DEFERJRARI : FIXED-SYSTEM MEASUREMENTS OF THE TIME- VARYING MULTIPATH 

 AND DOPPLER SPREADING 



which exhibits a deep fade. The phase of the signal (shown below) 

 goes through a 180-degree phase jump at the fade. For the pulse (bottom 

 figure) a small notch forms on the right side of the pulse which 

 slides across the pulse with increasing time. Precisely in the middle 

 of the CW fade the pulse power is also very low. Traveling with the 

 notch is a 180-degree phase jump. 



What this says about the CW fade is that at the instant of the 

 deepest part of the fade, the energy is equally split into two components 

 which are 180 degrees out of phase with each other and hence cancel. 

 On the other side of the fade the resultant vector shows up 180 degrees 

 reversed from before the fade. The only way this can happen is if 

 the perturbation that's causing it is causing all the arrivals — 

 there are 15 arrivals in the pulse — to slide relative to each other. 

 So it appears to be a broad-scale process rather than a localized 

 fluctuation. 



Figure 21 shows a model simulation where the gradient shifts slow- 

 ly with time. The pulse response is in the left column and the phase 

 is in the right column. A small notch forms in the pulse and slides 

 across the pulse, notching it out. Traveling with the notch is a 

 180-degree phase shift. These results contain 15 arrivals each with 

 slightly different travel times. 



An alternate representation of this fading is shown in Figure 22 

 in terms of a series of measured power spectra of successive pulses. 

 The carrier is the center line at 420 Hz. What happens here in time 

 is that we are going through a CW fade. Transmission is falling off 

 and coming back up again on the carrier. For the full spectrum it is 

 apparent that the fade slides across the band resulting in selective 

 fading. 



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