# Forum

## Link Budget ( SOM ) and Fade Margin

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

I have some doubts about Link Budget and Fade Margin.

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 ?

Are the other thoughts about Link Budget and Fade margin, also 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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