rosstalk penalty is quite high, of the order of 1.0 dB. 9.5.4.2.2 Residual Chromatic Dispersion, Self-Phase Modulation, and PMD The link as it is designed uses dispersion under-compensation to balance at least in part the SPM phase shift. The residual dispersion of 00ps/nm requires per channel disper- sion compensation. We repeat here the assumption already done for the 64-4QAM to maintain decision and ISI compensation independent and to adopt decision driven LO phase lock. Also in this case, a small number of states MLSE is suitable to correct the ISI coming from different dispersion sources and from SPM. A sampling rate of 75 Gsamples/s is assumed, which should be within the reach of very fast 22 nm CMOS electronics. Following the same procedure already used for 64-4QAM, the relation between the ISI index and the penalty after equalization is represented in Figure 9.53. Moreover, the ISI index due to PMD and to the interplay between DGD and SPM are reported in Figures 9.62 and 9.63. 0.25 (ps km 1/2 FIGURE 9.62 Pulse broadening due to PMD: ISI index versus the link length for different values -1,000 1,000 3,000 5,000 7,000 9,000 11,000 13,000 15,000 of the PMD parameter. System length (km) FIGURE 9.63 Inter-symbol interference due to interplay between SPM and DGD in the Ultra Long Haul 100 Gbit/s system considered in the text: ISI index versus the transmitted peak power per channel. Peak transmitted power per channel (dBm) Next Generation Transmission Systems Figure 9.63, obtained by the simulation of nonlinear pulse propagation, and considering that it takes into account the interplay between dispersion and SPM, indicates an optimum power for which the ISI index is minimum. In our case, the residual dispersion is quite high (1000 ps/nm) and the effective link length defined regarding nonlinear effects is of the order of 3598 km (257 spans and 14km per span). In these conditions, with a bilateral signal bandwidth of 50 GHz, both SPM and disper- sion have a strong broadening effect inducing chirps with opposite s