e to time availability: Per-hop time availability = 1.0000000 - 0.0000555 = 0.9999445 or 99.99445%. We recommend that for digital links, an additional 2 dB of fade margin be added to the Rayleigh values to compensate for path anomalies which could not be handled by automatic equalizers. The most common cause of fading is called multipath. Refer to Figure 9.14. As the term implies, signal energy follows multiple paths from the transmit antenna to the receive 9 Other texts call this "reliability." The use of this term should be deprecated because it is ambiguous and confusing. In our opinion, reliability should relate to equipment failure rate, not propagation performance. CONCEPTS IN TRANSMISSION TRANSPORT Ground surface A = layers of different refractive index Figure 9.14 Multipath is the most common cause of fading. antenna. Two additional paths, besides the main ray beam, are shown in Figure 9.14. Most of the time the delayed signal energy (from the reflected/refracted paths) will be out of phase with the principal ray beam. These out-of-phase conditions are what cause fading. On digital links we also have to concern ourselves about dispersion. This is delayed signal energy caused by the multipath conditions. Of course, the delayed energy arrives after the main ray beam pulse energy, spilling into the subsequent bit position, greatly increasing the probability that the bit position two will be in error. This is called intersymbol interference (ISI). To achieve the fade margin, we must overbuild the link which will increase the RSL above that of the RSL if we just designed the link to meet the objectives of the unfaded conditions. We must add to the link margin that number of decibels indicated by the Raleigh fading table above, plus the recommended additional 2 dB. Probably the most economic way to overbuild a link is to increase the antenna aperture. Every time we double the aperture (i.e., in this case, doubling the diameter of the parabolic dish), we increase the antenna gain 6 dB [see Eqs. (9.17a) and (9.17