Mitigation techniques for multi-beam broadcast satellite services

The transmission of huge quantities of data required today by the information society has brought to a compulsory need of new and very wide available frequency bands for both Multimedia and Broadcasting transmission. This, in turn, has implied to resort to very high and relatively new frequency bands. Presently the Ka-band, in perspective, the Q/V-bands are the most promising candidates to sustain the trend. This inevitably leads to the necessity to face the strong atmospheric attenuations (oxygen, water vapour, clouds and rain) encountered at these frequencies without resorting, especially in the case of satellite transmission, to excessive power or antenna gains (the so called “brute force” solution). According to the opinion of many operators today, the way out from these difficulties consists in the adoption of some form of Propagation Impairments Mitigation Techniques (PIMTs) that can assume various forms. One of these is certainly the adoption of adaptive multibeam antennas on-board of the satellite for the down link, a countermeasure that offers the possibility to radiate selectively the power privileging the sub regions affected, at a particular time, by strong atmospheric attenuation. The paper concentrates on the broadcasting application, surely the most demanding one in terms of beam-forming network and determination of the time-variant excitation coefficients which must be derived from available (and uncertain) meteo information. The driving procedures must face big problems related to the difficulty to workout, in a reasonably short time, the signals needed to drive the antenna in the two technologies foreseeable today for beam-forming networks: the multi-port amplifiers and the “flexible” travelling wave tubes. The technical solutions to approach this goal are presented in the light of the preliminary tests carried out so-far and the best optimization techniques which have been selected and finalized to this purpose.

I General

Ka-band propagation impairments such as signal attenuation and scintillations would lead, if faced with the classical approach to system design, to an oversized antenna front-end in order to cope with the attenuation exceeded for a given percentage of time all over the service area of a broadcasting satellite. However, due to the localized nature of many meteorological phenomena such as rain, bad propagation conditions never occur simultaneously across the whole service area. More frequent is the case in which only a small fraction of the service area is affected by severe fading such as the ones encountered in the meteorological fronts. Then a fixed front-end would require an excessive power expenditure to accomplish the required availability performance as if the bad weather conditions were present simultaneously over the whole served region. On the opposite, a reconfigurable antenna front-end could be better exploited since the power would be allocated only for the amount that is really needed thanks to the ability to spatially arrange power distribution according to the actual time-variant propagation needs. The expected result may allow a considerable economy in the management of the limited on-board available resources.