tic dispersion penalty induced on 8-DPSK transmission with self-beating detection has been analyzed in [71]. The procedure is the same outlined in Chapter 6. The ISI has to be taken into account by considering the memory effect introduced by chromatic dispersion. The ISI reduces the dynamic range of the modulation in a deterministic way, thus the penalty can be evaluated as average EOP. This penalty is plotted in Figure 9.48. From the figure it is derived that the line residual dispersion implies a penalty larger than 3 dB, which for sure cannot be considered small. Thus per channel dispersion compensation is needed. This compensation is realized in two steps: a fiber-based per channel dispersion com- pensator is placed in front of the transponder (physically in the ad hoc subrack where are allocated the 100 Gbit/s transponders) to compensate 200 ps of chromatic dispersion with the correct slope, the remaining correction is realized via an electronic dispersion compen- sator. Using a high dispersion dispersion-compensation fiber (DCF), 200 ps are attained for a 33 GHz wide signal with about 4 km of fiber; thus both cost and occupied space are not very large even if this is a per channel compensator. Average eye opening penalty (dB) FIGURE 9.48 Chromatic dispersion-induced penalty in a 100 Gbit/s 8-DPSK with self beating detection. (After Seimetz, M. et al., IEEE J. Lightwave Technol., 25(6), 1515, 2007.) Residual dispersion (ps/nm) Next Generation Transmission Systems This design is due to the fact that the electronic equalizer will be useful to compensate other ISI causes also, like PMD or some forms of nonlinearities, thus it has not to be over- loaded by the need of compensating a great residual dispersion. A study on design of electronic compensation in M-DPSK systems is reported in [72] where an optimized algorithm is proposed for small state number MLSE to be applied to M-DPSK systems. To adapt to our case the results in [60] requires passing from a bandwidth of 20 GHz for a 4-DPSK at 40 Gbit/s to a bandwid