ted until the Bit Error selective fading simply by substituting DFM for Fade Margin in the Ratio (BER) of the radio degrades to 10-3. The phase shifter is then equation normally used to calculate the outage time of analog radios adjusted SO that the notch appears at a different frequency and again due to fading. This allowed existing path design programmes to be the notch depth is adjusted for a 10-3 BER. This process is repeated easily modified for use on digital radio paths. In addition, it allows Variable attenuation Transmit Receive coupler coupler Fixed Variable Phase delay attenuation shifter Figure 50.20 Simulation of a multipath notch 50/10 Line of sight radio systems the susceptibility to selective fading of different radios to be com- The Composite Fade Margin may now be substituted for Fade pared on a one number basis instead of an entire signature curve. Margin in the equation for Fractional Annual Outage Probability as Dispersive Fade Margin is calculated using Equations 50.8 and in Equation 50.12. 50.9 from a Bellcore Technical Advisory, where B and B (f) represent the minimum and non minimum phase signature curves (50.12) respectively, and N is the number of f wide segments into which the curve is divided, in order to calculate its area. For example, with CFM = 35dB, average terrain (c=1), average (50.8) climate conditions (b=0.25), f=18.5GHz, and a path length of 10 miles, the resulting Outage Probability is, P=3.66x106. This is equivalent to 115 seconds per year. This outage time is due to the combined effects of Flat and Dispersive fading because we used (50.19) CFM in the calculation. 50.2.2.4 Conclusions about DFM For example, if the signature curve of Figure 50.21 for the 45Mbit/s radio is broken into 33, 1-MHz wide segments the result When designing 18 and 23GHz paths, Dispersive Fading may is as in expression 50.10 dominate when using the highest data rate radios (DS3, 45Mbit/s), with r.f. frequencies in the 18GHz band, in areas of the country that (50.10) are particularly dry (low rain rat