Research On Underwater Laser Communication Technology

With the development of underwater wireless communication technology,the requirement for communication is higher and higher.Traditional underwater acoustic communication can not meet the real-time transmission needs of picture video and other signals commonly used in current underwater communication,while underwater laser communication can make up for the shortcomings of large delay and slow speed of underwater acoustic communication.At the same time,laser has high modulation frequency,large data capacity and high confidentiality.Therefore,the research on underwater laser communication technology has important research significance and application prospect.The purpose of this paper is to realize a underwater laser communication with long distance and low bit error rate.Firstly,different information modulation technologies are studied,including On-Off Keying(OOK)modulation,pulse position modulation(PPM)and digital pulse interval modulation(DPIM).Then different information check code technologies are studied,including parity check code,hamming code and cyclic redundancy check code.Finally,the image coding technologies are studied,including entropy coding,predictive coding and transformation coding.Among them,the included Huffman coding technology,discrete cosine change(DCT)and stroke coding technology are selected to encode and decode the data.According to the above techniques,the sending and receiving systems of underwater laser communication are designed respectively,including hardware and software parts.In the hardware part of receiving system,image data are collected through the OV7670 camera in the transmission system,and then encoded by the FPGA development board.Finally,the information is loaded onto the 532nm laser by the photoelectric modulator.In the receiving system,PIN photodiode is used to receive the laser signal,and the data processed by the photoelectric receiving circuit and FPGA development board is restored.The software part of the sending system consists of three parts:data acquisition,data processing and data sending withl3 modules.The data acquisition part includes the camera configuration module,SCCB interface module and image acquisition module.The data processing part includes image segmentation module,color space conversion module,zigzag output module,discrete cosine transform module,RLE coding module,BIT coding module,Huffman coding module and serialization module.The data sending part is the packet sending module.The receiving system consists of three parts:data receiving,data processing and data restoration with 11 modules.The data receiving part is the receiving packet module;The data processing part includes deserialization module,Huffman decoding module,BIT decoding module,RLE decoding module,inverse quantization module,discrete cosine inversion module,anti-zigzag module and color space change module.The data restoration part includes image block Mosaic module and VGA driver module.The function of each module of sending system and receiving system,interface list and function simulation are introduced respectively.Finally,the verilog program is written to FPGA,and the communication function is realized by combining with other hardware.At last,three experiments were carried out under different underwater transmission distances,including the serial text communication experiment,the VGA video transmission experiment and the picture transmission experiment.Finally,in the case of the 12m longest underwater transmission distance,it realizes the serial text transmission with a bit error rate of 9.028×10-4 and the transmission of VGA video and pictures with a bit error rate of 5.031×10-4.The resolution ratio of the video and pictures is 640×480.Experiments show that the communication system designed in this paper can realize a kind of underwater laser communication with long distance and low bit error rate.