Fade Slope Analysis of Ka-Band LEO Satellite Links

Because the motion of a LEO satellite across the sky causes the earth-space path to pass through any rain cells in the vicinity very quickly, the degree of rain fading on such paths changes more rapidly and leads to steeper fade slopes than in the geostationary case. This has important implications for the performance of the power control algorithms and other techniques used to mitigate such fading. Because comprehensive measurement data have not yet been compiled for fading on LEO links at Ka-band, we must resort to simulation to predict the statistics of such fading. Here we use an approach based upon the well-known EXCELL model of horizontal rain structure and Goldhirsh´s method for obtaining the parameters of the EXCELL model from ITU-R rain zone parameters. As the satellite sweeps across the sky, we determine the intersection of the earth-space path with rain cells in the vicinity. Given the corresponding rain rates, and accounting for attenuation by atmospheric gases, cloud and fog, we predict the total attenuation along the path at each instant during the pass. Our results suggest that: (1) fade slopes observed on earth-space links to LEO can be between two and ten times greater than those reported previously for the geostationary case, (2) fade slopes will become steeper as the altitude of the satellite decreases, (3) fade slopes will become steeper with increasing carrier frequency, (4) rising and falling fade slopes of a given value are equally likely and (5) the probability of exceeding a given fade slope value tends to increase rapidly in regions with higher average rain rates.