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6.7  Summary and Recommendations

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). A model describing the reduced fading at equi-probability levels is given by (6-4), 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 which require near instantaneous two way responses.

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