gular frequency NF is the amplifier noise factor [E(t) + n(t)] ho(t) [E(t)+n(t)] ho(t) ) 2hh(t) E(t) + n(t) [E(t)+n(t)]@ h,(t)|| Equivalent Photodiode Equivalent optical square law electrical Decision filter detection filter Photocurrent electrical filter Signal noise spectrum optical filter FIGURE 5.70 Definition of the formal receiver model used for the FEC performance evaluation and of the related bandwidths. Telecommunication Networks and the unilateral power spectral density So does not present any factor beyond the terms related to amplifier noise On these grounds, the power of the electrical noise current corresponding to the optical noise is easily evaluated as (5.52) where E,(t) is the received field average n(t) is the optical noise process whose average is supposed to be zero Po, So are the signal power and the noise power spectral density, respectively The electrical bandwidth is called Be and it is often approximated with the symbol rate R and the optical bandwidth (unilateral as defined in Figure 5.69) is called Bo. Let us introduce the parameter = > 0.5. It is the ratio between the electrical and the optical bandwidths, thus it is also the number of samples at the Nyquist sampling rate of the optical signal that fit into the Nyquist sampling interval. This parameter will be quite useful in the analysis of transmission systems. As a function of M the electrical SN may be written as (5.53) where the approximation is valid in conditions of large SN, SO that it can be written as SNo > 1. In this case the SNs are equal if the electrical bandwidth is equal to half the uni- lateral optical bandwidth. This relationship is plotted in Figure 5.71 for different values of and it is the relation that has to be used to evaluate the code gain in systems with Gaussian noise. It is to be noted that Figure 5.71 is plotted with SNo as an independent variable; decreas- ing at constant SNo means that, for a constant value of the optical bandwidth, the elec- trical bandwidth shrinks SO that less optical noise power is d