Free space optics (FSO) technology has been used recently in many applications due to its various advantages such as high bandwidth, easy deployment, high security, free installation license and immunity to electromagnetic interference. However, this type of communication is vulnerable to meteorological conditions such as rain, haze and fog which significantly degrade the system's efficiency. To enhance the effectiveness of the FSO systems, Quadrature Phase Shift Keying (QPSK) modulation based on coherent detection constitutes a promising technique for achieving high spectral efficiency and long‐distance FSO links with the characteristics of low‐cost implementation and simple design. This work aims to investigate the performance of FSO communication links on image transmission under the presence of atmospheric attenuations. A comparative study has been made using QPSK modulation scheme and on–off‐keying (OOK) modulation scheme to estimate the maximum range that was used to recover an RGB transmitted image over an FSO link under different meteorological weathers. The obtained results clearly show that the quality of the received image has degraded significantly during transmission through free space yielding to limit the propagation range. Moreover, the proposed FSO communication system performs well when using QPSK modulation rather than OOK modulation in different environmental conditions. Thus, to improve the quality of the received image under adverse fog weather condition, high power and small beam divergence angle have been suggested to increase system's performance. The proposed solution increases the communication range of the FSO link by 230 m and 100 m for moderate and heavy fog, respectively.

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Free Space optics (FSO) has gained significant interest in recent years due to its ability to provide a more secure communication connection than conventional radio links, being relatively inexpensive compared to other wireless systems, and the fact that it operates without any interference from other wireless devices. FSO also has a downside in its susceptibility to certain environmental conditions, which can degrade the quality of the connection by absorbing, scattering, and refracting the optical beams. This paper discusses the rain attenuation effect on RGB image transmission through an FSO communication system. The study uses different metrics to analyze system performance and suggests increasing transmission power to improve performance during heavy rainfall. The findings suggest that for medium and heavy rain, an improvement in maximum link distances of 490 m and 340 m, respectively, is obtained as the transmission power increases to 15 mW. The paper also proposes a spatial domain image restoration technique to enhance image quality at the receiver using the median and wiener filters. Results show that the proposed solutions increase the communication range of the FSO link by 320 m and 205 m for moderate and heavy rain, respectively, when the transmitting power is about 10 mW.

Free-space optics (FSO) connects two distant points by a simple light beam. Currently, this technique is a complement to radio frequency systems and those based on optical fibers. However, its large-scale deployment is slowing down due to major drawbacks such as the short operating distance and its great vulnerability to atmospheric conditions. In this article, and in order to increase the connection distance of the FSO systems and maintaining an acceptable signal quality, we inserted in the transmission channel: a receiver and a transmitter that retransmit the received signal. We have chosen two weather situations. The attenuation values used are 85 dB/km and 170 dB/km. They represent two different fog densities. For an attenuation of 85 dB/km, the quality factor Q degrades and goes from the value 44.50 to 2.60 when the distance goes from 90 to 180 m. With the proposed technique, the quality factor has improved significantly since we obtained a value that is equal to 29.82 instead of 2.60 at the overall distance of 180 m. On the other hand, we increased the distance until we obtained a quality factor around 2.5. The distance obtained is equal to 360 m, which is twice the initial distance.

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