odel is placed before the threshold device is substituted by a discrete time integrator. In this digital receiver model, electrical current sampling happens at a rate of 1/T sam- ples a second where To has to be equal or shorter than the inverse of the unilateral optical filter bandwidth Bo, if the circuit has to fulfill the Nyquist sampling condition. Due to the Nyquist condition, no information related to the signal is lost due to sam- pling, SO that, for the performance evaluation model, adding thermal noise after the sam- pler and not before it as physically happens makes no difference. The optical signal incoming on the photodiode after optical filtering can be expressed in a useful form by introducing the so-called field quadratures. Let us select a reference frame to express polarization vectors; if PMD has to be taken into account, it is useful to consider as a base the output PSP, otherwise the base of the lin- early polarized modes can work; in any case, let us call p and p the two base unit vectors. Front Integrate and dump Sampling Decision Front Digital Sampling filter Decision FIGURE 6.19 (a) Analog model of the receiver detection chain and (b) digital model of the receiver detection chain. Telecommunication Networks Naturally the field component along each of these vectors will be a complex function: the set of four real functions constituted by real and imaginary part of the field components are called field quadratures along the chosen base [19]. Setting (6.55) the four field quadratures that we will indicate as E(t) with S = 1, 2, 3, 4 can be written as - N/2 (6.56) where the selected channel corresponds to k=0 and nk(t) are band-limited Gaussian pro- cesses summarizing ASE accumulation and any other effect that can be described as a Gaussian noise-whose bandwidth is limited by the optical selection filter as FWM and All the propagation effects causing signal distortion are summarized in the form of the received bit pulse gik(t). The pulse shape depends both on the position of the pulse in the b