Performance Analysis Of Underwater Wireless Optical Communication System Considering The Influence Of Ocean Turbulence

It is well known that the development and utilization of marine resources is inseparable from the acquisition and transmission of marine data.Therefore,the development of underwater communication technology is of great strategic significance to the development of human society.Underwater wireless optical communication(UWOC)has the characteristics of wide bandwidth,high available carrier frequency,and small transmission delay,which can well meet some of the needs of underwater communications in the new era.However,due to the existence of ocean turbulence,absorption and scattering and other factors,the energy of the laser beam carrying useful information in the transmission process will be attenuated,and the strength of useful signal detected at the receiver will decrease,which will seriously affect the performance of the communication system.Therefore,based on two new ocean turbulence channels—Generalized Gamma Distribution(GGD)channel model and Mixture Exponential-Generalized Gamma(EGG)channel model,this thesis mainly studies the performance of UWOC systems over composite fading channels,and employs orthogonal frequency division multiplexing(OFDM),space-time block coding(STBC),error correction coding,and relay assistance technologies to optimize the performance of UWOC systems.The main contents are as follows:(1)Considering the influence of ocean turbulence and absorption and scattering,based on the composite GGD channel model,the theoretical expressions for the average bit error rate of the serial relay-assisted OFDM underwater wireless optical communication(UWOC)system using the decode and forward(DF)protocol under OOK modulation,PSK modulation and QAM modulation are derived and verified by Monte Carlo(MC)simulations.The effects of different modulation methods,different temperature gradients,different relay nodes and different transmission environments on the performance of the communication system are analyzed.Furthermore,on the basis of the established serial relay OFDM-UWOC system,the RS error correction coding is used to further improve the error performance of the communication system.(2)The beam spread function(BSF)is used to describe the path loss model caused by the absorption and scattering of seawater,and the composite EGG channel model is constructed based on the BSF and the EGG channel model.Based on the composite EGG channel model and binary phase shift keying(BPSK)modulation,the theoretical expressions for the average bit error rate of the serial relay-assisted UWOC system under the conditions of Single-Input Single-Output(SISO),Multiple-Input Multiple-Output(MIMO),i.e.,using the STBC scheme,are derived.Since the GGD model can be obtained by changing the parameters of the EGG channel model,the theoretical expression of the average bit error rate of the UWOC system based on the composite GGD channel model under these two conditions is also given.In addition,MC simulation is used to verify the correctness of the expressions.The influence of the number of relay nodes,different antenna settings,different temperature gradients,different bubble levels,and different seawater conditions on the communication performance of the UWOC system is analyzed.Furthermore,on the basis of the established UWOC system,the RS code,cyclic code and convolutional code are used to further improve the performance of the system,and the degree of improvement of the performance of the system by these coding schemes is compared.This work is beneficial for the UWOC system design.