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5 posts by 2 authors in: Forums > CWNA - Enterprise Wi-Fi Admin
Last Post: April 7, 2007:
• Hi,

Link Budget is the total received signal considering FSPL, attenuation by cables, etc..., minus the transmit signal at the source or LB = TRS - TSS where TRS stands for total received singal and TSS stands for Transmited signal at the source. This numbers are rather negative(never positive) , so if i have a TRS of -60 and a TSS of -30 then LB = -60 - (-30) , LB = -30dmw

in the other hand Fade Margin is just an addition number to the already existant received sensitivity, if i have receive sensitivity of -40 and FM is 10, the i would have FM = -40 - ( 10 ) = -50, however i have seen that the FM can be negative or positive, if i have a FM of -10 then FM = -40 - (-10) = -30

by the case FM is negative i can say i am attenuating the signal and if it is positive i am increasing it, is this afirmation correct ?

i appreciate the colaboration of helping me out,
thanks

• In some text you will see a fade margin expressed as a positive (non-negative) number and others you will see it as a negative number (-10dBm). I can see how this would be confusing.

A fade margin is always a negative number. So if your link budget is at -68dBm and you have a 10dBm fade margin (notice I didn't call it a negative 10) then your actual signal is -78dBm.

A fade margin is not an actual loss. A fade margin is a value used to give a buffer, or margin of error, in case the signal degrades in some way. If there is high wind and your antenna moves slightly, there is more loss. If there is a snow buildup on your dish, that will induce loss.

Let us know if you need further clarification.

• Hi GtHill,

So, in the same example if we had a -10dbm, the receiver sensitivity would be :

-68 - (-10 ) in this case : -58dbm

correct ?

and if so, wouldn't i be shortning the power limit of the signal ?

thanks

• You said "-68 - (-10 ) in this case : -58dbm "

You are reducing the -68 by 10 dB more which would be -78dbm. As the number gets farther from 0 the signal gets worse.

Remember a fade margin is designed as a margin of error in case the signal gets less than ideal.

• So, because it is a margin, the number tends to be always positive, otherwise i would be taking the margin out instead of increasing it...

the -68 - (10) = -78dbm would be the case of the fade margin( i add a margin, signal gets weaker )

the -68 - (-10 ) = -58dbm would be the case where i limit the margin ( i subtract the margin, signal gets stronger )

right ?

You may be thinking that there's no point for the -10dbm margin ( but i think i have gone thru some exercises that showed it ).

thanks

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