Fading Channel Simulation in DVB
Fading is known from shortwave transmission, where the received field strength level may strongly vary due to atmospheric disturbances. In analog TV, the term 'fading' is practically unknown. Rather, one talks of 'antenna shadows' or 'ghost images'. The effect in question, however, is fading, ie constant reflection of the eletromagnetic waves emitted by the TV transmitter by walls of buildings, mountain slopes and similar reflecting natural or artificial obstacles. In analog TV, fading is of minor importance since the effects thereof can be eliminated almost completely through the directivity and exact orientation of the Yagi roof antenna for stationary TV reception at home. [[ br />Fading effects can also be observed in analog cable TV, for example in a block of flats linked to the cable network with one or several antenna sockets in every flat. If the taps for the sockets are not match-terminated, reflections with constant level and constant phase arise which may cause level reductions of several dB at exactly calculable points in the cable. [[ br />Moreover, the reception of TV signals broadcast via satellite can be impaired by fading. A known phenomenon is flickering of the received picture, produced by planes flying past or a drop in receive field strength caused by an approaching thunderstorm. All the above receive conditions have one thing in common: reception is stationary with a direct line of sight to the TV transmitter. [[ br />Looking at receive conditions in DVB, the effects in cable and satellite reception (DVB-C and DVB-S) are found to be similar as in analog reception. In these two modes reception is stationary, too. Terrestrial transmission (DVB-T) not only provides for stationary operation but also for portable and mobile reception. This considerably accentuates the effects of fading. [[ br />In this application note we investigate fading effects in DVB, with the emphasis on those in DVB-T signals.
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