s cause the received sig- nal to arrive via many separate paths. Figure 50.18 is a simplified Refracted ray Direct ray Fresnel zone Transmit Receive clearance Figure 50.17 DB from free space Figure 50.18 Multipath fading are lowered, portions of the wavefront will be obstructed by the representation of multipath fading where only two rays are shown. earth's curvature or an obstacle in the path. As cancelling zones are Since the two rays follow different paths, the distance travelled is obstructed the signal will increase and as phase zones are obstructed different and the two signals may be out of phase. The result is a the signal will decrease. When the obstruction reaches the edge of frequency dependent attenuation characteristic like that in Figure the first Fresnel zone, the signal will be twice that in free space. 50.19. For this reason multipath fading is often referred to as Raising or lowering the towers at this point will decrease the signal. frequency selective, or dispersive fading. Short haul and millimetre path design 50/9 at a number of frequencies resulting in a curve like that in Figure 50.21, known as a signature curve. The signature curve shows the depth of notch required, at a particular frequency relative to the centre of the r.f. channel, to cause the BER to degrade to 10-3. 45Mbit/s, 4-FSK 19Mbit/s, 4-FSK Freq. (MHz) Figure 50.19 Selective fading attenuation characteristic (20dB notch) The exact shape of the attenuation notch depends on the relative delay and attenuation of the two paths. The difference in level between the two rays determines how deep the notch is while the Freq. (MHz) difference in phase determines the frequency where the notch OC- curs. In addition to causing a notch in the attenuation characteristic, multipath causes distortion in the differential delay characteristic. Figure 50.21 Signature curve When the delayed ray's amplitude is smaller than the main ray's, a notch in the differential delay occurs. This is referred to as a minimum phase fade. A non-minimum phase fade