Define link margin

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29. Define link margin

In practical communications systems it is usually desired to have the received power level greater than the threshold level required for the minimum acceptable quality of service (usually expressed as the minimum carrier-to-noise ratio (CNR), or minimum SNR).

This design allowance for received power is referred to as the link margin, and can be

expressed as the difference between the design value of received power and the minimum threshold value of receive power:

Link margin (dB) = LM = Pr − Pr(min) > 0, where all quantities are in dB.

Link margin should be a positive number; typical values may range from 3 to 20 dB.

Having a reasonable link margin provides a level of robustness to the system to account for variables such as signal fading due to weather, movement of a mobile user, multipath propagation problems, and other unpredictable effects that can degrade system performance

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