 a quite high number. The effective value of the SNo estimated through the power budget and the noise power evaluation is 21.6 dBm that becomes 32.6 after the super-FEC gain. This leaves a margin of 5.5 dB for the allocation of the penalties. This margin is not very high, but should be sufficient in our case. Also in this shorter configuration, it is easy to demonstrate that with the figures reported in Table 9.13, the power budget equations are satisfied both at the receiver and at the transmitter. 9.5.3.2 Penalties Analysis 9.5.3.2.1 Intersymbol Interference (Residual DGD, SPM interplay, PMD, Optical Filtering) ISI is mainly caused by three factors: 1. SPM and residual dispersion interplay 2. PMD 3. Tight optical filtering In a regime of small pulse distortion we will assume that the overall ISI index can be obtained from the individual contributions. Dividing SPM and dispersion from PMD we can write (9.84) where the four operators acting on the envelop of the input pulse Pin(t) are: the transmitter AWG (Att), the receiver AWG (Are), the interplay between dispersion and SPM taking into account in-line compensation (K), and the PMD (P). To each process is associated a pulse spreading T SO that, for example, (9.85) where T is the width of the original pulse. Since NRZ pulses are symmetric around their axis and the first filter of the chain is Gaussian (thus symmetric too) the overall pulse broadening can be found in a first approximation using the formula (9.86) as graphically show in Figure 9.55. This rule is rigorously true for a cascade of linear pro- cesses with one nonlinear instantaneous process (see Appendix 3). Here we approximate our model assuming a concentrated effect for the interplay between SPM and dispersion, while they are distributed phenomena. This will allow us to proceed with the penalty analysis considering the different causes of ISI separately. Next Generation Transmission Systems FIGURE 9.55 Approximation used to evaluate the overall ISI index of the system. (a) Physically, all the ISI-rel