Study Of Carrier Recovery In Subcarrier Modulated Free-space Optical System

Free-space optical(FSO)communication is a kind of broadband communication method,which combines the advantages of radio frequency and optical communication.This thesis studies the carrier recovery techniques in FSO subcarrier modulator,including subcarrier modulation,coherent demodulation,orthogonal frequency division multiplexing and orbital angular momentum multiplexing.The main work and innovation of this thesis are as follows:(1)Phase and frequency offset of the subcarrier modulation signal in Gaussian channel,Gamma-Gamma atmospheric turbulence channel is numerical analyzed,modulated signal through different channel lead to signal frequency offset and phase offset will cause constellation diagram random perturbations.This thesis discusses the system bit error rate characteristics due to signal frequency and phase offset,and emphasis why the carrier recovery is necessary in FSO with experimental method.(2)Taking into account the nonlinear,non-Gaussian and time-varying characteristics of the signal when it’s through the moderate and strong turbulence channels,the feasibility of applying particle filter algorithm to carrier recovery is demonstrated.This thesis utilized particle filter in nonlinear,non-Gaussian and time-varying system parameter estimation and state filtering of subcarrier modulation,the frequency of subcarrier modulation signal is estimated.The simulation and experimental results show that particle filter can apply to the signal frequency estimation,beneficial to the subcarrier modulation signal demodulation.(3)Consider the subcarrier modulation signal is cyclostationary,and the cyclic spectral,high-order cyclic cumulants can able to restrain the Gaussian noise,this thesis proposed signal carrier frequency estimation by utilizes cyclic cumulants and cyclic spectral.Fourth-order cyclic cumulants and cyclic spectral of MPSK under Gaussian noise,Gamma-Gamma non Gaussian noise is simulated and analyzed,under the same data length,the relationship between signal to noise ratio,intensity fluctuation and carrier estimation is discussed.Experiments show that this method has the ability to noise suppression(4)The orbital angular momentum division multiplexing technique and the involved carrier recovery issue are studied.Different order of Laguerre-Gaussian beams is generated by spatial light modulator,the load signal with Laguerre-Gaussian beam orbital angular momentum superposition states is experimental studied.By using the cyclic spectral to separate the orbital angular momentum,the experimental basis is provided for orbital angular momentum multiplexing...