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

10-14
Propagation Effects for Vehicular and Personal Mobile Satellite Systems
reduced choice of potentially visible satellites near the equator (e.g., compare Figure 10-7
and Figure 10-8 with Figure 10-9).
The distribution labeled “2 Best Satellites” represents the joint distribution
associated with the two satellites giving jointly the “smallest fades”. At any instant of
time, different pairs of satellites may fall under the “2 Best Satellite” category. The
distributions labeled “3 Best Satellites” and “4 Best Satellites” are analogously defined.
The above joint distributions were derived assuming “combining diversity” where the
signals received are “added,” as opposed to “hand-off” where the satellite with the
“highest” signal is processed. It is apparent that relatively small differences in fade
margin reduction exist at the 10% probability in comparing the multiple satellite case
with the “Best Satellite” distribution at the three locations. For example, in comparing
the “Best-Satellite” with the “4-Best Satellite” distributions at the 10% probability, a less
than 5 dB improvement exists at all three locations. Somewhat larger improvements
occur at the 1% probability level, however the fade margins even for multiple satellite
diversity becomes prohibitively high.
Percentage of Locations Fade > Abscissa
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 Depth (dB)
Figure 10-10: Cumulative fade distributions for Tokyo, Japan at L-Band (1.5 GHz) for
the simulated Globalstar constellation. The joint distributions for multiple satellites
correspond to “combining diversity.”