Atmospheric effects on optical wireless links

The optical coherent wave propagation is affected by the atmosphere in many ways. Several theoretical models for propagation of light through the atmosphere are well known. To predict link availability in different climate zones it is necessary to do field tests for data acquisition. Therefore reliability- and availability-tests on free space optics (FSO)-systems have been done at the Department of Communications and Wave Propagation (by the research-group "OptiKom"). There have been obtained evaluation results for commercial available and also for self-developed optical point-to-point and point-to-multipoint FSO-systems. Test data at 155 Mbps (STM-1) were transmitted from one FSO-unit to a distant (2.7 km) FSO-unit. The received data were sent back (loop) to the first unit. The primary interest in this long-time investigation was the time of link failure, because it turned out that BERs be low in general, less than 10/sup -8/ at very bad weather conditions in winter and less than 10/sup -12/ at clear sky. In a second measurement campaign the influence of turbulences in the air was investigated. The measurements clearly show variations in the fluctuation of the incoming optical power during a day. In principle there are two periods with strong variations, during the day and during the night, and two periods of rather stable air, these are around sunset and sunrise. The power variations have the highest amplitude and show the fastest changes at noon and they are less distinct and show slower changes in the night. As a medium value power variations of 4 dB over the distance of 2.7 km were measured in summer. The duration of fades/scintillations was in the order of 4 to 60 milliseconds at daytime and about 10 to 150 ms in the night.