Research On Key Technology Of Underwater Wtreless Optical Communication Based On Orbital Angular Momentum

The gradual commercialization of fifth generation(5th Genaration,5G)communications has led to an explosion of data streams.Multimedia applications are growing exponentially every day,generating massive amounts of mobile data and requiring high data rate wireless connectivity.Optical Wireless Communications(OWC)technology technology can not only use extremely broad bandwidth resources,but also guarantee high quality communication.Compared with microwave communication,OWC has the advantages of high security,high communication rate and immunity to electromagnetic interference,and therefore has gained great attention from academia and industry.In recent years,the underwater environment has become an area of increasing human activity due to technological advances and economic demands.Underwater Wireless Optical Communications(UWOC)technology has gained more and more attention as an important application technology expansion of OWC in underwater scenarios.OWC technology based on Orbital Angular Momentum(OAM)multiplexing has been listed as one of the important technical alternatives for 6th Genaration(6G)communication due to its large transmission capacity and high frequency band utilization.In order to further improve the information carrying capacity of UWOC system,UWOC technology based on OAM is considered as a promising solution.However,in the UWOC environment,the orthogonality of OAM is impaired due to wavefront distortions caused by underwater turbulence.Due to absorption,scattering effects,the light intensity of the OAM beam will be attenuated and the spot of beam will be wandering.The gradual spread of the beam may provide an opportunity for eavesdroppers to eavesdrop or modify the transmitted signal,which means that the security of OAM-UWOC may become a potential pitfall in practical applications.To address the above problems,this paper analyzes and investigates the OAM beam modeling construction problem in underwater turbulent channel,the channel anti-impairment technique based on Adaptive Optics(AO),the OAM-UWOC propagation statistical distribution modeling and system performance analysis,the longitudinal orbital angular momentum multiplexing(LOAMM)system in UWOC and phase chaos encryption of OAM-UWOC.system,thus,the large capacity,high reliability and secure communication of OAM-UWOC system are realized.The main research works and innovations of this paper are as summarized as follows.1.For the problem of imperfect construction of turbulent channel in OAM-UWOC system,this paper proposes and improves an underwater turbulent channel model based on plane wave and spherical wave models,respectively.The model is composed by a Monte Carlo simulation consisting of a random phase screen framework.In this model,the coherence width and scintillation index of the wave beam are used to describe both the underwater turbulence strengths and the required phase screen properties located along the propagation path.The phase screens are generated by the Discrete Fourier Transform(DFT)method.In order to construct a phase screen that conforms to the underwater power spectral density characteristics,this paper compensates for the phase in the spectral domain of the low-frequency signal of the spatial phase screen by using a random sampling method in the frequency domain and by using independent and approximately logarithmically uniformly distributed points.In order to verify the accuracy of the numerical simulation results of the phase screen and the aberration spot,the obtained ensembleaveraged numerical results are compared with the corresponding resolved values in this paper.The results show that the turbulence channel model of the OAM-UWOC system constructed in this paper is valid and reliable.2.For the channel impairment problem caused by the underwater turbulent channel in the OAM-UWOC system,three different AO techniques are proposed in this paper to compensate for the distorted OAM beam.First,we propose a random amplitude masks amplitude-phase retrieval algorithm aided AO(RAMAPRA-AO)technique.This technique achieves the phase reconstruction of the beam by multiple amplitude modulation of the aberrated OAM beam,phase reconstruction of the beam was achieved using the phase recovery algorithm and the amplitude sampling information,thus completing the compensation of the aberrated beam in the far field.Secondly,this paper further proposes a defocus measurements aided AO(DMA-AO)technique based on probe beam,in which the beam intensity information of multiple defocusing planes are employed to recover the phase information of the distorted probe beam relying on a non-convex optimized phase retrieval algorithm,thus realizing the channel impairement compensation of the OAM-UWOC multiplexing system.In addition,this paper proposes a lens angular deflection measurements aided AO(LADMA-AO)technique based on the lens angular deflection measurements of the probe beam,which uses different angular deflections of the lens to achieve less correlated amplitude measurements,hence realizing the phase reconstruction of the probe beam through different amplitude measurements and phase retrieval algorithms,then the goal of distorted OAM beam compensation is accomplished.The simulation results show that the multi-amplitude based phase reconstruction method can achieve the channel impairement mitigation for OAM beams in underwater turbulent channels.3.In order to design robust and reliable OAM-UWOC systems,it is crucial to deeply investigate the statistical distribution model of OAM optical beams fluctuations due to underwater turbulent channels.To address the shortcomings of the existing OAM-OWC transmission statistical distribution models,this paper proposes a Generalized Gamma Distribution(GGD)distribution model with additional independent parameters and a mixture Generalized Gamma Johnson SB(GJSB)distribution model.The simulation results show that both models can effectively characterize the probability density function(PDF)of the optical radiation of the transmission channel of the OAM-UWOC system.Based on our proposed distribution model and the mixture exponentialgeneralized gamma(EGG)distribution model,which has been validated in the underwater optical communication environment,the performance metrics of the OAM-UWOC system,including channel capacity,outage rate and bit-error rate(BER),are analyzed and investigated in this paper.The simulation results show that the proposed statistical distribution model can effectively characterize the transmission fluctuations of OAM beams in single channel systems,diversity reception systems and multiplexed systems in underwater space optical communication,and can be used to overcome the computational complexity of Monte Carlo simulations,thus achieving analytical system performance evaluation.4.In this paper,the longitudinal orbital angular momentum multiplexing(LOAMM)system based on frozen waves(FW)is applied to underwater space optical communication for the first time to address the security issues in OAM-UWOC systems.By using the feature that different OAM modes located in different intervals in the propagation direction in the LOAMM system,it can not only effectively avoid the simultaneous eavesdropping of the multiplexed channel information,but also reduce the crosstalk between OAM modes induced by the turbulent channel.In this paper,we conceive the LOAMM-UWOC system and utilize the GJSB statistical distribution model to derive the probability of the strictly positive secrecy capacity(SPSC)of the system under a single channel.In addition,the average secrecy capacity(ASC)and SPSC performance of the multiplexed system are evaluated and analyzed based on different OAM mode purity and crosstalk models.Meanwhile,this paper proposes a chaotic phase encryption technique based on optical backward propagation(OBP)and random phase masks(RPM),and applies it to the OAM-UWOC system while evaluating the ASC performance metrics of the multiplexed system.By using RAMAPRA-AO technique to realize the precompensation of underwater turbulent channel,the system can not only effectively mitigate the impairement caused by the underwater turbulent channel,but also guarantee the secure communication of the system.