Appendix
5-10
602-95555-01 Rev A
NXE1-20 Digital Radio
5.1.2.4
Path Availability and Reliability
For a given path, the system reliability is generally worked out on methods based on the work of
Barnett and Vigants. The presentation here has now been superseded by CCIR 338-6 that
establishes a slightly different reliability model. The new model is more difficult to use and, for
most purposes, yields very similar results. For mathematical convenience, we will use fractional
probability (per unit) rather than percentage probability, and will deal with the
unavailability
or
outage parameter, designated by the symbol U. The
availability
parameter, for which we use
the symbol A, is given by (1-U).
Reliability
, in percent, as commonly used in the microwave
community, is given by 100A, or 100(1-U).
Non-Diversity Annual Outages
Let U
ndp
be the non-diversity annual outage probability for a given path. We start with a term r,
defined by Barnett as follows:
r
= actual fade probability/Rayleigh fade probability ( =10
-
F
/10
)
Where,
F
= fade margin, to the minimum acceptable point, in dB.
For the worst month, the fade probability due to terrain is given by:
r
m
=
a
x 10
-5
x (
f/
4) x
D
3
Where,
D
= path length in miles.
f
= frequency in GHz.
a
(terrain factor)
= 4 for smooth terrain.
= 1 for average terrain.
= 1/4 for mountainous, very rough, or very dry terrain.
