PDs is more extreme: generally an EPD is designed to compensate not only linear effects but also nonlinearities due to fiber propagation. For this reason, the signal distortion caused by an EPD is much more complex than that caused by an EBC. Telecommunication Networks 5.8.3 Electronic Dispersion Post-Compensation Among all the electronic distortion compensators, EDCs are the most diffused. Essentially there are three types of EDCs: 1. Continuous-time filters (CTFs) 2. FFE/DFE 3. Maximum likelihood sequence estimator (MLSE) equalization The working principles of CTFs are quite simple: the bandwidth of the receiver is sliced in a certain number of sub-bands and every sub-band is multiplied by a complex number. A feedback circuit regulates these multiplication factors SO as to maximize the eye opening or (in case an FEC is mounted in the receiver) the estimated error probability. This is a simpler type of EDC, but also the lowest performing one, and is going to be substituted in new systems by more performing algorithms. The most diffused EDC is the FFE/DFE. 5.8.3.1 Feed-Forward/Decision Feedback Equalizer The idea at the basis of the FFE/DFE is to compensate the dispersion via digital filtering of the received signal. The electrical current after detection can be in general written as )=R,Eb,8,(t-jT)+n(t) (5.44) where b;=0.1 is the transmitted bit gi(t - jT) is the received pulse, whose shape depends on the position in the bit stream due to the presence of dispersion T is the bit interval Assuming that the pulse received in the absence of distortion would be f(t) and neglecting the pulse shape dependence on the position in the bit stream, the distortion equivalent fil- ter Da(a) is a filter whose characteristic is to transform the ideal pulse in the real one, that is (5.45) where D(t) is the inverse Fourier transform of Da(a) The distortion equivalent filter is naturally only an approximation of the real channel behavior, which is nonlinear at least due to the photodiode. However, once Da(a) is intro- duced, the d