Research On Performance Of Multiple Apertures Receiver System For Free-space Optical Communications

Free-space optical(FSO)communication is a high-tech field of space communication.High-capacity ability,better security,anti-electromagnetic interference,less volume,less mass,less power consumption and license-free are all FSO benefits.So,since the invention of laser,FSO has gradually come from the theoretical and experimental study to the application stage with the development of the optoelectronic devices.As an important part of FSO link,satellite-to-ground link and terrestrial horizontal link will be affected by the atmospheric turbulence when laser beam transmits through the atmosphere.Atmospheric turbulence can cause atmospheric refractive index fluctuations which will influence the propagation of the laser beam,destroy the coherence of the optical field and result in a series of optical effects on the laser beam such as intensity fluctuation,phase fluctuation,etc,and may seriously affect the communication process of the signal-to-noise ratio(SNR),bit-error rate(BER),outage probability and other important performance.In order to eliminate the impact of the atmosphere turbulence on the performance of FSO systems,multiple apertures receiving technique is wildly recognized as a powerful fading-mitigation technology.There are three common signal combining methods for multiple apertures receiver system,maximum ratio combining(MRC),equal gain combining(EGC)and selection combining(SC).MRC combines the received signals from each aperture according to different weight to maximize the SNR of the combined signal.EGC combines the signals from each aperture according to the same weight.SC receiver system just keeps the signal of one aperture which has the largest SNR in real time and discards the rest.In this paper,based on the theory of atmospheric laser beam transmission and the Gamma-Gamma distribution model,BER and outage probability are studied for satellite-to-ground downlink optical communication with direct detection and coherent detection multiple apertures receiver system,SNR and BER are studied for fiber-based direct detection and coherent detection multiple apertures receiver system.Satellite-to-ground downlink FSO communication model with direct detection multiple apertures receiver system is established.Expressions of BER for satellite-to-ground downlink direct detection MRC,EGC and SC receiver system are presented,and expressions of outage probability for EGC and SC receiver system are presented.The BER and outage probability of MRC,EGC and SC direct detection multiple apertures receiver system and single aperture receiver system with the same total aperture area are comparative analyzed for satellite-to-ground downlink optical communication.The results show that the communication performances of MRC and EGC direct detection multiple apertures receiver system are better than that of single aperture receiver system,and the communication performance of SC multiple apertures receiver system is better than the single aperture receiver system when the BER or outage probability is lower than a certain threshold.At the same time,the communication performances of the three direct detection combining methods are decreased according to the order of MRC,EGC and SC.Satellite-to-ground downlink FSO communication model with coherent detection multiple apertures receiver system is established.Expression of BER and outage probability for satellite-to-ground downlink coherent detection MRC,EGC and SC receiver system are presented.The BER and outage probability of MRC,EGC and SC coherent detection multiple apertures receiver system and single aperture receiver system with the same total aperture area are comparative analyzed for satellite-to-ground downlink optical communication.The results show that the communication performances of MRC and EGC coherent detection multiple apertures receiver system are better than that of single aperture receiver system,and the communication performance of SC multiple apertures receiver system can be better than the single aperture receiver system under large zenith angle conditions.At the same time,the communication performances of the three coherent detection combining methods are decreased according to the order of MRC,EGC and SC.In the study of fiber-based FSO systems,in addition to consider the communication performance impact of intensity fluctuations caused by atmospheric turbulence,it is also necessary to consider the random variation of the spatial light coupling efficiency due to the wave-front phase distortion.The performances of fiber-based direct detection and coherent detection FSO with multiple apertures receiver system are studied based on the probability density function of fiber coupling efficiency under atmospheric influence.The structure models of horizontal link fiber-based direct detection and coherent detection multiple apertures receiver system are established.Expressions of SNR and BER for fiber-based direct detection and coherent detection MRC and EGC receiver system are presented.The BER performances of MRC and EGC fiber-based multiple apertures receiver system and single aperture receiver system with the same total aperture area are comparative analyzed for direct detection and coherent detection models.The results show that the communication performances of MRC and EGC direct detection and coherent detection fiber-based multiple apertures receiver system are better than that of single aperture receiver system.At the same time,the communication performance of MRC is better than EGC.In order to verify the correctness of the previous theory and simulation results and demonstrate the practical application of the multiple apertures receiver system,urban laser link of multiple apertures receiver system theoretical verification experiment is designed and established.Multi-channel simultaneous measurement of the received light intensity of multiple apertures receiver system is conducted under the influence of atmospheric turbulence.The experimental results of the intensity scintillation index,probability distribution and fading probability for different number of receiving apertures are presented.The Gamma-Gamma distribution theory and simulation results of multiple apertures receiver system are verified.The BER performances of MRC,EGC and SC multiple apertures receiver system are analyzed for direct detection and coherent detection models according to the experimental light intensity samples.The communication performance theory and simulation results of direct detection and coherent detection multiple apertures receiver system are verified.The research work of this thesis has laid a theoretical foundation for the application of satellite-to-ground downlink direct detection and coherent detection multiple apertures receiver system.Theoretical bases are presented for the development of fiber-based direct detection and coherent detection multiple apertures receiver system.And the experimental verification for system design and system performance analysis of FSO multiple apertures receiver system is provided.