DYER: FLUCTUATIONS: AN OVERVIEW 



probability density represents data taken over 2 days. The individual 

 densities are somewhat skewed as expected, but the individual densities 

 change with time. While the probability density of either entire 

 group has not been generated, the supposition is that it may approach 

 in and of itself a Gaussian distribution. Note that the means of the 

 distributions change with time as do the standard deviations. 



The final figure (Figure 12) shows one possible way to treat 

 this. The solid curve corresponds to a single population consisting 

 of phase-random multipaths. It is skewed with the 2.5-dB depression 

 in the mean. (That is, the most probable value is 2.5 dB higher than 

 the mean value.) If seven such processes are added, uniformly spaced 

 with a spread in means of 6 dB, the resulting distribution is easily 

 integrated (since it consists of a sum of delta functions) and leads 

 to the dashed curve in Figure 12. Two things have happened: First, 

 the standard deviation has increased beyond 5.5 dB (the dotted curve 

 is broader than the solid curve); and, second, there is less skew and 

 peakedness in the distribution. In general, as the spread becomes 

 larger, the dashed curve becomes more and more Gaussian in nature. 



SUMMARY 



In conclusion, there are many measures of fading. It is going 

 to be important to recognize various regimes of time for time series 

 and space for space series. It is equally important to indicate which 

 fluctuations are averaged out and which are included through the 

 length of the record. 



The understanding of the sub-processes is quite far along; how- 

 ever, it is difficult if not impossible to include everything that 

 is observed. A more likely approach is to formulate very clear 

 statements about the particular process being investigated at a 

 particular time, recognizing the diverse underlying mechanisms. 



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