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- and, 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 work-out, 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.