Research On Receiving Performance Of Distributed Antenna Array In Free–space Optical Communication Link

Free–space optical communication has the advantages of high capacitycommunication, high transmission rate, high anti-jamming capabilities, and receivedgreat attention throughout the world. However, turbulent atmosphere is one of themajor limitations of free–space optical communication systems. Turbulentatmosphere can bring a series of optical effects on the laser beam, such as angle ofarrival fluctuation, intensity fluctuation, phase fluctuation, beam wanders andwavefront aberration etc, seriously affect the receiving performance of thecommunication system. Analyze the effect of atmospheric turbulence to the receptionsystem and improve the overall power of the reception system are the key factors toincrease the reception performance of the communication system. So, we study theimpact which atmospheric turbulence on the reception performance of thecommunication system in this paper. The plane wave propagation horizontal and thepropagation in the slant path through the atmospheric turbulence are analyzed, andthe conclusion that the optimum receiving parameters in two different transfer modesis derived. Moreover，different atmospheric channel models are selected, withtheoretical and experimental research on the parameters of the receiving antennaarray and the bit error rate (BER) performance of communication system. Moredetails are as follows:Firstly, based on electromagnetic wave radiation theory, we study the relationshipbetween the receiving antenna array parameters and the method of improvingantenna array gain, and compare the receiving performance of a single receiverantenna and the antenna array. The receiving performance of the antenna array isbetter than a single large aperture antenna in the same receiving areas is obtained,and the antenna array gain is different with the different numbers of sub-antennas ofthe receiver. The optimum numbers of sub-antennas are different with the differentreceiving areas.Secondly, the performance of the intensity scintillation of the optical-wavepropagation horizontal and the propagation in the slant path through the different atmospheric turbulences are analyzed. Based on the modified Rytov method，thelog-intensity spatial covariance function of the optical-wave propagation in the slantpath through the turbulent atmosphere is derived. Moreover, we drive thatlog-intensity spatial covariance function of the optical-wave propagation horizontalthrough the turbulent atmosphere considering the inner scale and outer scale effects.Based on the aperture-averaging theory, we demonstrate the relationship between thereception performance of the distributed antenna array and the aperture-averagingfactor, and obtain the optimum aperture diameter, numbers of sub-antennas, and thesub-antennas interval of the antenna array receiver in intermediate turbulencepropagation horizontal and the propagation in the slant path.Finally, based on the Gamma-Gamma distribution channel model, we study theimpacts of amplitude fluctuation caused by atmospheric turbulence on the bits ErrorRatio (BER), and use the Mach-Zehnder-interferometer (MZI) based coherentdifferential-phase-shift keying (DPSK) modulation and adopt the antenna arrayreceiver using selection combining (SC) in the FSO communication system. TheBER expression by considering the inner scale and outer scale effects is derived.Moreover, the Inverse Gauss distribution is adopted, and the influence of intensityscintillation and the phase noise caused by atmospheric turbulence to the BERperformance are analyzed. And the receiving distributed antenna array usingMaximum Ratio Combining (MRC) is considered. The performance of bits ErrorRatio in free–space optical communication through the atmospheric turbulence isstudied, and the average BER of antenna array FSO links as a function of the phaseerror considering the inner scale and outer scale effects is derived. This dissertationcan benefit the performance analysis for the reception performance of free–spaceoptical communication system and experimental foundation for the design of thereceiving system.