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6.2 Depolarization
Effects
By making repeated measurements with co- and cross-polarization at 1.5
GHz for selected runs (elevation = 51°) during an Australian campaign,
equi-probability "cross-polar isolation levels, CPI" were determined
by Vogel
et al. [1992]. The cross polarization isolation is defined as
where COPS(P) and CRPS(P) represent the co-polarization and
cross-polarization signal levels at the equi-probability value P.
The CPI (in dB) is plotted in Figure 6-1
and was found to follow the linear relation
where A is the co-polarized fade (in dB) with a range between 0
and 11.8 dB.
The RMS deviation between the "best fit linear" relation (6-2)
and the data points for the corresponding runs was 0.4 dB. We note from
the plot in Figure 6-1 that the isolation severely
degrades as a function of fade level. For example, an approximate 11 dB
isolation is observed at a 5 dB fade. This result suggests that the simultaneous
employment of co- and cross-polarized transmissions in a "frequency re-use"
system is implausible because of the poor isolation due to multipath scattering
into the cross-polarized channel. Although the instantaneous isolation
is poor, polarization diversity may nevertheless be helpful in reducing
the statistical interference between two satellite systems that manage
to share the same frequency band by employing code-division multiple access
(CDMA). In that case, one system’s signals contribute to the other’s noise.
Cross-polarizing the alternate system would tend to reduce the noise at
the victim satellite reverse-link receiver while the alternate system’s
mobile earth terminals are in a clear line-of-sight condition. Estimating
the net benefits of such a scheme is not straightforward, however, because
the impact of power control has to be factored in.
Figure 6-1: Cross polarization isolation (CPI) as a function of
co-polarization fade at equi-probability levels.
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