Analysis Of Optical Waves Underwater Propagation And Channel Characteristics On Communication Performance

In order to ensure the accuracy and timeliness of the propagation of data,images,video and other information underwater,propagation and channel characteristics of underwater laser have become the focus of research.When laser is transmitted underwater,laser beam energy is greatly attenuated because of ocean turbulence effect and attenuation characteristics of underwater suspended particles,which caused a series of turbulence effects such as beam expansion,beam wander and intensity fluctuations.These affects channel characteristics and communication performance of propagation.In severe cases,communication may be interrupted.The main work of this article is as follows:1.We theoretically analyzed propagation characteristics of optical waves in isotropic ocean turbulence,including intensity distribution of laser,beam centirod effect,long-term beam expansion and scintillation.Based on the spectral inversion method of simulated random phase screen,we gave random phase screen model in ocean turbulence and improved the statistical characteristics of the phase screen through fourth harmonic compensation.Also we simulated and analyzed the changes of the laser intensity distribution,beam wander,and scintillation after Gaussian beam passed through ocean turbulence,and compared it with the theoretical value.The results show that with the increase of?,?_T and the decrease of?,the intensity of turbulence increases,these causes beam wander,long-term expansion,and scintillation increasing.The simulation results are in good agreement with the theoretical results and larger than theorectial value.2.Based on OOK modulation method,we used Gamma-Gamma and exponential Weibull distribution as turbulent channel model,and gave the probability density function of underwater wireless optical channel which considerd three factors of attenuation effect,aiming error and turbulent.And we also calculated closed channel expressions of average channel capacity and outage probability.It is found that the greater?and anisotropic factor,the smaller?and?_T,the weaker ocean turbulence.It leads to an increase in average channel capacity and a decrease in outage probability.The increase in beam source size and quantum efficiency also leads to an increase in channel capacity.Seawater is turbid,attenuation coefficient of particles increases,average channel capacity decreases and outage probability increases.Average channel capacity of EW distribution is higher than GG distribution.Outage probability of EW distribution is higher than GG distribution when the normalized SNR is larger.The transmitted power increases,and system performance is improved.And the larger the normalized jitter standard deviation,the lower system reliability and the smaller average channel capacity.Normalized beamwidth has a non-linear relationship with average channel capacity and outage probability,and the difference in transmit power will affects value of normalized beamwidth.When _LW/R is about 3,average channel capacity reaches the maximum;when it is about 7,outage probability is optimal.In actual use,larger transmission power,proper normalized beam width and smaller jitter standard deviation can improve system reliability.3.In order to improve laser underwater propagation performance,several methods have been proposed.Firstly,based on on-off keying modulation method,we analyzed the expression of average bit error rate when using direct detection and coherent detection systems in the combined channel of Gamma-Gamma distribution turbulence,attenuation and aiming error.After that,we introduced the common modulation methods such as OOK,PPM,PSK and DPSK modulation and derived expressions of average bit error rate of OOK and DPSK modulation in coherent detection.Then we studied average bit error rate of SISO,SIMO and MIMO systems.Finally,the large aperture receiving technology is introduced.Research indicates that transmitting system uses spatial diversity technology and modulated,and the receiving end uses coherent detection and large aperture to receive,which can effectively improve the system communication performance.