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Post subject: Fade margin estimation for spread spectrum systems
Unread postPosted: Tue Mar 15, 2005 10:46 pm
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
A = Rough ness factor
B = Climate factor
R = Reliability
Cannot use Barnett-Vignant reliability equation for spread spectrum
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 .