0.07504 0.06898 1.0995 1.0663 0.1237 0.1125 1.0604 1.0308 0.1864 0.1673 1.0202 0.9974 0.2632 0.2341 0.9789 0.9630 0.3504 0.3104 0.9394 0.9293 0.4426 0.3922 0.9040 0.8981 0.5346 0.4755 0.8735 0.8705 0.7039 0.6347 0.8266 0.8263 0.8440 0.7735 0.7943 0.7948 0.9552 0.8888 0.7719 0.7723 1.0432 0.9832 0.7557 0.7558 1.1142 1.0603 0.7434 0.7434 1.2218 1.1766 0.7255 0.7257 1.3293 1.2886 0.7080 0.7091 1.4126 1.3764 0.6930 0.6948 1.3737 1.3665 0.6862 0.6869 1.3163 1.3059 0.6840 0.6849 Microwave Propagation Atmospheric multipath Smooth Surface Figure 3.51 Multipath fading signal. The received indirect signal consists of the signal reflected from the surface of the earth and atmospheric layers. When the direct and indirect signals arrive at the receiving antenna with a half-wavelength phase difference, fading will take place. The severity of fading depends upon the amplitude and phase of the indirect signals. Thus, the probability of fading exceeding a given fade depth depends upon two factors: (a) Amplitude of the indirect signals (b) Percentage of time for which fading is present Due to the interaction between the direct and indirect signals, the resulting signal will consist of con- structive peaks and destructive troughs. There are two types of fading here, minimum and nonminimum delay phase fading. Assume that the direct signal level is unity and when the amplitude of the indirect signal is less than unity, e.g. b, then the 'notch' depth B (shown in Figure 3.52) would be given as B = -2010g10(1-b) (3.50) If the indirect signal has an amplitude greater than unity, then the amplitude of the peaks and troughs will be b + 1 and b - 1 respectively. The former is called minimal phase fading while the latter is called nonminimal phase fading. For a given notch depth and the frequency distance from it, there is a critical fade depth which, if exceeded, would result in a BER greater than the threshold level. In detailed link budget calculations, the term 'signature' is used to take in the effect of the above-mentioned phenomenon. 