th of 33 GHz from an 8-DPSK at 100 Gbit/s; this can be done by scaling the penalty taking into account the different distances on the phase circle between nearby constellation point and scaling the dispersion corresponding to a certain penalty of 1.65², which is about a factor 2.7. The resulting penalty curve is shown in Figure 9.49, and for the residual dispersion of 75 ps the penalty is 1.25 dB. 9.5.1.2.2 Polarization Mode Dispersion Under a qualitative point of view, the impact of PMD is the same suffered by other modu- lation format. A detailed analysis has been carried out in [74,75], taking into account both the average penalty and the outage probability due to instantaneous bad relation among the SPS and the transmitted polarization. Also PMD of a standard SSMF has to be compensated and since an electronic dispersion compensator is already foreseen for the residual dispersion, the easier way is to design the compensator also for PMD compensation. This is done in [72], adopting the MSLE already described in the previous section, the PMD related penalty results as in Figure 9.50. Thus, for At = 0.1V1500 = 4 ps, the associated penalty is 0.25 dB. 9.5.1.2.3 Kerr-Induced Degradation (SPM, XPM, and FWM) The effect of SPM is considered in [71] in great detail. Naturally, the overall penalty depends both on the span input power (that is assumed the same for each span) and on the number of spans. The SPM penalty plot deduced in the regime of small penalty per span is reported in Figure 9.51 taking into account also the mitigating effect of residual dispersion. The Penalty (dB) No compensation Coordinated 1.25 dB FIGURE 9.49 Penalty curve after optical and electronic residual dispersion compensation in the case of a 100 Gbit/s 8-DPSK system transmitted on a line designed for 40 Gbit/s. (After Salehi, J.G. and Proakis, M., Digital Communications, 6th Residual dispersion (ps) edn., McGraw Hill, s.l., 2008.) Telecommunication Networks Penalty (dB) FIGURE 9.50 Penalty curve after electronic PMD compensation in the case 