na. On this ground, an FWM induced capacity limit can be introduced, that is the limit to the product distance by bit rate imposed by the FWM effect. In general, this is defined as the point on the envelope of the maxima of a figure similar to Figure 6.17 at which the FWM causes a penalty of 1 dB. This limit can be evaluated starting from the FWM total power as follows. Let us define the FWM to noise ratio FN as the ratio between the FWM power and the optical noise power in the optical selection filter bandwidth: FN = Pf/oo. If we consider a system limited by the optical noise (that is the most interesting case) we assume the FWM power as an additional optical noise source, it results (6.43) Then, since ideal SNo is given by the FWM induced penalty can be written as (6.44) The analysis of the XPM is a bit more complicated due to the fact that the expression of the XPM induced crosstalk power has to be carried out by dealing with the nonlinear propa- gation equation in a more complicated way with respect to FWM. This study is carried out in detail in [22] while a simple but sufficiently accurate model in almost all the practical cases is reported in [23]. We will use this last model, since all the WDM designs we will do will always be verified by simulation. The conclusion of the model presented in [23] is that also XPM can be considered like a power dependent noise due to the combined effect of modulation, walk-off, and polariza- tion fluctuations. Transmission Systems Architectures and Performances In analogy with FWM, an XPM to optical noise ratio can be defined XN=P/o where Px is the XPM equivalent noise power SO that, the XPM induced penalty can be written as = + 1 (6.45) The XPM power affecting the kth channel of the WDM comb can be evaluated by the fol- lowing equation (6.46) where k is the index of the selected channel (P) is the average optical power S;(w) is the interfering channel normalized power spectral density that S;(w)dw Hik(w) is the representation of the interaction between the jth and the kth c