| Free space optical communication(FSO) has attracted significant attention for avariety of applications with a high bandwidth, low cost, fast and easy deployment,license-free. In recent years, many countries have a growing increase in research anddevelopment activites. However, the free space optical communication is vulnerableto the impact of atmospheric turbulence which resulting wavefront randomfluctuations, beam wander, intensity fluctuations. Deterioration of the beam qualitymake performance of free space optical communications worsen such as bit errorrate increased, channel capacity decreased, even the communication interrupted,which seriously affect the stability and reliability of communication links.This article have focused on the impact of atmospheric turbulence on FSO andhave analyzed performance of FSO system over atmospheric turbulence. To mitigateeffects of atmospheric turbulence, a novel adaptive optics compensation technologyhave been used in FSO.First, we investigate atmospheric attenuation effects and atmosphericturbulence effects on the beam propagation. We have researched mechanism ofatmospheric turbulence and models of refractive index fluction. Based on theoryof Gaussian beam transmission in turbulent atmosphere and modified Rytov method,the scintillation index is given define and the scintillation been derived within zeroinner scale and outer scale effects. Several intensity fluctuation probabilitydistribution models have been given.Second, in order to analyze link performance of FSO system over atmosphericturbulence, we consider a FSO system using IM/DD with OOK. The laser beamspropagate through a gamma-gamma turbulence channel with additive whiteGaussian noise (AWGN). The channel is assumed to be memoryless, stationary andergodic, with independent and identically distributed (i.i.d.) intensity fadingstatistics. We also consider that the channel state information (CSI) is available atboth the transmitter and the receiver. The closed-form expressions of bit errorrate(BER), outage probability and the average capacity (ergodic capacity) have beenderived and coupling efficiency has been given. How performance of FSO areaffected by the atmospheric is analyzed in simulationsIn order to mitigate atmospheric turbulence effects, expressions of BER andcoupling efficiency for FSO system based on adaptive optics compensation isderived using theory of adaptive optics and atmospheric turbulence. Adaptive opticscompensation effects for FSO are analyzed in simulations. Although adaptive optics have a good correction results, it has some limitations.First, the conventional adaptive optics can not measure directly the distorted phase.It require wavefront sensor to measure the aberration information and reconstructwavefront which increase complexity and time-consuming calculation affectingseriously the real-time. Second, it is difficult to measure distorted wavefront instrong turbulence causing intensity scintillation. Third, the general wavefront sensoris bulky, expensive, cost and large. We propose blind optimization adaptive optics tocompensate the FSO systems. Blind optimization adaptive optics system is a systemwithout wavefront sensor which measure and reconstruct wavefront. It canmaximize the performance metric by using the optimization algorithm to mistigateturbulence effects. We establish FSO system based on blind optimization adaptiveoptics. According to the system model, numerical simulation compensationexperiments are done, the results show that not only phase can be corrected, but alsointensity scintillation can be corrected by the blind optimization adaptive optics. Inthe optimization process, if the gain value is too large, then the system convergenceis faster, but it may cause oscillation and low accuracy. If the gain value is too small,then accuracy is high, but the system convergence is slow. In order to improve theconvergence speed and accuracy, an adaptive gain stochastic parallel gradientdescent optimization algorithm is proposed. |
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