| Information society has an urgent need for high-speed and secure communication.Free space laser communication has the advantages of broadband and high speed,and chaos with characteristics like initial value sensitivity and pseudo-randomness is naturally suitable for information encryption.Applying chaos encryption technology to space laser communication can encrypt information at the physical layer and ensure the safety of information,which is a feasible technology to realize secure and high-speed communication.However,the atmospheric turbulence in the free-space channel will seriously degrade the quality of the transmitted signal,which limits the development and application of space laser communication.Up to now,there are relatively few studies on the effect of atmospheric turbulence on free space chaos laser communication,it is quite necessary to carry out related researches.In this thesis,the design of free space chaos laser communication system including atmosphere channel is presented,based on an optoelectronic feedback chaos generation system using nonlinear optical devices.And the effect of the mismatch noises caused by atmospheric turbulence on communication performance is investigated both theoretically and experimentally.Firstly,we analyze the effect of atmospheric turbulence on the BER in the uplink and downlink respectively by numerical simulation.In the downlink,the effect of atmospheric turbulence is mainly the intensity scintillation effect,and there is also obvious beam wander effect in the uplink.We compare the BER with and without atmospheric turbulence,simulation results illustrate that communication performance deteriorates significantly under the influence of atmospheric turbulence.After that,we analyze the influence mechanism of atmospheric turbulence on the BER of system.Simulation results demonstrate that the larger or smaller received optical power corresponds to a worse instantaneous BER,and although the probability that received optical power causes larger mismatch noises is small,its corresponding larger instantaneous BER deteriorates the ensemble average BER significantly.In the uplink,the beam wander effect leads to an overall attenuation of the received optical power,so that the probability that received optical power at smaller level furtherly increases,which leads to that the instantaneous BER within this range dominates the ensemble average BER.Subsequently,we compare the independent effects of internal and external mismatch induced atmospheric turbulence on the BER,internal mismatch deteriorates the BER more than external mismatch on the whole.But unlike in the downlink,there exists an optimal divergence angle in the uplink to minimize the system BER,which also leads to a greater impact of external mismatch on the BER than that of internal mismatch when divergence angle is at small level.After the numerical simulation analysis,we optimize the system structure in detail and build an indoor space chaos laser communication system.After preliminary and precise commissioning,system is under well-synchronization state.Then the numerical simulation results are primarily validated by simulating the effect of atmospheric turbulence with flowing water vapor.The experiments test the effect of atmospheric turbulence on the BER at different bit rates,as well as simulate the independent effect of internal and external mismatch on the BER by changing the parameters.Overall experimental results are consistent with the simulation results.In addition,we further test the BER of the eavesdroppers to verify the encryption effect.This thesis presents a theoretical and experimental study of the effect of atmospheric turbulence on performance of space laser chaos communication systems.The results of this study lay the foundation for understanding the effect of atmospheric turbulence on free-space chaotic laser communication and have reference significance for the design of actual space chaos laser communication systems. |
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