| The free-space optical communications(FSOC)refers to transmission in unguided propagation media through the use of optical carriers,i.e.,visible,infrared,and ultraviolet bands,which is used for point-to-point high-speed communication.Compared with the traditional radio frequency communications,the FSOC has high carrer frequency,wide spectrum,good directivity,high transmission rate,and strong security performance.Compared with the optical fiber communications system,the FSOC system has the benefits of flexible link erection and low cost.Therefore,FSOC systems are appealing for a wide range of applications such as inter-satellite and satellite-to-ground communications,emergency communications,last-mile access,and back-haul for cellular systems.However,the optical beam propagating through the atmospheric space will be affected by various phenomena,especially atmospheric turbulence.Atmospheric turbulence is the most important factor that causes the deterioration of the FSOC channel.This paper is focused on the characteristics of the atmospheric turbulence channel and technologies combating against the turbulence.The main contents of this paper are as follows:1.The simulation model and emulation platform of laser beam transmission through an atmospheric channel are established based on the Kolmogorov and Von Katman turbulence model.Based on the emulation platform,the transmission of high-speed QPSK signals is demonstrated.The average received power and its fluctuation under different turbulent strength and different transmission distances are studied.The distance of 0.5 m,1 m,and 2 m indoor is used to emulate the link length of 0.5 km,1 km,and 2 km respectively.The experiment shows that the received power decreases with the increase of turbulence strength and the transmission distance.The fluctuation of the received power first increases and then decreases with turbulence strength and the transmission distance because of the aperture averaging.2.A 320 Gbps Nyquist channel transmission with OTDM-QPSK signal through emulated 4 km free-space turbulence link is demonstrated to study the effect of atmospheric turbulence on the bit error ratio(BER)and outage probability.3.The collection of free-space light by few-mode fibers is studied.The coupling efficiency of different kinds of fibers under turbulence is measured.Under moderate-to-strong turbulence,utilizing three-mode fiber or six-mode fiber instead of SMF can improve the average free-space-to-fiber collection power by 4 dB or 7 dB,and the relative standard deviation can be decreased by 34%or 49%.4.The adaptive optics(AO)system of free-space light coupling into a three-mode fiber is established.The experiment shows that the coupling efficiency of FMF can be further improved?10 dB by utilizing an AO system under strong turbulence.5.The concept of FSOC based on mode diversity coherent receipt is proposed and established.The performance and implementation of combining are analyzed.A 40 Gbps QPSK signal transmission based on three-mode diversity is demonstrated.Considering the same BER or interruption probability,the power benefit of three-mode diversity under moderate-to-strong turbulence is 4?5 dB.And the benefit reduces to 2 dB under extremely strong turbulence.6.The improvement of system performance by combining the spatial diversity and mode diversity technology is studied on the simulation of the satellite-to-ground transmission link.Simulation results of geostationary Earth orbit(GEO)to earth downlink transmission show that utilizing three-mode diversity or four-aperture diversity technology can improve the transmission performance by 6?9 dB or 7.5?9 dB,respectively.The combination of three-mode and four-aperture diversity technology can further improve the transmission performance by 2 dB. |
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