s low, a constellation with a lesser number of degrees of freedom can perform sufficiently well. 9.3.3 Multilevel Modulation Receivers Since all possible optical field modulation formats can be seen as modulations of one or more of the field degrees of freedom, a receiver that is able to detect all the field quadra- tures is a universal receiver for optical modulated fields. Since the quadratures have to be detected with their own sign, to distinguish points in similar positions, but in different quadrants in the quadrature space, coherent detection needed. This is performed by beating the incoming signals with a local laser (called local oscillator [LO]) at a slightly different wavelength to obtain an intermediate frequency sig- nal that can be processed with standard intermediate frequency (IF) electronics. This has also the advantage of allowing very effective electronic compensation of fiber propagation effects, first of all polarization fluctuations. Minimum normalized distance (d/\P) Constant power M-4QAM M-QAM M-4QAM FIGURE 9.28 M-DPSK Comparison on different modulation formats based on quadrature modulation on the ground of the mini- mum distance between couples of points. Number of levels Next Generation Transmission Systems Naturally, this will not be the optimal receiver for all the cases, and sometimes, bet- ter performances could be achieved with receivers tailored specifically for the considered modulation format. This is the case of both M-DPSK and M-PolSK, where better receivers can be designed with respect to the universal receiver. On the other hand, the quadrature receiver can be demonstrated to be the optimum receiver for both M-4QAM and M-QAM modulation formats, independently from the cho- sen constellation [28]. Thus, in this section, we will first describe the universal quadrature receiver and then specific optimal receivers for M-DPSK and for M-PolSK. 9.3.3.1 Four Quadrature Receiver The block scheme of the four quadrature receiver is reported in Figure 9.29. The scope of the receiver is t