ntegrated circuits (ASICS). Increasing the number of levels, in order to transmit 100 Gbit/s using 512-4QAM with constant power (obtained by selecting 512 levels from the 600-4QAM reported in Table 9.4), the required sampling rate goes down to about 55 GHz, a speed of the order of that used in experimental systems with electronic nonlinear precompensation (experiments with sampling rates around 50 GHz are reported as an example in Chapter 6). After baseband filtering, the four photocurrents are proportional to the quadratures of the field arriving at the receiver, but for the shot noise contribution. A part the ASE noise, the quadratures of the received field, is related to the transmitted field quadratures by the fiber propagation characteristics causing chromatic dispersion, polarization rotation and PMD, and nonlinear distortions. Let us assume in a first moment that fiber propagation is perfectly linear. The role of the electronic postdetection processing is to compensate as much as possible linear propagation effects to recover the transmitted signal quadratures, to drive the fre- quency lock of the LO, and to estimate the transmitted symbol. These functions can be recognized in the scheme of Figure 9.29b, where fiber-induced polarization rotations are compensated by multiplying the received quadratures by the inverse of the fiber polarization rotation matrix (the Jones matrix) in the Jones Matrix Inversion (JMI) block, and pulse distortion by residual chromatic dispersion and PMD are compensated via the Pulse Equalizer (PE). Next Generation Transmission Systems The Jones matrix evolution at the first order (i.e., without the W dependence that causes PMD) is tracked by the Jones Matrix Tracking (JMT) block, while the frequency lock loop driving signal is generated by the Frequency Control Circuit (FCC); finally the transmitted symbol is estimated by the Decision Circuit (DC). The key elements of the electronic section of the receiver are the JMT and the FCC; elec- tronic techniques to compensate linear pulse