25. For the only three cases in which the optimum constellation is approximately known, it is evident that the gain in terms of minimum distance is relatively small. This will bring us to consider from now on the regular configurations listed in Table 9.3. For each regular constellation, the power spectral density of the transmitted signal along two reference orthogonal polarizations can be evaluated with standard methods and the complete expression is reported in [37]. The way in which the transmitted power is divided among the two polarization components depends obviously on the choice of the polarization reference frame and, if the field at the output of a transmission fiber is considered, on the fiber instanta- neous Jones matrix. However, the power spectral density in any case exhibits a central zone comprised between the first two spectrum zeros, which are always symmetric with respect to the origin, where almost 92% of the power is comprised. The two considered zeros are at W = + 2R/M thus confirming also for M-PolSK the general rule that the unilateral optical bandwidth can be assumed equal to Bo = R/M SO that the spectrum width is 2R/M. Under the point of view of modulation feasibility, a simple polarization modulator is needed to produce a PolSK signal. A polarization modulator can be realized using elec- tro-optics materials exactly the same way a phase modulator is realized (see Chapter 5) [36,40]. Polarization modulation can be also implemented in conjunction with differential encoding. In this case, the information is coded into the change of the polarization vector in the Stokes space between two adjacent symbol intervals [41,42]. This further coding does not alter the structure of the transmitter, but for the addition of the differential encoder, and in a few cases is quite useful to simplify the receiver. Next Generation Transmission Systems 9.3.2.4 Multilevel Four Quadrature Amplitude Modulation (M-4QAM) This modulation format exploits the possibility of modulating all the optical field degrees 