s detected. In particular, Pk indicates the reflec- tance of the surface for circular or unpolarized light, Kk exp(i):) is the ratio between the complex reflection coefficient for linearly polarized light along parallel and perpendicular directions to the local incidence plane. In all the cases we have considered up to now, a direct detection polarimeter produces four photocurrents that changes while the polarization of the received field changes. It is possible to demonstrate that in all the cases we have considered, there is a linear relation between the Stokes parameter and the currents at the output of the polarimeter that does not depend on the received polarization, thus it is possible to characterize the polarimeter with a matrix A SO that, if the four currents at the polarimeter output are collected in a vector C, we can write [58,59] (9.46) Since the current at the output of the electrical demodulation in the coherent receiver shown in Figure 9.34 is also proportional to the Stokes parameters and the proportionality is a particular type of linear dependence, we will assume Equation 9.46 as valid for any type of optical front end in an M-PolSK receiver. As far as the electrical processing section is concerned, it is in charge to perform both fiber polarization tracking and, in the coherent case, of generating the driving signal for the frequency lock of the LO. Different algorithms have been proposed for these functions and we will not review them in detail. Some algorithms use the same ideas we explained dealing with the quadrature receiver. In this case, the modulation is eliminated by using a decision driven algorithm that inverts the modulator operation starting from the estimate of the transmitted symbol [35]. Other algorithms use the idea that the constellation can be designed SO that the average value of the Stokes vector over the entire constellation is not the null vector. Next Generation Transmission Systems In this case, the modulation can be eliminated by simply averaging the modulated field o