Research On Multi-channel Underwater Optical Communications System Based On The PWM

Underwater communication technology is one of the most important means for human to explore marine resources and understand marine environment.Traditional underwater long wave communication and underwater acoustic communication technology propagation long distance,but it has some disadvantages such as low rate,poor anti-interference ability and lack of confidentiality.In recent years,laser underwater communication has attracted wide attention because of its good confidentiality,fast communication speed and large capacity of communication.The laser communication mainly uses OOK and PPM modulation now,compared with OOK and PPM modulation,PWM modulation has the advantages of strong anti-interference ability,low bandwidth demand,and analog/digital modulation,so the PWM modulation method plays an important role in the field of underwater optical communication.In this paper,a multi-channel underwater optical communication system with PWM modulation method is designed.Firstly,this paper analyzes the attenuation characteristics of light in seawater channels,and the light source wavelength of the underwater communication system is520nm.By analyzing t existing research results,The attenuation coefficient of seawater is estimated to be 0.15-0.25m^-1.This article analyzes the principles of commonly used analog/digital signal modulation methods,and characteristics of various methods are compared.Based on the analysis result and modulation method of seawater attenuation characteristics,this paper chooses?PWM?method as the modulation method of laser.This method has the advantages of simple system structure,strong anti-interference ability and strong laser driving ability.Secondly,because of the large attenuation of underwater laser communication and the strong interference,a multi-channel underwater optical communication system with PWM modulation method is designed.The underwater laser communication of the system is tested with sine wave and square wave with a frequency of 100Hz-5MHz.According to the light attenuation coefficient of seawater and the minimum response power of detector,the maximum communication distance of the system in the low attenuation coefficient sea area is calculated.At last,the real-time communication of analog video,audio,digital image and text signal is carried out respectively by the underwater laser communication system and system parameters are optimized according to signal characteristics.The results from the experiments of underwater optical communication can be seen that the PWM modulation underwater laser communication system has good communication effect,very low error rate(10^-5)and high anti-interference ability for analog signal and digital signal.This makes PWM modulation become one of the most attractive modulation method of underwater laser communication.