The need for much better data rate in support for the applications having the high speed has gathered the attention to the development of Free space Optical communication technology (FSO). But the widespread usage of this technology is restrained by the several ill effects caused by the atmosphere on the propagation of optical signals. In order to mitigate such problems in the FSO communication, relay assistance has been employed and this serves the alternative to the spatial diversity. Free

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... ce optical (FSO) communication is a cost-effective and high bandwidth access technique, which has been receiving attention to recent commercialization successes. FSO get affected by the weather turbulence and it leads to severe problems in the transmission channels. In this paper, we proposed the Centralized Relay selection and Optical Filtering (CROF) for processing the signals in the FSO system which overcomes the environmental challenges. Initially, an effective relay is selected with the centralized switch and stay algorithm. This algorithm selects suitable relay for serial path transmission. Here, the serial path is considered for signal transmission in a free path and the distributed Greedy algorithm with serial transmission is utilized to obtain more information. Then atmospheric turbulence induced scintillation noise presented at the signal is eliminated through modified Kalman filtering. Finally, the performance of the proposed FSO system is evaluated in MATLAB simulation platform and these evaluations are analyzed for different atmospheric conditions like fog, haze, rain and clear weather. The performance evaluation shows that the proposed algorithm outperforms the noncooperative scheme and an existing relay selection protocol and reliable communication with improved SNR and BER is achieved. they provide good agreement between theoretical and experimental data [4] . FSO communication is an important area in free-space information systems [5] . This technology is utilized for ship to ship communication, up-and-down connections among space platforms, aircraft, satellite-to-satellite cross links, other ground platforms, and between mobile terminals inside the environment [6]. In the free space environment, the various techniques have been adopted for turbulence management over FSO systems. If the medium includes broadcast over the environment and numerous applicants are distributed in the identical optical channel, the medium reveals arbitrary activities that relate compactly through protocols in the upper layer [7]. A different system designs are required to be enhanced through numerous system layers towards moderate system performance [8] . FSO is considered as the highly secure transmission medium and the main challenge of this technology is posed by the atmospheric weather condition [9] . Here signal scattering occurs due to the optical photon and this is caused by the small particles present in the air. If the particle size is comparable to the wavelength of optical wave, then scattering will occur. Since the particles are having larger size and it is more effective to the shorter wavelength of light [10] . In case of atmospheric attenuations like haze and fog, the visibility (km) is considered as a significant weather parameter and it causes power loss [11] . If snow and rainfall are combined, then the dominant attenuation will occur. Additional experiments were performed in which the attenuation is described as a consequence of scattering [12] . FSO link unavailability is mainly caused due to heavy rain and dense fog in the tropical regions [13] . The most influences of a data link in FSO is due to variations of turbulence such as arbitrary optical power variation, optical absorption and scattering, displacement of the beam centroid and wavefront alterations. Additional influences happening due to environment (i.e. Weather data) are used for further processing [14] . In traditional processing, some methods are used for eliminating the challenge in FSO broadcast. The methods are Kim Model, Kruse Model, Al-Naboulsi Model and Ijaz Model [15] . The Kruse model has been widely used in recent years as the unique model that predicts the attenuation from the visibility data. One of the unique models for predicting attenuation problem is Kruse model that provides the visibility of data. Fog attenuation for Kim model is V < 500 m which is considered as wavelength independent [16] . Attenuation of fog has the spectral band of 0.69-1.55 µm and it has the visibility in the range between 50 m and 1 km. Attenuation wavelength V > 15 m for Ijaz Model and compared to infrared wavelengths, visible wavelengths are more attenuated [17] . The major challenge in the research is environmental condition which produces problems in the overall performances. The clear air turbulence also induces some problems and efficient coherent systems are needed to eliminate these problems. They also reduce the cost of 30 Centralized Relay Selection and Optical Filtering Based System Design for Reliable Free Space Optical Communication over Atmospheric Turbulence