M-DPSK M-4QAM PoISK M-4QAM CP M-4QAM FIGURE 9.45 Sensitivity of different multilevel modula- tion formats in ASE-limited conditions at Number of level (log2(M)) BER = 10-4. Telecommunication Networks In order to have better trends, it seems that either the number of degrees of freedom has to be increased, or the space metrics has to be changed (passing, e.g., from a plane to a sphere), the first step being more effective than the second. 9.3.5 Coherent Receivers Sensitivity to Phase and Polarization Fluctuations At the end of this section devoted mainly to the performances of multilevel transmission systems, it is necessary to consider an important element regarding coherent detection that comes into play any time the transmission system cannot be considered ideal. We will take as a reference the receiver presented in Section 9.3.3.1, that is, the univer- sal quadrature receiver, but what we will present in this section can be adapted at almost any coherent receiver with marginal modifications to take into account the receiver structure. The main advantage of coherent detection is that the set of electrical currents at the out- put of the receiver are proportional to the field quadratures, but for the noise. This prop- erty allows information to be coded into any degree of freedom of the optical field thus fully exploiting its transmission potential. On the other hand, this also means that any form of random fluctuation of the field is detected by the receiver and constrains its performances. Here we will consider the two main noises that can limit the performance of a coherent receiver while in a direct detection receiver these noises can be generally neglected: the phase noise and the polarization noise. 9.3.5.1 Phase Noise Penalty for Coherent Quadrature Receiver Several causes can generate phase noise on the optical signal before detection: generally, the most important are the frequency fluctuations of the transmitting laser and of the LO and the nonlinear phase noise coming during propagation from Kerr effe