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

12-8
Propagation Effects for Vehicular and Personal Mobile Satellite Systems
Table 12-2: Listing of polynomial coefficients characterizing phase fluctuation
distributions of the form (12-16) for road types exhibiting “moderate” and “extreme”
shadowing and a 5 dB fade threshold.
Road Type
Polynomial Coefficients
α o α 1 α 2 α 3 α 4 α 5
Moderate 56.51 -6.516 7.325x10 -2
Extreme 54.23 -4.242 -1.0897x10 -2
2.380x10 -2
6.425x10 -3
2.059x10 -4
2.082x10 -5
-3.985x10 -5
-4.258x10 -6
The above results demonstrate that the influence of phase excursion on demodulation
techniques are small over the range of probabilities from 5% to 95%, since the phases are
within ± 15° relative to the median.
12.5.4 Suggestions for Further Work
It is suggested that the above results be extended to other frequencies such as UHF,
S-Band, and K-Band as well as to elevation angles smaller than 20° where existing
databases are available. Extension of the formulations to hilly and mountainous terrain is
also suggested.
12.6 Polarization, Antenna Gain and Diversity Considerations (Chapter 6)
Fading effects are examined at UHF and/or L-Band using antennas with multiple
polarizations and different antenna gains. Fade reduction experienced by changing lanes
is described. Diversity gains are characterized as a function of antenna spacing and
satellite pointing
12.6.1 Frequency Re-Use
Co- and cross-polarization measurements at 1.5 GHz have demonstrated that insufficient
isolation exists for mobile-satellite scenarios. For example, when a fade of 5 dB was
experienced, isolation of only 11 dB existed in the cross polarized channel. Hence,
frequency re-use through the transmission of multiple polarizations is not a viable
solution.
12.6.2 Multipath Effects for Variable Antenna Gains
Where multipath reflections from the ground do not arise, small fade differences occur
when using antennas of different gains. On the other hand, for earth-satellite geometries
in which ground multipath may arise, the use of higher gain antennas may substantially
reduce fading by as much as 10 dB. The former and latter conclusions are based on
measurements at 1.5 GHz and 20 GHz, respectively.