Outage Probability Analysis of FSO Links Over Foggy Channel

Outdoor free-space optic (FSO) communication systems are sensitive to atmospheric impairments, such as turbulence and fog, in addition to being subject to pointing errors. Fog is particularly severe because it induces an attenuation that may vary from few decibels up to few hundreds of decibels per kilometer. Pointing errors also distort the link alignment and cause signal fading. In this paper, we investigate and analyze the FSO systems performance under fog conditions and pointing errors in terms of outage probability. We then study the impact of several effective communication mitigation techniques that can improve the system performance including multi-hop, transmit laser selection, and hybrid radio-frequency (RF)/FSO transmission. Closed-form expressions for the outage probability are derived and practical and comprehensive numerical examples are suggested to assess the obtained results. We found that the FSO system has limited performance that prevents applying FSO in wireless microcells that have a 500-m minimum cell radius. The performance degrades more when pointing errors appear. Increasing the transmitted power can improve the performance under light to moderate fog. However, under thick and dense fog, the improvement is negligible. Using mitigation techniques can play a major role in improving the range and outage probability.