Performance Analysis Of Relay-Assisted Underwater Wireless Optical Communication Systems Under Ocean Turbulence Fading Channels

In recent years,with the increase of ocean explorations,high speed and real-time data transmissions are required.Comparing with the traditional radio frequency and underwater acoustic communication technology,underwater wireless optical communication(UWOC)has higher bandwidth and speed,and better security,and has attracted much attention.However,many of the recent research activities in this area show that due to three main degrading effects,including absorption,scattering,and turbulence-induced fading,the present UWOC system is applicable in short ranges that are typically less than 100 m,thus hindering its widespread usage.Therefore,it is of great significance to study how to overcome the adverse effects of the above three factors in order to improve the communication quality of UWOC system.Based on the ocean turbulence channel model and considering the combined effects of absorption,scattering and the misalignment,the performance of relay-assisted UWOC system with low density parity check code(LDPC)and multiple pulse position modulation(MPPM)considering the dynamic decision threshold(DDT)method is studied.The main contents are as follows:(1)Based on the decode-and-forward protocol,the performance of a multi-hop parallel UWOC system is investigated with the combined effects of absorption,scattering,the misalignment,and the ocean turbulence-induced fading.Firstly,the expressions of the Probability Density Function(PDF)and Cumulative Distribution Function(CDF)under log-normal distribution are deduced.With the max-min criterion as the best path selection scheme,the CDF for identically and independently distributed and non-identically and independently distributed links in intensity modulation/direct detection systems are derived,respectively.Then,the analytical ABER expressions of binary phase shift keying and multiple phase shift keying subcarrier intensity modulation schemes are deduced with the help of the Gauss–Laguerre quadrature rule.Finally,the effects of different various water types,link length and turbulence conditions on ABER of system are analyzed.Monte Carlo(MC)simulation is used to verify the correctness of the model and results.In addition,LDPC codes are applied in the simulation to improve the system performance.(2)The symbol error rate(SER)performance of a multi-hop parallel UWOC system with MPPM scheme has been investigated over aggregated channel model.Firstly,based on a new ocean turbulence channel model,the mixture exponential generalized Gamma(EGG)distribution model,which effectively describes the air bubbles and temperature gradient-induced irradiance fluctuations,and considering the absorption,scattering and the misalignment,a multi-hop parallel UWOC system with MPPM is established.On the basis of the expressions for the CDF of aggregated channel gain and multi-hop parallel system gain,the closed-form SER expressions for DDT scheme is derived.And it is verified by Monte Carlo simulation.Finally,the SER performance is further analyzed systematically under different bubbles level,temperature gradient and structure parameters.The results show that the EGG channel model can accurately capture the combination of air bubbles,temperature gradient and salinity fluctuation gradient,making it a unified model to better study the system performance.In addition,DDT scheme can bring better performance to the system.This thesis can provide references for the design and implementation of relay-assisted UWOC system.