Quantifying the Effects of Weather on Free Space Optic (FSO) Link Availability

摘要

Free Space Optics (FSO) has become a viable, high-bandwidth wireless alternative to fiber optic cabling. The primary advantages of FSO over fiber are its rapid deployment time and significant cost savings. The disadvantage of FSO over fiber is that laser power attenuation through the atmosphere is variable and difficult to predict, since it is weather dependent. This factor limits the distance at which FSO should be deployed. Using historical weather data collected at airports, the link availability as a function of distance can be predicted for any FSO system. These availability curves provide a good indication of the reasonable link distances for FSO systems in a particular geographical area. FSO link distances can vary greatly from desert areas like Las Vegas to heavy-fog cities like St. Johns NF. An analysis of link budgets and visibility-limiting weather conditions indicates that to meet the carrier-class availability of 99.999%, FSO links should generally be less than 140 m. However, there are cities like Phoenix and Las Vegas where the 99.999% maximum link distance increases significantly. A more practical solution to extending the high availability range would be to back-up the FSO link with a lower data rate radio frequency (RF) link. This hybrid FSO/RF system would extend the 99.999% link range to longer distances and open up a much larger metro/access market to the carriers. To show the geographical dependence of FSO performance, the first map of FSO availabilities contoured over North America is presented. This map is the first step to developing an attenuation map for predicting FSO performance, which could be used in a similar fashion to the International Telecommunication Union (ITU)/Crane maps for predicting microwave performance.