ion distance. The value of the maximum PMD transmission distance is reported in Figure 9.10 versus the bit rate for different values of the PMD parameter. Dots are values calculated with the model of Chapter 6 and assuming linear propagation. The Telecommunication Networks 10,000 1,000 0.2 ps/km 1/2 ps/km 0.8 ps/km 1/2 FIGURE 9.10 Maximum PMD transmission distance ver- sus the bit-rate for different values of the PMD parameter. Bit rate (Gbit/s) curves are interpolated with a function inversely proportional to the bit rate through a constant. From the figure it is quite evident that the fit is very good, almost in all the interesting range of values for the PMD parameter. Thus we can conclude without a big error that the maximum PMD transmission dis- tance is inversely proportional to the bit rate. From Figure 9.10 it is also clear that the impact of PMD is very important at 100 Gbit/s: the maximum PMD transmission distance for a PMD parameter of 0.2ps/km ¹/2 is about 100 km. Also in this case, since the dependence of the PMD penalty on the bit rate is due to the relationship between the bit rate and the optical bandwidth, both adoption of multilevel modulation and coherent detection are beneficial for the same reasons discussed in the above section. 9.2.3.2 Polarization Mode Dispersion Compensation PMD can be naturally compensated both electronically and optically. The main issue in PMD compensation is the fact that it is a random phenomenon, thus requiring adaptive compensators to follow its fluctuations. Electronic compensation circuits generally have such a feature, thus being suitable for compensating PMD, besides other ISI-related impairments like chromatic dispersion. For example, in Figure 5.53, it is shown as both feedforward equalizer/ decision feedback equalizer (FFE/DFE) and maximum-likelihood estimation (MLE)-based compensators can reduce the PMD-induced penalty for 10 and 20 Gbit/s signals. Scaling the bit rate at 100 Gbit/s poses however a great challenge to electronic compensa- tion, due to the gr