The filter is specified by the roll-off factor aa, describing the frequency excess of the filter, and by its symbol span ६. While aa is a system-dependent parameter usually chosen between o.25 and o.35, ६ denotes the number of symbols taken into account for smoothing the rectangular pulse sequence to limit the bandwidth and has a strong influence on the amplitude distribution of the signal. Values above six have been found to have no significant influence on the amplitude histogram, while values below that strongly distort it. For the modulation scenario to have a physical setting, a data rate Rc has to be additionally defined to set the bandwidth.