t. The results presented here can be compared with those in [79] taking into account that, in a very first approximation, the ISI-induced penalty depends only on the ratio between the pulse broadening and the pulse width and on the distance between first neighbors in the signal constellation. This is evident also from the data reported in [79]. Thus, the penalty due to ISI for quadrature coherent detection can be plotted introduc- ing the ISI index already used in the previous section as the ratio between the pulse broad- ening At and the pulse width Ts = 1/Rs. Considering a 16 states MLSE and a 4 states MLSE, as described in [79], the ISI-induced penalty is represented in our case in Figure 9.53 for different numbers of MLSE states. In order to use data in Figure 9.53, the ISI index has to be related to the physical parameters of the link, taking into account the interplay between SPM and chromatic dispersion. In our case, we are propagating an NRZ signal. The pulse broadening due to the inter- play between SPM and dispersion can be simulated and since this effect depends on the pulse energy, the broadening will be different from pulse to pulse since different energies are transmitted in correspondence to different symbols. This effect could be taken correctly into account by evaluating the broadening for each pulse of the constellation and defining a different error probability for different symbols. For sake of simplicity, and also since a very detailed analysis of this effect is not SO useful due to the great number of other approximations, we will simply consider the worst case broadening. The worst case could be not relative to the most powerful pulse, depending on the partial compensation between SPM and dispersion at the different powers; all the constellations have to be considered in any case (compare Chapter 6). The ISI is plotted versus the power at the input of a span in Figure 9.54 that is obtained by simulating the propagation of a single pulse through the line. From the figure, it is evi- dent tha