillations, i. e. pdf of amplitude and intensity; long and short term statistics of amplitude scintillation distributions. The relationship of scintillation with link parameters and meteorological parameters are also studied. The second part of the thesis investigates the dynamic characteristics of scintillation. It examines the observed effects of wind and cloud presence on the intensity and power spectrum of tropospheric scintillation and then moves on to present results relating to the analysis of the instantaneous frequency scaling of scintillation. In the third part of the thesis scintillation fade and enhancement duration statistics are presented for various threshold signal levels, and their use in fade countermeasures is examined. Finally, a study of adaptive fade countermeasures (FCM) that could be used for systems operating at Ka-band and above for mitigating the effects of scintillations and rain attenuation is presented. The last part, presents an improved global prediction model for both the long-term standard deviation and the signal level distribution of tropospheric scintillation. The model is validated using measurement results from satellite links in Europe, the United Sates and Japan at frequencies from 11 GHz to 50 GHz, and path elevation angles 5.8° to 40°.