. On the other hand, since it is applied on the signal after detection, even if fiber propagation is perfectly linear, the channel as it is viewed by Transmission Systems Architectures and Performances Penalty (dB) With pre-chirp No pre-chirp FIGURE 6.9 Dispersion related power penalty of a 10 Gbit/s system using NRZ modulation a SSMF fiber and pre-chirping technique. System parameters are reported in Table 6.1 but for fibers parameters that are reported in Table 4.1. (After Boyd, R.W., Nonlinear Optics, Academic Press, San Diego, CA, 2008.) Link length (km) the electronic dispersion compensator (EDC) is a nonlinear channel due to the presence of square law detection carried out by the photodiode. Since a large number of EDC are digital linear filters, we can expect only a partial compensation. Of course, this is not true for nonlinear compensation compensator, that in principle can attain much better performances and that, due to the advance in microelectronics technol- ogy, is for the first time a practical possibility. To have an idea of the possible performance of feed forward equalizer-decision driven equalizer (FFE-DFE) compensators, in Figure 6.10 the optical signal to noise ratio SN needed for a BER of 5 X 10-4 (i.e., the requirement for systems using a standard forward error correcting code [FEC], see Section 5.8.5.5) is plotted versus distance for propagation over an SSMF and detection using a PIN photodiode [17]. The FFE-DFE compensator allows the dispersion limit (about 36km in this case with- out dispersion compensation) to be pushed up to about 70km in this case, and slightly better results are reported in literature. In the figure also the performances of a Viterbi algorithm-based maximum-likelihood-sequence-estimator (MLSE) compensator are reported. As expected, it is more effective in compensating extra errors coming from sig- nal distortion, pushing the dispersion limit in this case up to about 90 km. Optical signal to noise ratio (dB) No comps FFE (6) FFE (4) DFE (2) FIGURE 6.10 OSNR needed for