Design And Implementation Of Underwater Visible Light OFDM Communication System

With the rapid development of ocean observation networks,people have accelerated the mining of materials in the ocean.The ocean observation network is composed of countless tiny sensor nodes,and data exchanges between each node and between the node and the underwater receiving device through wireless communication technology.An efficient and real-time wireless communication scheme is a prerequisite to ensure the normal operation of the ocean observation network.Commonly used underwater wireless communication schemes include radio frequency communication,underwater acoustic communication and optical communication.Compared with the transmission characteristics of light waves under water,electromagnetic waves have serious attenuation in underwater transmission,the communication distance is shorter,the transmission rate of sound waves under water is low,and the communication performance is easily affected by the environment.Therefore,there is an urgent need to find a solution suitable for high-speed underwater information interaction in ocean observation networks.Relying on the strong penetrating characteristics of blue light with a wavelength of in seawater,wireless optical communication technology has brought real-time and efficient data interaction methods for ocean observation networks.In order to further improve the transmission bandwidth and anti-channel interference capability of the underwater optical communication scheme,the OFDM technology in broadband wireless communication can be applied to the field of optical communication.Since the input signal of the LED modulation terminal needs to meet the requirements of real positive numbers,optical OFDM modulation can only obtain signals that meet the requirements by sacrificing the frequency band utilization and structural complexity of the system.This article starts from the four unipolar schemes applied to optical OFDM modulation technology.First,the basic principles of the four schemes are explained in detail through theoretical derivation,and then the four schemes are analyzed and summarized through simulation experiments under underwater background noise conditions.The communication performance of the solution.Next,in order to solve the problem of low frequency band utilization in the traditional optical OFDM modulation scheme,an optical OFDM modulation scheme that uses discrete Hartley transform instead of discrete Fourier transform is proposed.According to the nature of the discrete Hartley transform,two new optical OFDM modulation schemes are proposed to further improve the error performance of the system.Finally,a set of underwater high-speed optical OFDM communication system is designed and built based on the discrete Hartley transform.The work of this paper is listed as follows:(1)Analyze and summarize the advantages and disadvantages of various technologies applied in underwater communication at this stage,and introduce the research status of underwater wireless optical communication technology and optical OFDM(O-OFDM)technology.(2)Through the study of the absorption and scattering phenomena of underwater light waves during propagation,the general model of light attenuation in the ocean is derived,and the feasibility of applying blue light to underwater optical communication is proved through simulation experiments.(3)Four unipolar schemes applied in the field of visible light OFDM communication are elaborated in detail,and the relationship between underwater communication distance and bit error rate of the four systems is analyzed through simulation experiments in underwater background noise.The relationship between underwater communication bandwidth and bit error rate and the PAPR performance of the system.(4)According to the nature of Discrete Hartley Change(DHT),two new schemes were designed.Scheme 1: DHT-based O-OFDM modulation scheme using PPM mapping.Scheme 2: DHT-based composite O-OFDM(ADO-OFDM)modulation scheme using QAM mapping and complex-to-real conversion.Subsequently,the communication performance of the two schemes is studied through the simulation experiment of underwater background noise.(5)The underwater high-speed optical OFDM communication system is designed and implemented using the DSP platform.Including DSP development board design,MOS tube LED switch drive circuit design,digital-analog and analog-digital module design,fast Hartley transform(DIT-FHT)software design,system overall transceiver software design,etc.After testing experiments with different underwater water quality and different communication distances,the communication performance of the system has reached the expected effect.