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

Polarization, Antenna Gain and Diversity Considerations 6-15
146°, 49°. Hence, these satellites were separated in azimuth and elevation by 102° and
25°, respectively.
Probability (%)
99
98
95
90
80
70
60
50
40
30
20
10
5
2
1
Shadowed
Blocked
Clear
TDRS Satellite
T7 (24 o El, 248 o Az)
T1 (49 o El, 146 o Az)
Max. of T1 and T7
-4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
Fade Exceeded at Probability (dB)
Figure 6-11: Single and joint probabilities for “clear”, “shadowed” and “blocked”
scenarios derived employing S-Band (f = 2 GHz) TDRSS measurements [Vogel, 1997].
The joint distributions in Figure 6-11 correspond to the “hand-off” mode (maximum of
two satellite signals). Fading for the “clear” case at the smaller percentages is a result of
ground specular reflections and shadowing by the head. At the 1% level, the joint
distribution is shown to reduce the fade margin from a maximum of approximately 6 dB
to 2 dB. The “blocked” distributions show smaller fading than the “shadowed” case for
the following reasons. The “blocked” environment distribution is in part representative
of an “on” or “off” switch where the line of sight is either “not blocked” or “blocked,”
respectively. In addition, this distribution also includes diffraction and multipath from
the buildings, and some shadowing from a few trees. The distribution is therefore
strongly dependent upon the density and height of the structures. On the other hand, the
shadowed distribution represents an almost continuum of tree fading measured inside a
grove; especially relative to the T7 satellite. The diversity gains at 1% from the
“shadowed” and “blocked” environments are noted to range from 8 to 13 dB and 8 to
10 dB, respectively. These diversity gains have similar magnitudes to those calculated
employing the optical measurements described in the previous section.