of a 100 Gbit/s 8-DPSK system transmitted on a line designed for 40 Gbit/s. (After Freckmann, T. et al., Joint electronic dispersion compensation for DQPSK, Proceedings of Optical Fiber Conference/International Optical and Optoelectronics Conference OFC/IOOC 2008, San Diego, CA, IEEE/OSA, s.l., 2008, p. OTuO.6.). Differential group delay AT (ps) sensitivity to the nonlinear phase distortion due to SPM is greater for M-DPSK with respect to IM (see Chapter 6) due to the number of levels and to the fact that the information is coded in the phase; thus it is directly affected by the Kerr-induced phase fluctuations. Nonlinear induced crosstalk can be classified as in case of IM systems in XPM-induced and FWM-induced crosstalk. In both cases, the decrease of the spectrum width with respect to the 80 GHz for which the system is designed helps alleviate the effect. XPM-induced crosstalk is studied in [75] where it is demonstrated that, in our condi- tions, SPM is dominant. Nevertheless, XPM is particularly dangerous for this modulation format, due to the fact that the power is constant. This can be intuitively understood looking at Equation 4.41 that expresses the nonlinear phase in the case of a WDM comb. While in the case that the intensity of the channels fluctuates, the walk off due to dispersion generates a mixed situation in which some chan- nels do not contribute due to the absence of power; this does not happen in the case of a constant power modulation format. Naturally, polarization fluctuations and phase differences due to walk off still work to reduce XPM efficiency, but in the same general situation, XPM is more efficient for constant power modulation formats. Penalty (dB) number FIGURE 9.51 SPM-induced degradation in the case of the 100 Gbit/s 8-DPSK transported on a 40 Gbit/s line. In the curves, the interplay of SPM with the residual dispersion is taken into account. (After Seimetz, M. et al., IEEE J. Lightwave Technol., 25(6), 1515, 2007.) Optical power at the span input (dBm) Next Generation Transmissio