Note: I moved this from
the Employment forum to here - Kirt B.

I am an RF Engineer, developing Spread Spectrum
Radios to be used in Point to multipoint fixed telecommunication
systems .

Fading estimation of Signals (unspreaded)
in LOS link :

Propagation channel characteristics
of our system is very much similar to that of Line-of-sight
(LOS) Digital microwave link used in telecommunication
systems, where the fading is mainly due to multipath
caused by the gradients of atmospheric index of
refraction and ground reflections. The Barnett-vignant
reliability equation (equation –1) below holds good
for estimating fade margins of LOS link where the
signal is not spread.

FM = 30 log d +10
log ( 6ABf ) - 10 log ( 1-R ) – 70 -----------------------
( 1 )

FM= Fade margin in dB

d = Path
length in Km

f = Carrier frequency in GHz

A = Rough ness factor

B = Climate factor

R = Reliability

Cannot use Barnett-Vignant
reliability equation for spread spectrum system
:

Since the signals in our system is spreaded
the above equation cannot be used to estimate the
fade margins. It is well known that Spread spectrum
systems are resistant to multipath fading , but
I do not have definite figure by which I can relax
from the figures I get using the equation-1. For
eg: for one of our links, fade margin when estimated
using equation-1, gives a figure of 13 dB , as our
system is spread spectrum system 13 dB fade margin
is not required but I am not sure by how much I
can come down on this figure.

Lookig forward
for simple equation /empirical formula:

Hence,
I am looking for any simple equation or empirical
formula to estimate fade margins for spread spectrum
system , similar to the equation-1 . Coming down
on the fade margin figures will bring down the cost
of our systems but I want to be sure by how much
I can relax on the fade margins, so that the performance
of the link is not affected .