For example,
heavy rain results in signal loss of about ??“4 dB/km, which means an FSO system operating
at 0.5 km would experience an additional signal loss of ??“2 dB during heavy rain.
The dB loss/km for the IR signal of FSO is also correlated to visibility during these
weather conditions. A sample data correlating visibility and weather conditions with
dB/km attenuation of 850 nm signal is provided in Table 9.1. The data can be used
to compute the link margin of a 850-nm FSO link in various weather conditions. For
example, according to the table, a moderate fog, which has a visibility of 500 m, causes
an attenuation of about ??’21 dB/km. This means an FSO link operating at 1 km would
need 21 dB of link margin for it to be able to overcome atmospheric attenuation during
conditions of moderate fog. An FSO link at 500 m would need half that amount, 10.5 dB
of margin, for it to be able to overcome the same weather condition.
In order for an optical wireless system to be immune to all weather conditions, its
link margin during the worst condition has to be at least 0 dB. From Table 9.1, it is
evident that the worst weather condition, dense fog, can produce a loss of as much as
??“270 dB/km.
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