Research On The Key Technology Of Underwater Long-Reach Wireless Optical Communication

Compared with the underwater radio communication and acoustic communication,underwater wireless optical communication is able to provide high-capacity and low-latency communication.In the latest research,large capacity underwater wireless optical communication has been developed rapidly.It shows that the transmission rate can reach gigabits per second.However,the development of long-reach underwater wireless optical communication is delayed.With the increasing of ocean activities,the distance of underwater wireless optical communication is the problem to be solved.As with long-reach underwater wireless optical communication,the key technologies including ocean optical channel analysis,optical signal transmitting,optical signal receiving,and modem technique are studied.Based on the modular simulation platform of underwater wireless optical communication,the system parameters of underwater wireless optical communication are analyzed and optimized.By using flat-topped beam at transmitting module and MRC at receiving module,the distance of the communication system can be extended.The feasibility of underwater quantum key distribution is studied for security weaknesses caused by long-reach wireless optical communication.The primary research work in this paper is as follows:1.On the basis of previous studies,the attenuation models of various marine substances are summarized and analyzed.To select the system working wavelength,the different types of seawater are subdivided according to the single scattering albedo.Then,three kinds of seawater optical channel models are analyzed.By comparing the pros and cons of the channel models above,a seawater optical channel model based on Monte Carlo method is selected.2.The system framework of the whole communication system is analyzed.Then,the modeling of functional modules and the whole simulation platform of the underwater wireless optical communication system are built.According to relevant specifications of the commercial system,the simulation platform of underwater wireless optical communication system is validated.Finally,the parameters of the underwater wireless optical communication system are optimized in order to improve the communication distance of the system.The results show that the maximum communication distance of the optimized wireless optical communication system is 16 attenuation lengths and 25attenuation lengths in clear seawater and mixed seawater when the BER is 10^-4.3.In order to improve the communication distance of the underwater wireless optical communication system,it is proposed that using the flat-topped beam at the transmitting and the maximal ratio combining at the receiving.The simulation results show that,when the BER is 10^-4,the distance of underwater wireless optical communication system with flat-topped beam and maximal ratio combining is 25%longer than the distance of the traditional underwater wireless communication system.4.As with the potential security weakness caused by long-reach underwater wireless optical communication system,a secure communication technology scheme combining with the underwater quantum key distribution is proposed.The application scenarios of underwater quantum key distribution are analyzed.Then,based on the simulation platform of the underwater wireless optical communication system,a model of underwater quantum transmission channel is built by using the vector Monte Carlo method.According to the measured data of Muller matrix,the theoretical model of Muller matrix is modified.Finally,the performances of the underwater quantum key distribution system are analyzed,including the transmission distance,the quantum fidelity,the quantum bit error rate and the sifted key generation rate.The results show that when the quantum bit error rate is 25%,the sifted key generation rate is about 17kbps,and the communication distance is about 89m.In the practical application,the secure communication technology requires the relay or multi-node networking to achieve a wide range of underwater secure wireless optical communication networks.