Research On The Performance Of Wireless Optical Communication Systems Under The Combined Effects Of Atmospheric Turbulence And Pointing Errors

Various types of information are exploding with the rapid development of the internet and communication technology.Due to the limitation of spectrum resources,traditional radio frequency(RF)communication does not have the capability to meet the ever-increasing demand for future high-capacity transmission,and a new communication technology is needed urgently to relieve this pressure.Wireless optical communication,also known as free-space optical(FSO)communication,has attracted significant attention recently in academia and industry as it offers the advantages of high bandwidth and capacity.When a laser beam propagates through the atmosphere,it will be affected by the atmospheric turbulence,which causes the degradation of the beam quality and link performance.It has been shown that intensity scintillation and pointing errors are major performance limiting factors for a long-distance FSO system.In this dissertation,we investigate the performance of FSO systems in different scenarios and the obtained results provide a theoretical guidance for practical engineering.Specifically,the main research contents and innovations of this dissertation are summarized as follows:(1)The challenges of parameters estimation for a composite lognormal-Rician turbulence and pointing errors channel is addressed.A novel estimation algorithm,namely saddlepoint approximation(SAP)estimator is formulated.It is shown that the proposed estimator can achieve a trade-off between the computational cost and accuracy.The mean square error(MSE)and normalized mean square error performance(NMSE)of SAP estimator are investigated by Monte Carlo simulation.(2)The SAP estimator is extended to study the estimation performance over GammaGamma turbulence channels with pointing errors in view of its successful application of parameter estimation in the composite lognormal-Rician turbulence and pointing errors channels.The MSE and NMSE performance for the considered channel models are evaluated by Monte Carlo simulation.Moreover,the effects of the diameter of the receiver aperture on the estimation performance are analyzed.(3)The performance of multiple-input multiple-output(MIMO)systems employing equal gain combining(EGC)over Lognormal-Rician turbulence channels with pointing errors is investigated.Based on the series representation and algebraic approximation,a novel probability density function expression(PDF)expression for the sum of the considered channel models is developed.Also,the impacts of the number of apertures and beam width on the approximation error are performed in detail.To reveal the importance of proposed approximation,new approximate closed-form expressions of the ergodic capacity,outage probability,and bit-error rate(BER)are derived.Monte Carlo simulations are present to validate analytical results.(4)The performance of MIMO systems employing EGC technique over GammaGamma turbulence channels with pointing errors is present.By applying Jensen's inequality and algebraic approximation,a novel PDF expression for the sum of a composite fading channel is developed.Note that the obtained result is more conductive than the existing results in the literature from the viewpoint of parameter estimation.Based on the obtained PDF,new approximate closed-form expressions of the ergodic capacity,outage probability,and BER are established.Monte Carlo simulation results verify the accuracy of derived expressions.(5)The performance of low-density parity-check(LDPC)codes over atmospheric turbulence fading channels with pointing errors is studied.Considering the advantages of neural networks in function approximation,this paper improves the check node message update rule in the traditional belief propagation(BP)decoding algorithm and proposes a new low-complexity decoding algorithm,namely neural network decoding.To illustrate the validity of the proposed method,we present a comparison between neural network decoding and BP decoding over the composite channels using the Monte Carlo simulation.Moreover,the Shannon limit for the MIMO systems employing the maximal ratio combining(MRC)is calculated.