Research On Modulation And Demodulation Of Underwater Wireless Optical Communication

Underwater wireless optical communication generally refers to the use of light waves(especially lasers)as a carrier of information,Transmission of video,image,voice and other information in point-to-point,point-to-multipoint,multi-point-to-multipoint communication in seawater.Due to the low frequency and low transmission rate of the baseband signal without modulation,it is easier to receive interference during the propagation process,Therefore,the baseband signal needs to be carrier-modulated,and the low-frequency signal is mounted on the high-frequency signal for transmission.After being transmitted through the seawater optical channel,the modulated signal is demodulated by the demodulation system and finally the baseband signal is obtained.Underwater wireless optical communication technology is very suitable for transmitting important information under water due to its high bandwidth,fast transmission rate and good confidentiality.This paper mainly studies the modulation and demodulation of underwater wireless optical communication.The main contents of the thesis are as follows:1.The optical channel characteristics of seawater are analyzed.The scattering and absorption effects of seawater optical channel on light wave transmission are analyzed.Through simulation analysis,it is concluded that blue and green light with wavelength between 450 nm and 570 nm has the smallest attenuation coefficient in the seawater optical channel.It provides the theoretical basis for using blue and green light as the light source.The composition of underwater wireless optical communication system is analyzed and the design of transmitter and receiver in the circuit of underwater wireless optical communication system is completed according to the actual requirements.2.C++programming language is used to realize the design of the OOK modulation and demodulation program based on STM32 single-chip microcomputer.That is,the base band OOK modulation is realized by controlling the duty ratio of PWM signal.When the baseband signal is 0,the system sends the square wave with low duty cycle;when the baseband signal is 1,the system sends the square wave with high duty cycle.In addition,the program of FSK modulation and demodulation is designed.In other words,the baseband signal is used to control the sinusoidal wave with two different frequencies to realize FSK modulation,and the demodulation of the modulation signal is realized by using two captures.3.Designed the scheme of linear sweep signal modulation LED light intensity and correlation demodulation,namely,generated multiple groups of frequency conversion wave data groups by MATLAB software,which were stored in static memory.DAC module generates frequency sweep signal from data at a certain frequency.Through linear frequency sweep signal,LED luminescence intensity is controlled to conduct modulation at a certain frequency.The demodulation of modulated signals is carried out by using the characteristics of different signal correlations of different frequencies.The demodulation of modulated signals is carried out by using the characteristics of different signal correlations of different frequencies.4.Conducted underwater wireless optical communication system test,conducted bit error rate test on three different modulation and demodulation methods OOK modulation and demodulation,FSK modulation and demodulation of linear sweep signal,and compared the advantages and disadvantages of the three methods.