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6.4 Fade Reduction
Due to Lane Diversity
We examine the extent by which fades are reduced (or increased) when switching
lanes for LMSS configurations. Figure 6-3 shows
vehicles on the inner and outer lanes, respectively, where the satellite
is to the side and the propagation path passes through the tree canopy
on the side of the road. We note that the path length through the canopy
is greater when the vehicle is closest to the tree line (inner lane). A
fade reduction should therefore be experienced by switching lanes from
the inner to the outer lanes. The authors measured this effect at UHF (870
MHz) [Goldhirsh
and Vogel, 1987], and L-Band (1.5 GHz) [Goldhirsh
and Vogel, 1989]. Repeated measurements were executed employing a helicopter
as the transmitter platform, and corresponding cumulative fade distributions
were derived for inner and outer lane scenarios at fixed path elevation
angles of 30°, 45°, and 60°. To characterize the increase in
signal power by switching from the inner to the outer lanes, a quantity
known as the "fade reduction, FR" was defined. This quantity is
obtained is obtained by taking the difference between equi-probability
fade values from distributions pertaining to inner and outer lane driving.
Figure 6-3: Mobile satellite scenario showing larger intersecting path
length through tree canopy for inner lane driving relative to the outer
lane case.
The fade reductions at UHF and L-Band are plotted in Figure
6-4 and Figure 6-5 respectively, for the
indicated elevation angles as a function of the maximum fade as derived
for inner lane driving. These curves fit the third order polynomial expressed
as
 ,
|
|
where FR (in dB) represents the fade reduction obtained by switching
from the inner to the outer lanes (Figure 6-3)
and A represents the maximum fade (in dB) derived for the inner
lane driving scenario. The coefficients in (6-4) are
tabulated in Table 6-2 and Table
6-3 for the UHF and L-Band frequencies, respectively The
"best fit polynomials" agree with the FR values derived from the
measured distributions to within 0.1 dB RMS.
Figure 6-4: Fade reduction due to switching lanes at 870 MHz versus equi-probability
attenuation at the indicated path elevation angles.
Figure 6-5: Fade reduction due to switching lanes at 1.5 GHz versus equi-probability
attenuation (inner lane) at the indicated path elevation angles.
Table 6-2: Coefficients of the fade reduction
formulation at f = 870 MHz.
|
El. Angle (°)
|
a0
|
a1
|
a2
|
a3
|
dB Range
|
|
30
|
-5.020 x 10-3
|
0.3354
|
-2.439 x 10-2
|
7.764 x 10-4
|
2-20
|
|
45
|
-0.8193
|
0.8430
|
-5.758 x 10-2
|
1.222 x 10-3
|
2-12
|
|
60
|
-0.2305
|
0.2288
|
6.773 x 10-2
|
-3.608 x 10-3
|
2-11
|
Table 6-3: Coefficients of the fade reduction
formulation at f = 1.5 GHz.
|
El. Angle (°)
|
a0
|
a1
|
a2
|
a3
|
dB Range
|
|
30
|
0.3181
|
0.26153
|
-1.573x 10-2
|
3.734 x 10-4
|
3-25
|
|
45
|
-1.073
|
0.8816
|
-4.651 x 10-2
|
7.942 x 10-4
|
3-17
|
|
60
|
-8.127 x 10-2
|
0.2044
|
5.781 x 10-2
|
-2.235 x 10-3
|
3-15
|
It is interesting to note that larger fade reductions occur at the greater
elevation angles. This arises because at the larger angles, a change of
lanes may radically alter the earth-satellite path from a shadowed to a
non-shadowed state. At the lower elevation angles, this change of state
becomes less likely. It is noted from Figure 6-5 that the L-Band fade is
reduced from 10 dB to approximately 8 dB, 6 dB, and 4.5 dB at 30°,
45°, and 60°, respectively. At UHF (Figure
6-4), the 10 dB fade is reduced to 8 dB, 7 dB, and 5 dB at 30°,
45°, and 60°, respectively.
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