Hi,
I think the link margin, is the difference between the receiver’s sensitivity (i.e., the received power at which the receiver will stop working) and the actual received power.
Probably the single most significant parameter in describing the performance of an FSO link is the link margin.
Basically, this is the amount of light received by a terminal over and above what is required to keep the link active
(hence the term “margin”). The higher the LINK MARGIN is, the higher the quality of the received signal and the farther we are from the cliff. The closer to 0 dB, the worst the quality of the received signal and therefore the closer to the point where the system stops to operation.
When the LINK MARGIN is 0 dB, we are in the limit of the correction. The services can be displayed, although occasionally can be seen some artifact, but only a slight signal degradation can cause the drop through the cliff. The system will stop to work immediately.