timum performances are achieved by a joined design of the MLSE and the FEC SO that the interaction between the two algorithms takes into account the correct noise statistics and optimizes the performances of both FEC and MLSE [87]. At present this design is greatly constrained by memory requirements both in terms of memory quantity and memory-processor communication speed. However, it is possible that further progress in electronics will make this technology more accessible. 5.8.4 Pre-Equalization and Pre-Distortion Equalizers A simple technique to avoid the intrinsic limitation of post equalization due to the pres- ence of the nonlinear photodiode response is the implementation of electronic pre- compensation, that is, signal distortion in the transmitter before propagation. This technique can be implemented, for example, by using an I/Q modulator, or a dual- drive Mach-Zehnder modulator. The simpler scheme of signal pre-distortion that is meant for compensation of chromatic dispersion impairments is shown in Figure 5.63. The principle of operation of this circuit is simple: the modulator is driven by two sig- nals that are built in such a way that the modulator output coincides with the signal to be transmitted filtered by a filter whose spectral response is the inverse of the fiber dispersion spectral characteristics. In general, this way of compensating linear distortion is prone to several mathemati- cal difficulties related to the fact that the inverse frequency function can be impossible Telecommunication Networks Modulator Receiver FIGURE 5.63 Block scheme of an electronic dispersion pre-compensator. to define when the distorting filter has a big attenuation, SO as to almost cancel the signal at those frequencies. Fortunately this is not the case with fiber dispersion, whose char- acteristic function is a pure phase filter, whose module is equal to 1 at all frequencies. Thus, the driving currents for the modulator can be easily evaluated with simple calcula- tions [88]. The pre-distortion filter implement