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

6-16
6.7 Conclusions and Recommendations
Propagation Effects for Vehicular and Personal Mobile Satellite Systems
The following salient conclusions and recommendations are based on the results of this
chapter.
1. Simultaneous employment of co- and cross-polarized transmissions in a “frequency
re-use” system should not be used except to potentially help isolate different CDMA
systems because of the potential for poor isolation caused by multipath scattering into
the cross-polarized channel (Figure 6-1).
2. Where ground multipath effects do not arise, small equi-probability fade differences
of 1 to 2 dB may occur (at L-Band) for mobile antennas having different antenna
gains (Figure 6-2). At low elevation angles where ground multipath effects may
arise, higher gain antenna has been found to substantially reduce fading by as much
as 10 dB at 20 GHz.
3. Changing lanes may substantially reduce the equi-probability fading because of the
shortening of the path length through roadside tree canopies (Figure 6-3). At a 10 dB
fade at UHF and L-Band, lane changing has been found to reduce the equi-probability
fade for 60° elevation by approximately 5 dB (Figure 6-4 and Figure 6-5). Equation
(6-4) gives a model describing the reduced fading at equi-probability levels, where
the coefficients are summarized in Table 6-2 and Table 6-3.
4. For a 5 m separation between antennas and single antenna fades of 3 dB and 6 dB,
diversity gains at L-Band (based on simulations using measurements made in
Australia) are approximately 1.5 dB and 3.5 dB (Figure 6-8). These results are within
0.5 dB of those derived from measurements made in Japan (Table 6-4).
5. Satellite diversity measurements at L-Band for urban areas gave rise to diversity gains
as high as 14 dB for percentages as low as 1% (Figure 6-10). Nevertheless, even
applying diversity gain methods, fade margin levels have been found to exceed 17 to
20 dB at the 1% probability (Figure 6-9 and Figure 6-11). Such high fade margins for
urban areas may preclude direct earth-satellite voice communications that require
near instantaneous two-way responses.
6.8 References
Akturan, R. and W. J. Vogel [1997], “Path Diversity for LEO Satellite-PCS in the Urban
Environment,” IEEE Transactions on Antennas and Propagation,” Vol. 45, No. 7,
July, pp. 1107-1116.
Goldhirsh, J. and W. J. Vogel [1987], “Roadside Tree Attenuation Measurements at UHF
for Land-Mobile Satellite Systems,” IEEE Transactions on Antennas and
Propagation, Vol. 35, No. 5, May, pp. 589-596.
Goldhirsh, J. and W. J. Vogel [1989], “Mobile Satellite System Fade Statistics for
Shadowing and Multipath from Roadside Trees at UHF and L-Band,” IEEE Trans.
Antennas Propagation, Vol. AP-37, No. 4, April, pp. 489-498.
Hodge, D. B. [1978], “Path Diversity for Earth-Space Communication Links,” Radio
Science, Vol. 13, No. 3, pp. 481-487.