In order for an FSO link at 1 km to be immune from all weather conditions,
it needs to have a clear weather margin of 270 dB. Just to put this number in perspective,
for a typical commercially available FSO system to have a margin of 270 dB at 1 km,
it needs to be transmitting at least 1020 watts of optical power, a number that is not
even a theoretical possibility. Therefore, such FSO systems at 1 km may never be immune
from all weather conditions.
Weather
Condition Precipitation Amount (mm/hr) Visibility dB Loss/km
Dense fog 0 m??“50 m ??“271.65dB
Thick fog 200 m ??“59.57dB
Mod. fog Snow 500 m ??“20.99
Light fog Snow Cloudburst 100 700 m??“1 km ??“12.65, ??“9.26
Thin fog Snow Heavy rain 25 1.9 m??“2 km ??“4.22, ??“3.96
Haze Snow Mod. rain 12.5 2.8 m??“4 km ??“2.58, ??“1.62
Light haze Snow Light rain 2.5 5.9 m??“10 km ??“0.96, ??“0.44
Clear Snow Drizzle 0.25 18.1 m??“20 km ??“0.24, ??“0.22
Very clear 23 m??“50 km ??“0.19, ??“0.06
TABLE 9.1 Signal Loss Due to Various Weather Conditions [1]
Optical Wireless Mesh Networks 245
Now consider a link at 100 m. The loss due to the worst weather condition at 100 m
is ??“27 dB. Therefore, in order for the link to be immune from all weather conditions, it
needs to have a link margin of 27 dB.
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