The Research On Channel Capacity And Phase Recovery Algorithm For Atmospheric Laser Communication Faster-than-Nyquist System

Atmospheric laser communication can achieve information transmission without using optical fiber,and it has the characteristics of no frequency license requirement,high security and large transmission capacity that is considered a feasible method to realize wireless access in the future.Because the data is directly transmitted in the atmosphere,then,the transmission rate of the system is affected by atmospheric turbulence that will lead the actual transmission rate is difficult to reach the theoretical level.In view of this,It is very urgent to how to increase the transmission rate of the atmospheric laser communication system.Faster-than-Nyquist improves the transmission rate by shortening the transmission interval between adjacent pulses,and is compatible with multiple input multiple output,orthogonal frequency division multiplexing and other technologies.It provides an effective method to solve the transmission rate bottleneck problem of atmospheric laser communication system and has a wide development prospection.Atmospheric laser communication faster-than-Nyquist system are affected by turbulence intensity,receiving aperture and other internal parameters of the system.So studying and exploring the methods of enhancing system performance is very significant and practical value.There is inter-symbol-interference in the atmospheric laser communication faster-than-Nyquist system,meanwhile,some factors such as the fluctuation of light intensity caused by atmospheric turbulence,phase noise caused by laser linewidth,and thermal and scattering noise raised in space can cause the received signal constellation diagram dispersive.Consequently,these will lead the system reliability get poor.Therefore,in this thesis,the theoretical ergodic channel capacity and outage capacity of atmospheric laser communication faster-than-Nyquist system are derived,and analyzed the effects of various factors to enhance the system effectiveness.Subsequently,the constant modulus algorithm and Viterbi-Viterbi phase estimation algorithm are cascaded and used for phase recovery,which are improving the reliability of the system.The details are as follows.(1)The expression of the instantaneous signal-to-noise ratio at the receiving end is derived based on the exponetiated Weibull turbulence channel.Thus,the probability density function and cumulative distribution function of the instantaneous signal-to-noise ratio are obtained.Then the expression of the instantaneous channel capacity of the system is derived simplified by the logarithmic and trigonometric formulas.The generalized hypergeometric method is used to obtain the closed expression of the system ergodic capacity and outage probability.Numerical simulation and Monte Carlo simulation are carried out through the computer.the results show that the Monte Carlo simulation curve are very close to numerical simulation curve,which proves the theoretical expression is accurate.Finally,the effectiveness of the system is improved by analyzing the influence of turbulence intensity,receiving aperture,roll-off factor and acceleration factor on ergodic capacity and outage capacity.(2)After proving the atmospheric laser communication faster-than-Nyquist system can bring channel capacity gain,the reason for reducing the system reliability are also analyzed.the effects of inter-symbol-interference,atmospheric turbulence,laser linewidth,thermal noise and shot noise on the received signal are studied.Meanwhile,a reasonable mathematical model is established for each influencing factor.Simulations show that the received signal constellation diagram of the atmospheric laser communication faster-than-Nyquist system occurred dispersive,which affected the signal decision.To address this problem,a phase recovery algorithm is proposed in this thesis.It firstly uses constant modulus algorithm to compensate the amplitude of the received signal,then,cascadeing the Viterbi-Viterbi phase estimation algorithm algorithm to solve the phase rotation problem.The simulation results show that the proposed scheme can effectively recover the signal phase and improve the convergence of the constellation diagram.