Home 
Forward
3.2
Time-Series Fade Measurements
In the analysis of times-series roadside fades for Land-Mobile-Satellite
Service (LMSS) scenarios, the attenuation levels are represented by the
dB ratio of the non-shadowed power received under conditions of negligible
multipath relative to the shadowed levels. Figure
3-1 and Figure 3-2 are examples of relative
power measurements depicting nominal characteristics of fading and phase
variations for non-shadowed and shadowed line-of-sight cases, respectively.
These measurements were performed by Vogel
and Goldhirsh [1995] in Bismarck, North Dakota and in central-Maryland
where L-Band transmissions (1.5 GHz) emanating from the MARECS B-2 satellite
were received at elevation angles of 22° (Figure 3-1) and 40° (Figure
3-2). The fluctuations due to receiver noise were within 1 dB (rms). The
non-shadowed environment (Figure 3-1) may be
characterized as an open rural road and the shadowed case (Figure
3-2) a tree-lined highway where the line-of-site path was obstructed
by the roadside trees.
In Figure 3-1,
fluctuation in the relative power (±
1 dB) and in phase (± 15°) are due
to both system noise and to a small extent multipath. Nominally, peak-to-peak
variations of less than 5 dB of power and 25° of phase may arise from
multipath for non-shadowed cases. The shadowed case
(Figure 3-2) has fades which are highly variable
with fluctuations exceeding 15 dB. Some of these deep fades are also accompanied
by rapid phase shifts. Time-series of fade and phase of the above types
were obtained for various LMSS scenarios and environments and corresponding
cumulative distributions were derived as described in the following paragraphs.
Figure 3-1: Time-series of 1.5 GHz fade (top) and phase (bottom) over a
one second period at a sampling rate of 1 KHz. Measurements were taken
of transmissions from MARECS-B2 at 22° elevation along an open road
near Bismarck, North Dakota.
Figure 3-2: Time-series of 1.5 GHz fades (top) and phases (bottom) over
a one second period at a sampling rate of 1 KHz. Measurements were taken
of transmissions from MARECS-B2 at 40° elevation along a highway with
roadside trees in central Maryland where the satellite line-of-sight was
shadowed.
We show in Figure
3-3 an example of the variability of the minimum, maximum, and average
fades over an approximate 10 minute period (approximately 10 km) pertaining
to 20 GHz measurements of transmissions from the Advanced Communications
Technology Satellite (ACTS) in Bastrop, Texas. The trees genus was predominantly
Loblolly Pine (evergreen tree) and the elevation angle was approximately
55°. The central, red curve corresponds to the average signal level
of 1000 samples taken over a one second period (1 KHz sampling rate). The
upper and lower curves correspond to the maximum and minimum signal level
values obtained from the 1000 samples measured each second, respectively.
Maximum values of three to four dB relative to the non-shadowed line-of-sight
case arise because of constructive interference caused by multipath scatter
by the foliage and branches. Large signal level drops to –20 dB and lower
are due predominantly to canopy attenuation. The cumulative distribution
for this run is described in Section
3.4.3 (Figure 3-12).
Figure 3-3: Relative K-Band signal time-series of maximum, minimum, and
average levels for a 10 minute run (approximately 10 km) along a road lined
with evergreen trees in Bastrop, Texas in February 1994.
Home
Forward