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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 :
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
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 .