FSO Path Loss Model Based on the Visibility

Free-space optics (FSO) is attracting renewed attention in the frame of 5G, which demands wireless backhauling technologies with extremely high data rates over distances up to a few kilometers. Link availability is a FSO well-known issue due to the large path loss expected during poor visibility, such as in case of fog. Several path loss models, based on the visibility, are assessed through an analytical approach. The simple relationship valid in the visible spectrum can be extended to the first optical window, while, at 1.550 μm, the effect of fog microphysics cannot be ignored. Fog is definitively less challenging in the mid-IR (10.6 µm) even at very low visibility values (less than 0.5 km), which was not evident from older studies. Lower and upper bounds for the extinction coefficient are calculated and substituted into the FSO link budget equation. The combined effect of the microphysics and of the accuracy of visibility measurements may result in large uncertainties on the maximum achievable path length. The latter is also influenced by the microclimate, as links are usually deployed in urban areas while visibility data are collected outside. Path lengths in city areas can double, for the same link availability.