Study Of The Properties Of Flat-topped Beams Propagating In Atmospheric Turbulence And The BER Of Communication Channel

Atmospheric laser communication systems are becoming increasingly popularbecause of their many advantages. However, the performance of atmospheric lasercommunication systems is affected by atmospheric turbulence. In order to ensure thestability of communication system, it is significant to research the influence ofatmospheric turbulence on atmospheric laser communication systems.Atmospheric turbulence has a significant impact on the quality of a laser beampropagating through the atmospheric, which can severely impair atmospheric lasercommunication systems. In this paper, the fundamental of atmospheric turbulence isintroduced, the formula for calculating the intensity distributions, beam spreading,power in bucket (PIB) and spectral characteristics are analyzed in atmosphericturbulence from weak to strong. The effects of the beam order, the spatial coherence,and the turbulent parameter on the intensity distributions, beam spreading, spectralcharacteristics and PIB have been discussed in detail. Using flat-topped beams as lightsource and different modulations, the influence of the scintillation and beam spreadingon the bit error rates (BER) in laser communication system is analyzed. Based on thecorrecting error ability of RS code, the performance of communication system iseffectively improved by using RS code in PPM modulation scheme. The results showthat the on-axis intensity of partially coherent flat-topped (PCFT) beams decreases withincreasing turbulence and decreasing coherence of the source, whereas the on-axisintensity of PCFT beams in the far field decreases slightly with increasing beam order.The spreading of PCFT beams is smaller for beams with a higher order, a lowercoherence degree of the source and a weaker atmospheric turbulence. The power inbucket (PIB) of PCFT beams both in turbulent media and in free space decreases byincreasing beam order, decreasing the correlation length and increasing turbulence.Furthermore, if the scaling law is valid, the normalized spectrumS (ω)of PCFT beamspropagating through turbulent atmosphere is the same as the normalized sourcespectrumS0(ω). This similarity is valid for any arbitrary order of the flat-topped “N”.If the scaling law is not valid,S (ω)of PCFT is different fromS0(ω). The on-axisscintillation index is effectively reduced and the BER of atmospheric lasercommunication systems is decreased by using flat-topped beams as source. Thus, theinfluence of atmospheric turbulence on BER is decreased and the performance of laser communication systems is effectively improved by using flat-topped beams as sourcewith PPM modulation scheme. The BER of laser communication systems is decreasedand the performance of laser communication systems is effectively improved bychoosing RS code as channel code method of atmospheric laser communicationsystems.These studies have great significance in the development of atmospheric lasercommunication systems.