ess area, where distances are not SO long and the cost is a key issue. In addition to phenomena impacting the single channel, channel crosstalk is another relevant degradation source in WDM systems. Assuming an NRZ signal, the received field can be written as follows: (6.39) here k is the channel index that is assumed to run from -N/2 to N/2 being N+1 the chan- nel number. Since the channels interact one with the other, the received pulse in the jth bit interval, not only depends on its position in the bit stream, but also on the considered channel; this is the reason why the pulse gjk(t) has both the channel index k and the slot index j. The channels will be asynchronous in time, since their group velocity is different, SO that the propagation delay TK brings the channel index. Similarly to the delay, the polarization also will be different from channel to channel, justifying the channel index on the polarization unitary vector Last but not least, the detected power also depends on the considered channel, due to the nonuniform wavelength response of optical components, and it is labeled as Pk. In the usual hypothesis that the transmission impairments are small and can be treated as perturbation of the ideal system, we will go on analyzing the interference in form of a power penalty, to add to penalties coming from other sources. Two kinds of interferences are potentially present in WDM systems: linear and nonlin- ear interference. The term linear and nonlinear depends on the fact that the first type is always present, even when linear propagation can be assumed, while the second is related to nonlinear fiber propagation. Transmission Systems Architectures and Performances 6.2.6.1 Linear Interference in Wavelength Division Multiplexing Systems Linear interference arises due to the fact that there is an overlap between adjacent channel optical spectra, SO that the power of a channel unavoidably interferes with the adjacent ones, as shown in Figure 6.15. This effect has to be limited by optical filtering, since aft