es). At intermediate data rates, 19Mbit/s for example, in the same dry climate at 18GHz, the dispersive outage may be about the same as Since the minimum and non minimum phase signatures are the the rain outage. At the lower data rates (DS1, DS2) rain outages should dominate in all climates. same, Sw is twice this value, 1.876. Putting this into Equation 50.8 results in DFM = 36.9dB. It also appears that rain outage will still be the largest factor in all 23GHz paths. When Dispersive Fading is dominant, increasing FFM beyond 6dB above DFM will not significantly improve outage 50.2.2.3 Composite fade margin time. When Rain Fading is dominant, increasing FFM beyond this point will improve outage time. The significance of DFM depends on its value relative to the normal In summary, the Dispersive Fade Margin and specification is not system fade margin which is often referred to as Flat Fade Margin as significant at 18 and 23GHz as it is at 2 and 6GHz because a (FFM). To see this, DFM and FFM are combined into a single term typical path's outage time is still dominated by rain fading at the known as Composite Fade Margin (CFM) using Equation 50.11. higher frequencies. CFM = - (50.11) 50.3 Digital and analogue microwave A plot of CFM vs. FFM for DFM = 30 and 40dB is shown in systems Figure 50.22. Note that when FFM = DFM the Composite Fade Margin is 3dB less than FFM or DFM. Also note that increasing Though it is in everyday use in the Common Carrier Services, FFM beyond DFM + 6dB results in very little further improvement digital microwave and multiplex is beginning to replace analogue in CFM. microwave and multiplex in the Private Operational Fixed Micro- wave Services. The Private user, however, may question the need to replace his analogue system or to acquire a new digital system. There are system cost differences and system performance dif- ferences. This section will address mainly the system performance differences. DFM=40dB Hybrid systems transmitting data on analogue voice channels on analogue microwave are n