Line of Sight (LOS)
FSO system operation requires line of sight (LOS). Line of sight simply means that the transmitter and the receiver at both networking locations can see each other. Because IR beams propagate and expand in a linear fashion, the line of sight criteria is less strict when compared to microwave systems that require an additional path clearance to account for the extension of Fresnel zones.
The easiest way to find out if line of sight exists between two remote locations is visual observation. For distances that are longer than a mile, this might not be trivial. Field glasses and telescopic lenses might be necessary in these scenarios. Many FSO vendors incorporate an alignment telescope into the FSO terminals to accomplish this task. Some organizations prefer to use more sophisticated GIS maps before sending a field crew out to assess a site. A variety of GIS mapping software vendors exist whose programs can load high-resolution and three-dimensional topology maps. These maps include information regarding buildings and their specific locations. This allows for determining whether line of sight exists between two known locations.
Although rooftop to rooftop is one of the more typical deployment scenarios for FSO, it might be possible to locate the transceivers behind windows in the building when roof access is not available. However, you must take care to determine whether line of sight can be achieved. In addition, the angle the beam makes with the window is critical. The angle should be as perpendicular as possible, yet slightly angled (5 degrees) to reduce bounce-back of the beam to its own receptor. At some angle, no light will be transmitted at all. This complete internal reflection is what keeps light inside fiber-optic cable. Also, some windows contain glass or glass coatings that reduce glare. Because these windows are often specifically designed to reject infrared, the coatings can reduce the signal by 60% or even more. Sometimes, window connections have no alternatives because roof rights cannot be obtained. Decreasing the link distance (which increases the power of the signal at the receiving telescope due to decreased geometrical loss) or increasing beam intensity can often solve the problem.