Fade margin estimation for spread spectrum systems

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 .

Fade margin estimation for spread spectrum systems - RF Cafe Forums