t an optimum system working point exists, as anticipated in Chapter 4, in corre- spondence of which the pulse broadening is minimum due to the interplay between SPM and DGD. In this case, the optimum transmitted power is about 0 dBm. Penalty (dB) 4 States 16 States FIGURE 9.53 Penalty due to ISI versus the ISI index for different types of low number of states MLSE. (After Alfiad, M.S. et al., IEEE J. Lightwave Technol., 27(20), 4583, 2009.) Normalized ISI index (ATRs) Next Generation Transmission Systems FIGURE 9.54 ISI index versus the power per channel at the Peak transmitted power per channel (dBm) input of a span. The system however cannot work with this low transmitted power, which does not assure a sufficient SN to be attained. Thus a greater ISI has to be accepted. At the power at the span input equal to 2 dBm, the ISI factor is about 0.45, that is, the pulse width increases almost 50%. From Figure 9.53, if we select an MLSE with 16 states, this ISI factor implies a penalty of 0.1 dB. 9.5.2.2.7 Penalties Analysis Summary The summary of the penalty analysis relative to this case is reported in Table 9.12, the resulting pre-FEC SNo is 18.65 dB, slightly smaller than the 19 dB deriving from the draft design, thus suggesting that the system is feasible. In this case, simulation is particularly important due to the small margin between the draft design and the result of the penalties analysis. As usual we have performed a simulation of all deterministic phenomena adding the noise through a theoretical model. The simulation result completely confirms the analysis, providing a situation slightly better than that depicted by the penalty analysis, confirming that the bulk of the adopted approximations are worst cases. 9.5.3 Practical Multilevel Systems: Transmitting 100 Gbit/s on a 10 Gbit/s Line by PolSK In the above section, we have designed the 100 Gbit/s system SO that the spectrum fits into the available optical bandwidth. This has brought us to use a great number of levels, with the consequent complexity at t