Gbit/s is around 28. In Figure 6.13 the sensitivity (input optical power needed to attain a BER of 10-12) is shown versus the APD gain in the absence of dispersion and nonlinearity. In this ideal condition, the sensitivity gain due to the APD is around 9 dB, passing from a PIN sensitivity of about -24.5 dBm to an APD sensitivity of about -33.5 dBm. The impact of the other performance decreasing factors on APD receivers are not ferent from what we have seen in the case of the PIN receiver, but for the sensitivity gain. The only factor that is sensibly impacted from the presence of an APD is the role of SPM. As a matter of fact, for a fixed fiber link, the presence of an APD at the receiver allows the optical power to be maintained much lower, SO reducing the effect of SPM. 6.2.5 Performance of Non-Regenerated Return to Zero Systems Considering RZ systems, the performances in condition of ideal propagation are the same with respect to NRZ, since they depend only on the signal and noise energy received in the bit interval. The situation is completely different in the case of presence of dispersion. In the case of chromatic dispersion, two things happen when decreasing the pulse width. On one side, the dispersion is more efficient, the relative pulse broadening being bigger, on the other side, the pulse does not occupy the whole bit interval; thus, some broadening can be accepted if no energy is transferred in the nearby intervals. As usual, when two different phenomena tend to balance we have to expect an optimum value of the pulse width at which the penalty due to chromatic dispersion is minimum. As we have mentioned in Chapter 4, the pulse propagation problem in the presence of chromatic dispersion can be solved exactly in the case of Gaussian pulse. In this case, it is possible to derive the following expression of the output pulse root square medium width (that is also half the width at 1/e below the maximum), obtaining [2] (6.38) Transmission Systems Architectures and Performances that is Equation 6.17 in the l