 and 26.2 nm optical bandwidth over SSMF. The system can support 64 channels with 50 GHz spacing. The Q factor is plotted versus the spectral position of the selected channel in Figure 6.24. From the plot it is clear that chirped RZ is efficient in taking under control the interplay between dispersion an XPM/SPM. On the other hand, when DWDM is considered, the wider optical spectrum of RZ mod- ulation impacts on channel interference. An example assuming ideal modulation and demodulation with square filters can clarify the effect. The linear sensitivity penalty for the central channel of an NRZ 10 Gbit/s comb with a spacing of 50 GHz is 1 dB, while in the case of RZ transmission with 30% duty cycle it is as high as 4 dB and to reduce the penalty to 1 dB the channels have to be separated by a gap of about 120 GHz. In-line dispersion management can also be combined with pre- or post-electronic dis- persion compensation. Careful system design can achieve good results with time domain electronic compen- sators like FFE-DFE, but better results can be attained especially at high speeds using a more advanced solution. It consists in a linear EDC working on the baseband electrical pulse before modulation. It consists of three steps: FFT, multiplication for the inverse fiber frequency response, and inverse FFT [39]. This solution requires electronic processing with a very high sampling rate, arriving up to 80 Gsamples/s if applied to channels at 40 Gbit/s and it is possible only with the more recent generation of CMOS circuits. An example of comparison between a system exploiting standard pre-compensation via DCFs and a system substituting the DCF at the transmitter with the EDC is reported in Figure 6.25 [40]. At 10 Gbit/s the performances of the traditional compensation method is superior, but at 40 Gbit/s this is no more true, due to the better accuracy with which the electronic control manages very-high-speed signals compensating the dispersion channel by channel. A different technique based on electronic pre-compensat