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

Polarization, Antenna Gain and Diversity Considerations 6-13
reference, the fade is considerably reduced by switching to the other diversity modes.
For example, at the 20% probability, a 17 dB fade for the highest satellite may be
compared to 6 dB for the “Best” satellite scenario, giving rise to an 11 dB diversity gain
at that probability. We note that the higher diversity combinations (e.g., 2, 3, and 4 Best
Satellites) do not significantly contribute to an increased diversity gain at percentages
greater than 20%. Figure 6-9 shows that using the “3 Best Satellite” diversity mode, 1%
probability gives rise to a 20 dB fade margin for an urban environment. This
substantially high fade may preclude voice communications for urban environments at
small probabilities even with satellite diversity.
Probability P (%)
98
95
90
80
70
60
50
40
30
20
10
5
2
1
Satellite Scenario
Highest Satellite
Best Satellite
2 Best Satellites
3 Best Satellites
4 Best Satellites
-5 0 5 10 15 20 25 30 35
Fade (dB) Exceeded at Probability P
Figure 6-9: Cumulative fade distributions at L Band (f ≈ 1.6 GHz) for the simulated
Globalstar constellation with combining diversity for Tokyo, Japan [Vogel, 1997,
Akturan and Vogel, 1997].
6.6.3 Satellite Diversity Gain
Figure 6-10 depicts the diversity gains for the distributions given in Figure 6-9 associated
with the “combining diversity” mode. Also shown are the diversity gain results
associated with the “hand-off” mode as defined above. For any fixed probability, these
two processing modes differ by approximately 1 dB (at most), but also note that any