Probability of Exposure of Optical Receiver to Direct Sunlight in FSO-Based Wireless Mobile Communications

Free-space optics (FSO) communication systems can be applied to cellular mobile communication networks to offer high-data-rate services to mobile users. In such networks, the exposure of optical receivers to direct sunlight could degrade the system performance and damage light-sensitive photo-detectors permanently. We investigate the average probability of the exposure of optical receiver to direct sunlight in FSO-based wireless mobile communication networks. We first estimate the probability of exposure to direct sunlight in open areas and then extend the theoretical analysis to the urban areas where the direct sunlight can be blocked by buildings. The validity of the analyses is evaluated through Monte Carlo simulations over a virtual city. The results show that the probability of exposure to direct sunlight ranges from 10⁻⁶ to 10⁻², depending upon the receiver's field-of-view (FoV), latitude, average building heights, and the height of base stations (BSs). We also find out that the probabilities increase with the receiver's FoV to the power of 1.8 when the receiver's FoV is larger than the angular diameter of the sun. In urban areas, the probability decays exponentially with the average height of buildings exceeding the half of the BS height. The results also show that the probability rises with the BS height to the power of ∼2.6 under the same conditions. Finally, we estimate the outage time of FSO-based wireless mobile communications occurring by the exposure to direct sunlight. When the receiver's FoVs are 1 and 10 degrees, the outage times are estimated to be <1 and <35 hour/year, respectively, for the BS height of 28 m. The findings of this work could be used to estimate the probabilities of exposure to direct sunlight in various optical systems operating outdoors.