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guest 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 Lineofsight (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 Barnettvignant 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 ( 1R ) – 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 BarnettVignant 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 equation1. For eg: for one of our links, fade margin when estimated using equation1, 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 equation1 . 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 .
Posted 11/12/2012
