Research On Visible Light MIMO Beamforming For Underwater Turbulence Channel

Underwater Visible Light Communication（UVLC）is a technology that uses light waves as a carrier to transmit information in an underwater medium.Compared with traditional underwater acoustic communication and radio frequency communication,UVLC has the advantages of less pollution,high safety,low delay,and fast transmission rate.However,light beams are affected by water absorption,scattering,and turbulence when transmitting information in underwater environments,resulting in reduced system transmission efficiency.Aiming at the problem of signal attenuation and fading in the process of underwater visible light high-speed data transmission,this dissertation studies the propagation characteristics of visible light in turbulent channels,and builds an underwater visible light Multiple Input Multiple Output（MIMO）communication system.Use spatial diversity to reduce the signal’s exposure to deep fading.Due to the increase of turbulence intensity,the signal is gradually affected by fading,and only using spatial diversity technology is not enough to obtain good reliability.In this dissertation,proposes to use beamforming and a beamforming algorithm based on maximum ratio combining is proposed to optimize the control of the beam at the transmitter.The optical power is redistributed and the fading signals that have experienced different paths are combined at the receiver,so as to achieve the purpose of resisting strong turbulence.The main research work completed in this dissertation includes as follows:（1）Aiming at the problem of signal fading caused by turbulence,spatial diversity technology is used to resist the influence of turbulence.Firstly,an underwater visible light MIMO spatial diversity communication system is built,and the random fading coefficients obeying the mixed exponential generalized gamma distribution are generated by the acceptance rejection sampling algorithm.Then,the inhibitory effects of three common receiver mergers on turbulence are analyzed based on the mixed exponential generalized gamma distribution model.In addition,beamforming technology is introduced at the transmitter to redistribute the optical power to resist the signal fading caused by the scintillation of the light intensity caused by the turbulence.The simulation results show that the turbulence intensity is proportional to the system bit error rate,and the maximum ratio combining can effectively suppress the signal fading caused by the turbulence and reduce the system bit error rate.The beamforming introduced at the transmitter is also effectively resist the influence of turbulence.（2）Aiming at the serious signal fading problem caused by strong turbulence,beamforming technology is introduced at the transmitter,and a beamforming algorithm based on maximum ratio combining is proposed to improve the reliability of the system.Through spatial diversity,the received signals are combined through different paths,and the beamforming optimization model with the goal of maximizing the signal-to-noise ratio is established.In a non-negative optical signal and total power constraint condition,the optimal beamforming vector is solved,and sent to the transmitter through the feedback link,the optical power of each LED is redistributed optimally,and more optical signals are allocated to the channels with small fading,forming a target beam with the maximum SNR.In the underwater visible light MIMO system,the bit error rate performance under different turbulence intensity is simulated and analyzed to verify the effectiveness of the proposed algorithm for system improvement.The results show that the bit error rate is under the order of10^-6magnitude,in the case of weak turbulence and strong turbulence,compared with the 2×2 system,the signal-to-noise ratio of the 4×4 system is improved about 6d B and 4.6 d B respectively.Furthermore,in the system of 4×4 with weak turbulence and strong turbulence,compared with the beamforming algorithm and the maximum ratio combining algorithm,the signal-to-noise ratio of the proposed algorithm in this dissertation improves about 0.6 d B,1.8 d B and 2.2 d B,4 d B respectively.It is proved that the algorithm can effectively overcome the fading caused by turbulence and improve the reliability of the system,and the greater the turbulence intensity,the more obvious the gain.