The second quantity discussed above is called the average fade time duration (AFTD) which is
defined as the average time the received signals??™ envelope is below the normalized specific signal
level, R. The mathematical definition is given as
(3.17)
(3.18)
where T the total time under consideration. The resulting expression is given as
(3.19)
A quick observation of this result shows the AFTD is inversely proportional to the vehicle speed
or Doppler frequency. For example, for fast fading vehicle speeds, the average time the signal is
below the specified value is small when compared to the case when a slow moving vehicle is
considered.
Given the above relationships we will choose two values of Doppler frequencies to calculate some
typical LCR and AFTD quantities. Let us assume two Doppler frequencies, fm200 Hz and fm50 Hz.
Also assume our interest lies in the statistics centered around a normalized 20 dB fade. Below we plot
both of the quantities of interest where we extract the 2 points to compare against.
Using the plot and the previous LCR definitions (see Fig. 3.7), it is easy to extract the LCR for a
20-dB fade. For maximum Doppler spreads of 200 and 5 Hz, we encounter this fade level for 49.
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