 bits, we have (6.18) where represents the probability to estimate the middle bit of the pattern equal to bk2 when tbk2 was transmitted and bk2 # tbk2. The BER is evaluated on the middle bit of the pattern since this covers all the pos- sible cases; the other bits of the pattern only serve to correctly represent the channel memory. 6.2.3.2 NRZ Signal after Propagation In the NRZ case, we will assume the transmitted pulse perfectly squared. We will assume also that both Raman and Brillouin scattering are negligible. Raman scattering has a high threshold, and we have to avoid a very high power that goes beyond it. As far as Brillouin is concerned, the threshold is quite low and the effect has to be elimi- nated somehow. Telecommunication Networks In order to increase the Brillouin threshold, the dependence of the Brillouin gain on the signal bandwidth (see Equation 4.51) is exploited. The spectrum of an amplitude modu- lated signal can be divided in the sidebands whose width is approximately equal to the bit rate and the central carrier (see Figure 6.3). Since optical systems transmit multigigabit signals, the sidebands' width is much greater than the Brillouin gain bandwidth and their effect can be neglected with respect to the central carrier. In general, the central carrier power is above the threshold if effective transmission has to be carried out. However, it is sufficient to operate a direct phase modulation of the transmitting laser (often called dithering and operated by a modulation of the bias current) to increase the carrier spectral width well beyond the Brillouin linewidth 22 12 MHz [7]. In this condition the Brillouin gain becomes [8] (6.19) where B is the linewidth of the Brillouin gain curve B is the optical bandwidth of the signal pumping the Brillouin scattering S(@) is its power spectrum Ps is its optical power GBO(a) is the Brillouin gain for monochromatic pump WB is the frequency of the maximum gain Moreover it is assumed B >> From Equation 6.19 it is derived that a dithering of about 250 MHz 