ultiplexing Systems Nonlinear interference depends on nonlinear fiber propagation. The effects that give the main contribution to this interference term are Kerr induced four wave mixing (FWM) and Kerr induced cross phase modulation (XPM), assuming as always that the Brillouin effect is neutralized with a sufficiently fast source dithering [10]. Equations 4.45 and 4.46 allow us to evaluate the FWM power on the various frequen- cies arising due to this effect. The main hypothesis here is that the channels can be considered monochromatic, that means the bit rate R is much smaller than the channel spacing. This condition is verified several times, but not always. For example, it is not verified in the case of a DWDM system at 10 Gbit/s and a channel spacing of 25 GHz. Fortunately, in the case of 10 Gbit/s systems with 100 or GHz spacing, the monochro- matic channel assumption is quite reasonable. In principle, knowing the FWM power hitting the signal bandwidth does not allow a complete performance evaluation. As a matter of fact, the FWM instantaneous power in a signal bandwidth fluctuates with time due to the channels modulation, walk-off, and polarization changes. The channel walk-off is due to the fact that different channels have different group velocities; thus, the alignment of the transmitted bit streams varies with time while one "slides" with respect to the other. Since an FWM term arises only when a "one" is transmitted on all the involved channels, walk-off causes the FWM power to change in time. A similar variation is due to polarization fluctuations. Since we have seen in Chapter 4 that the bandwidth of the PSP is about GHz, if the channel spacing is on the order of 100 GHz or greater, the polarization of different channels evolves independently. FWM happens only when the polarization of the involved channels is almost the same; thus, polarization fluctuations cause FWM power to change in time. These FWM fluctuations have fast and slow components (as fast as twice the bit rate and as slow as polarizatio