Fade Countermeasure Options in Tropical Rain Climate

Abstract

Satellite communication systems operating at frequencies above 10 GHz in tropical climates are subjected to many fade occurrences due to heavy rain. Service providers need to consider the use of appropriate forward error correction codes, the choice of modulations, diversity techniques, and the range of uplink/downlink power controls to use during severe rain fade periods in the overall design of their communications networks. The probable duration of a rain fade at a given level, what time of day it is likely to occur, how frequently it happens, and when it might next return are all important aspects that will contribute to customer perceptions. This paper presents the performances of annual and seasonal sitediversity and time-diversity measurements. Statistics of fade duration and fade slope variations are provided to give the detailed insight for the design of VSAT systems, in particular, in choosing an economical link margin and a suitable adaptive power control subsystem. Further more, the variations in the received clear-sky level of the satellite beacon show evidence of a diurnal variation that is a solar day due to the atmospheric thermal tide. There is also evidence for annual periodicity in absorption levels through the atmosphere on top of the diurnal tidal effect. In rain-fade countermeasure design for Ku/Ka-band systems in the tropics, the compensation for signal impairment due to tropospheric/atmospheric thermal tide should also be included in the fade countermeasure algorithm. For a satellite using on-board processing, the thermal tide element of the algorithm could be programmed into the down-link power control on board of the satellite. I. Introduction Direct-to-home very small aperture terminals and small office home office (DTH/VSAT and SOHO) systems operating at Ku/Ka-bands have typically less than 10 dB link margin and yearly availabilities of 99.5 %. The corresponding outage of 0.5 % is equivalent to 44 hours per year. These 44 hours of outage are not usually distributed evenly but are more likely to occur at certain times of the day and are quite often concentrated within a particular season. Such variations will affect customers’ service perception if long and frequent outages are encountered at critical times. Quality of service (QoS) and customer perceptions are important issues. In tropical, high rainfall rate regions of the world, not only will the average annual cumulative path attenuation statistics indicate that the link margin will likely be exceeded for more than 0.5 % of an average year but also the timing of the outages by season and clock-hour may cause the link to appear even worse. Of equal significance are the duration of the outages, and the interval between them. If power control is an option, the fade slope characteristics should be studied.