Fade and Interfade Durations on Earth-space Links in Temperate and Tropical Locations

Fade and interfade duration distributions derived from Kuand Ka-band slant-path measurements in several temperate and tropical locations and different path geometries are used to get insight into the dependence of distributions on fade threshold, frequency, elevation angle and climate. Comparisons of average lognormal fits to duration distributions were used to investigate these dependencies. At Ku-band, durations of events at tropical sites were consistently greater than those found for temperate locations. Results of tests of prediction models with measured data are discussed. INTRODUCTION Attenuation at frequencies above 10 GHz, due mainly to rain, leads to outages that compromise the availability and quality of service, making this one of the most critical factors in satellite link design. Thus, in some cases the use of suitable compensation techniques to counter excessive attenuation will be needed to maintain reliable system operation. The proper design of fade mitigation techniques on satellite links requires not only knowledge of long-term statistics, but also of second-order statistics describing the dynamic behavior of attenuation, such as duration of fades, duration between fades and fade rate. Analyses leading to the characterization and modeling of these phenomena are particularly needed nowadays as commercial systems start operating at Ka-band (20/30 GHz), and the use of even higher bands is being explored. In tropical regions the application of mitigation techniques might be even more demanding. Fade and interfade duration statistics have been derived from Kuand Ka-band measurements of beacon signals or radiometric sky noise temperature collected over the last 15 years in temperate and tropical locations. Statistics were produced at Ku-band for four sites located in Southeast Asia, and sites in Ottawa and Belgium, and at Ka-band for sites in Ottawa and Vancouver in Canada, Tampa in the U.S., and Belgium. The sites are located in different climatic regions and the range of elevation angles, from 7.4o to 52o, enabled insight into the dependencies of fade dynamics on these factors. The North-American data were collected during experiments with the ACTS satellite beacons at 20.2 and 27.5 GHz, and the Belgian data were from Olympus beacon signals at 12.5, 19.8, and 29.6 GHz. Ku-band attenuation data in Asia and Ottawa were derived from radiometric measurements at 12 GHz. Fade duration distributions for all locations and at both frequency bands obeyed a general lognormal behavior for durations above 30-50 seconds. Comparisons of average lognormal fits to the distributions at every site and frequency suggested a dependence of durations on frequency and elevation angle. At Ku-band, the median values of the Southeast Asian distributions were consistently longer than those recorded at Ottawa and Belgium, suggesting a dependence on climatic characteristics. Interfade duration distributions from the ACTS and Olympus data showed a clear dependence on fade threshold and a general lognormal behavior within given duration regions. FADE AND INTERFADE DURATION DISTRIBUTIONS Fade duration statistics are usually presented as conditional distributions of the number of fades exceeding certain durations, given that a specified fade threshold has been exceeded. This representation provides information on the number of outages and system availability due to propagation on a link, given a fade margin and an availability specification. Conditional distributions of number of fades for Vancouver and Tampa, shown in Fig. 1, were derived from the 27.5 GHz ACTS beacon data with link elevation angles of 29o and 52o, respectively. Tampa displays a larger annual average number of fades than Vancouver for thresholds above about 4 dB; below this margin however, the number of fades is comparable. A running-average filter with 10-second time constant was applied to the data in order to suppress rapid fluctuations related to scintillation and receiver system noise. Fade Duration (sec) 100 101 102 103 104 105 N um be r o f F ad es 100 101 102 103 104 105 2 dB 4 dB 6 dB 8 dB 10 dB 12 dB