User's Manual

3 Detailed Description
34 APCD-LM043-4.0
You can predict the amount of path loss for each of these cases, as illustrated in Figure 21.
Figure 21 Path Loss Calculation
As shown in Figure 21, the path loss for each case is quite different:
• Case 1 (Unobstructed Path): Over the length of the path, the signal drops as 1/R
2
,
where R is the distance from the CCU. The range, R
case1
, is determined by the
distance at which the signal reaches threshold plus the desired fade margin.
• Case 2 (Path Obstructed in Vicinity of EUM): From the CCU, the signal initially
drops as 1/R
2
until it reaches the obstructions in the vicinity of the EUM. Through
these obstructions, the signal drops more steeply than it does in the unobstructed
case, more like 1/R
4
. Once again, the range, R
case2
, is determined by the distance at
which the signal reaches threshold plus the desired fade margin. As shown above,
R
case2
<R
case1
, which intuitively makes sense. If the path to the EUM is unobstructed,
you would expect to be able to serve EUMs that are farther from the CCU, and to
provide better fade margin to those that are in closer.
• Case 3 (Path Obstructed in Vicinity of CCU): From the CCU, the signal initially
drops as 1/R
4
until it leaves the obstructing clutter and terrain in the vicinity of the
CCU. Once the signal leaves these obstructions, it drops as 1/R
2
since the remainder
of the path is clear. Once again, the range R
case3
, is determined by the distance at
which the signal reaches threshold plus the desired fade margin. As shown above,
R
case3
<R
case1
. Although it shows R
case3
<R
case2
, this may or may not always be the
case; however, it is always true that the margin is greater for Case 2 than Case 3, in
the coverage area indicated by the shading in Figure 21. In this area, the probability of
successful indoor installs is likewise higher for Case 2 than Case 3.
Range
Case 1
Unobstructed Path
Free Space Loss
Case 2
Path Obstructed
in Vicinity of EUM
Case 3
Path Obstructed
in Vicinity of CCU
Tx O/P
Rx Threshold
Fade Margin
R
case 3
R
case 2
R
case 1
Probability of successul indoor installation is
greater for Case 2 than for Case 3, in this
region