Handbook of Propagation Effects for Vehicular and ... - Courses

Earth-Satellite Propagation Effects Inside Buildings 8-33
Vogel and Torrence [1995a, 1995b] have characterized the short-term frequency
variability as a function of the fade slope dS/df expressed in dB/MHz. This descriptor,
which characterizes the change in the received power level over a 1 MHz bandwidth, is
quantified in Figure 8-12. Values of the mean fade slopes range between 0.5 to
1.3 dB/MHz for the various building locations. These slopes should only be used over
bandwidths of approximately 1 MHz.
8.7.2.2 Cumulative Distribution Dependence on Frequency
Vogel and Torrence [1993] developed a model giving the frequency dependence on the
cumulative probability distribution of the relative signal loss for each of the sites. This
model is given by Equations (8-1) through (8-3) where tabulations of the indicated
coefficients are listed in Table 8-5.
8.7.2.3 Distortion Due to Bandwidth
Vogel and Torrence [1993] described the signal distortion in terms of the standard error
relative to the mean signal loss over bandwidths ranging from 2 MHz to 90 MHz. Figure
8-6 shows that at 2 MHz, the standard error is 0.5 dB whereas the deviations range from
2.5 to 3.5 dB at 90 MHz. These results imply that relatively small distortions of the
signal should be expected for bandwidths between 1 to 10 MHz. These results are
consistent with time delay calculations as given in Figure 8-2.
8.7.2.4 Frequency Correlation
The autocorrelation of the relative signal loss was described as a function of the lag in
frequency from 0 to 20% of the frequency (Figure 8-20) by Vogel and Torrence [1995c].
The median decorrelations (between 0.5 GHz and 3 GHz) were found to vary between
1.2 to 20.2% of the frequency with an overall average of 5.4%. Excluding the extreme
case (Farmhouse), the median decorrelation varied between 1.2% and 3.8% with an
overall average of 2.4% (Table 8-16). Such information is useful for establishing the
efficacy of uplink power control at one frequency given knowledge of the power at
another frequency.
8.7.3 Fading Effects Due to Antenna Position
8.7.3.1 Antenna Spacing between Maximum and Minimum Signal Levels
Vogel and Torrence [1993] determined the median antenna spacing between antenna
positions giving maximum and minimum relative signal levels. The median position was
found to be between 35 and 45 cm independent of frequencies between 0.7 and 1.8 GHz.
The example in Figure 8-5 shows signal variability approaching 30 dB over this position
interval. The spacing interval (between minima and maxima) was found to be Gaussian
distributed with a mean of approximately 41 cm and a standard deviation of 17 cm. This
mean distance suggests an optimum spacing that might be employed for space diversity
systems.