it stream, due to the influence of the nearby bit through dispersion and similar effects introducing memory in the fiber response, and on the position of the considered channel in the WDM comb, due to the interference with nearby channels. The photocurrent is easily evaluated on the ground of the expression of the field quadra- tures. After sampling, a sample of the photocurrent can be expressed as (6.57) where ny(t) comprises all the receiver noise (i.e., thermal noise, dark current noise, and in case other noises introduced from the receiver). The digital integrator performs the sum of a certain number of samples to produce the final value to compare with the threshold. Let us call M the number of samples summed up by the integrator. The integration bandwidth, that is by definition the electrical band- width B, can be evaluated as the inverse of the integration time. Thus, B = (MoeTo) where T is the sampling rate. Transmission Systems Architectures and Performances Since the bandwidth of the photocurrent before electrical filtering is Bor due to the Nyquist condition we can assume a sampling rate To = 1/B. It is to be noted that in real receivers, the Nyquist condition is often not fulfilled, due to the fact that it is not necessary to reproduce exactly the shape of the pulse, but it is enough to estimate the energy in the bit time. It derives = Bo/Ber where represents the ratio between the unilateral optical band- width and the unilateral electrical bandwidth (frequently approximated with the bit rate R) and it is the same parameter we introduced to relate the optical and electrical SN ratio in Chapter 5. Let us also remember a convention we have done in Chapter 5 and that we will use throughout the book. The noise spectral density in the optical bandwidth So is defined SO that multiplied for the unilateral optical bandwidth B gives the noise power. Thus, in the case of a single EDFA amplifier So(a) = hw NF(G - 1) being NF the amplifier noise factor (this is doubled with respect to the more common definition f