Link Margin The link margin of an FSO system represents the margin on the amount
of received optical power available for the system to perform to its specifications. It is
expressed in terms of dB and is computed as 10 times the Log of the ratio of the available
received power and the minimum required power. For example, a link with a 0 dB
link margin has just enough optical power at its receiver to perform to its specifications.
A link with a 3 dB link margin has twice as much optical power at its receiver as would
be necessary for it to perform to its specifications.
Link margins for FSO systems are often specified for various weather conditions and
link distances. However, the actual available link margin may be different as weather
conditions or installation properties change. For example, a system specified to have 9 dB
of margin in clear weather conditions when operating at a distance of 200 m would
most likely have a margin of 8 dB during heavy rain. The same system would have only
3 dB of margin when operating at a distance of 400 m in a clear weather conditions.
Having a extra link margin allows a system to operate normally even in conditions
that can reduce the amount of optical power received by the system.
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