Analysis of Suppressing Turbulence-induced Noise and Fading in Terrestrial Free-space Optical Communications

Terrestrial FSO Communication is an alternative and promising telecommunication technology in addition to existing technologies, such as fiber optics and microwave, to provide flexibility in point-to-point telecommunications services. Even though it has flexibility in deployment and has a relatively larger bandwidth than microwave technology, the terrestrial FSOC has major weaknesses in the signal propagation matter due to constantly fluctuating atmospheric conditions in the form of scattering and beam wandering, and simultaneously causes temporal and spatial noise in the receiver side. It is clear that the main weakness of the terrestrial FSOC is the influence of the Turbulence-Induced Noise and Fading effect. Many papers propose suppressing the fading effect with various methods. On the receiver side, a spatial filter technique has been developed using pinholes to suppress fluctuations due to atmospheric turbulence. Multiple receivers are also proposed to compensate for the fading effect. Statistically based detection techniques applied to sampling-and-hold circuits are also widely discussed. This paper proposes a new method of suppressing fading effects by using a CCD sensor and processing algorithm on the receiver side to track the focus point in the receiver, which constantly moves due to beam-wandering by atmospheric turbulence. Solving the problem of intensity fluctuation due to atmospheric turbulence cannot only be solved on the receiver side. Due to the fading effect, the transmitter side must have a feedback channel from the receiver and bit rate setting control to compensate for random intensity fluctuation. With this feedback control, it is expected FSOC becomes feasible technology as an alternative to the existing terrestrial telecommunication technologies.