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

5-2
Propagation Effects for Vehicular and Personal Mobile Satellite Systems
line of sight between the mobile and the satellite intersects the canopy of the respective
tree over the spatial intervals dd1, dd2 … ddN, and that the non-intersecting intervals are
nfd1, nfd2, … nfdN. It is also assumed (as an example) that the signal level falls and
remains below the indicated relative threshold of -5 dB during each intersection of the
line-of-sight ray and the canopy. Given a knowledge of the vehicle speed at any instant
of time, the distance durations over which the 5 dB attenuation is exceeded (i.e., dd1, dd2,
… ddN) may be calculated, and density and cumulative distributions determined subject to
the condition that the fade exceeds 5 dB. Distributions for the non-fade durations (i.e.,
nfd1, nfd2, … nfdN) may also be calculated for the condition that the fade is smaller than
5 dB. In this manner, conditional fade and non-fade distance duration distributions may
be determined. This normalized representation may then be converted to time duration
distributions by division of the vehicle speed. It is apparent that the fade duration
distributions will be dependent upon the canopy dimensions of the individual trees, the
density of foliage, the vehicle offset from the trees, the elevation angle of the line-of-sight
ray to the satellite and the tree heights.
0
-5
dB
nfd1 nfd2 nfd3
dd1 dd2 dd3 dd4
Distance Traveled (m)
Figure 5-1: Idealized scenario showing line of roadside trees (top) and relative signal
levels (bottom) for the case in which a vehicle receiving satellite signals drives parallel to
the roadside trees and the line-of-sight ray to the satellite intersects the tree canopies.