Scintillation Fade and Enhancement Duration Statistics at 20, 40 and 50GHz

Statistics of observed durations of scintillation fades and enhancements of the ITALSAT satellites at 20, 40 and 50GHz are presented for various threshold signal levels, and their use in fade countermeasures is examined. The analysis shows that most signal amplitude deviations from the mean level are of short duration and do not depend on the threshold level. Distribution of fade durations at thresholds 0.1 and 0.5dB were reasonably well approximated by the lognormal function. Introduction The saturation of C-band and the ever-increasing demand for new services that require greater bandwidth has led to the exploitation of higher frequencies. The higher frequencies offer various advantages such as, increased bandwidth, smaller antennas, and smaller satellite footprint that give higher EIRP and permit frequency reuse. The main obstacle however is that they are subject to stronger 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 [1]. Scintillations are rapid fluctuations in amplitude and phase of the received signal arising from fluctuations in the atmospheric refractive index due to turbulence. Increasing the transmitted power or the receiving antenna diameter to provide an adequate fade margin is often not feasible in VSAT systems. An alternative scintillation countermeasure, which is being investigated, is the use of Forward Error Correction (FEC) codes and adaptive modulation schemes. Link budget engineers use annual and worst month cumulative distribution functions to calculate the link budget for a new satellite communication system. The use of digital signal processing however requires the knowledge of the dynamics of tropospheric scintillation, including the distribution of scintillation fade duration and inter-fade interval. Fade duration statistics are particularly important for the design of high frequency satellite systems. In such high frequency systems, operating at a fixed small fade margin, the best way to mitigate propagation impairments is by introducing adaptive techniques, adaptive fade countermeasure strategies. Fade and interfade duration statistics provide the system designer with useful information for evaluating various mitigation techniques that will be employed to ensure a given system availability and quality of service. In this paper attention is focused on the analysis of scintillation fades and enhancements at 18.7, 39.6 and 49.5 GHz, which will be hereafter referred to as the 20, 40 and 50GHz, respectively, using propagation data from the ITALSAT F1 and ITALSAT F2 satellites. Experiment and Analysis ITALSAT was Italy's first operational communication satellite launched on the 16 of January 1991 by an Ariane booster and stationed in geostationary orbit at 13.2 degrees east. The design life for the ITALSAT vehicle originally was only five years, but ITALSAT F1 operated beyond its expected life, facilitated by the adoption of a propellant saving option in which the North/South station keeping was abandoned. As a consequence it became necessary for the beacon receivers to track the satellite position. Towards the end of 1997 the 50 GHz beacon receiver at Sparsholt was equipped with a tracking unit to counter the problem. The same goal was attained for the 40 GHz receiver in August 1998 [2]. The data examined can be divided into two sets: a) Data set 1: This contains data measurement from the ITALSAT F2 satellite, operating at 20 GHz and covering a 1-year period from September 99 to August 2000. b) Data set 2: This contains data measurement from the ITALSAT F1 satellite, operating at 40 and 50 GHz and covering a 1-year period from September 96 to August 97. 20th AIAA International Communication Satellite Systems Conference and Exhibit 12-15 May 2002, Montreal, Quebec, Canada AIAA 2002-1906 Copyright © 2002 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. 2 American Institute of Aeronautics and Astronautics Both data sets were recorded at Sparsholt (51.0814°N, 1.3947°W), UK at a path elevation of 30° and at a sampling rate of 1 Hz. Cassegrain antennas of diameters 1.22m for the 20 GHz and 0.61m for both the 40 GHz and 50 GHz beacons were employed. The propagation data together with a range of meteorological measurements recorded at the same sampling rate were archived to a compact disk once a month. Pre-processing of the raw propagation data was carried out. This involved visual inspection of graphs of the data to identify gaps and spurious samples, and high-pass filtering using a 6 order Butterworth filter with a cut-off frequency of 0.04 Hz to extract scintillation [3]. Distribution of Scintillation Fade & Enhancement In Figure 1, the observed annual cumulative distributions of scintillation fades and enhancements at 20, 40 and 50GHz are presented. It can be seen from this figure, that for 0.01% of the time the fade levels at 20 and 50GHz are 0.8 dB and 1.5 dB, respectively. This represents an increase in the scintillation signal amplitude by a factor of 1.88.

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