spacing is about 6. In this condition, repeat- ing the procedure carried out in Chapter 6 where the linear crosstalk is considered like an additional noise source, a penalty of 0.2 dB is obtained. 9.5.2.2.3 Phase Noise Both the used optical sources, the transmitting laser and the LO, are external cavity lasers, with a linewidth of 500 kHz. Next Generation Transmission Systems The global linewidth to symbol rate ratio is then X 10-5. In this condition, the phase noise is completely negligible. 9.5.2.2.4 Nonlinear Depolarization Effect The depolarization occurring during fiber propagation due to Kerr effect is analyzed in [65,77]. Applying the method reported in [65] to our case, the propagation of a 16.6 GHz sig- nal along 3000 km with a span length of 70km and a power at the in-line amplifier output of 2 dBm, the depolarization factor Spol is about 0,0025. Equation 9.81 can be rewritten as a function of the SN, obtaining for our system working with a pre-FEC SNo equal to 19 dB a plateau due to nonlinear depolarization equal to 26 dB and a penalty due to depolarization equal to 0.9 dB. The plateau is sufficiently far from the system working point, but the penalty is not SO small, considering the small margin we have. 9.5.2.2.5 Nonlinear Crosstalk Even if the power propagating along the line is quite high, the nonlinear crosstalk is lim- ited by the increased channel spacing. The impact of FWM can be estimated by the model detailed in Chapter 6 obtaining a penalty of 0.15 dB, while XPM results to be negligible 9.5.2.2.6 Residual Chromatic Dispersion, Self-Phase Modulation and PMD Residual chromatic dispersion, SPM and PMD are the relevant ISI sources in this system. As far as PMD is concerned, using the model of Chapter 6, it is easy to determine that the penalty due to PMD is completely negligible in this case (below 0.1 dB). The situation is quite different regarding the interplay between SPM and DGD. The system is designed optimizing the dispersion map for a transmission at 10 Gbit/s. Thus postcompensation is us