y matrix like M and it can also be interpreted as the random rotation induced by the combination between linear and nonlinear effects. The expression of the photocurrent is formally similar to that derived for the case of phase noise, thus following the same derivation and assuming perfect linear polarization tracking yields (9.82) where SN eo is the ideal value of SN in the same circumstances. The effect of depolarization is very strong, as shown in Figure 9.47, introducing a plateau in the SN. 9.4 Alternative and Complementary Transmission Techniques We have seen that multilevel modulation formats constitutes a very promising transmis- sion technique to design long distance very high capacity optical links. Besides multilevel modulation, there are a set of transmission techniques that can fur- ther improve the system performances, either helping to decrease multilevel sensibility to some transmission impairment or providing the way for a further ideal performance improvement. The most important of these techniques is Orthogonal Frequency Division Multiplexing (OFDM), a technique for low speed channels multiplexing that simultaneously greatly reduce the sensitivity to linear causes of ISI and increase sensibly the bandwidth efficiency exploitation. Besides OFDM, PDM and polarization diversity are often used to exploit the polarized nature of the electromagnetic field without an explicit polarization modulation and decrease the sensitivity of the system to channel interference, respectively. Telecommunication Networks Also pulse polarization diversity is sometimes used, especially in RZ systems, to pre- vent the jitter caused by interaction between nearby pulses. In this section, we will briefly introduce all these techniques. 9.4.1 Orthogonal Frequency Division Multiplexing OFDM is used extensively in broadband and radio communication because it is an effective solution to ISI caused by a dispersion during signal propagation [66]. This becomes increasingly important as data rates increase to the point that the m