Research On Modulation And Demodulation Technology Of Underwater Photon-counting Wireless Optical Communication System

In order to achieve long-distance underwater wireless optical communication(UWOC),a high sensitivity Single Photon Avalanche Diode(SPAD)has been studied as an optical signal detection device.Since SPAD outputs discrete single photon pulse signals,the signal demodulation technology of the traditional wireless optical communication system cannot be applied to the photon-counting wireless optical communication system.In addition,underwater channel variability,optical quantum effect and the random process of photon detection will affect the communication link and performance of long-distance photon-counting system.Therefore,according to the needs of long-distance communication of photon-counting UWOC system,this paper focuses on the modulation and demodulation technology and reliable data transmission of photon-counting wireless optical communication.The main research contents and results are as follows:1.We proposed a method for recovering clock and data directly from discrete random pulse sequences output by SPAD.On this basis,a new communication model is established,taking into account not only fluctuations in photon flux and quantum efficiency of photon detection in a time-slot,but also the phase difference of the recovered clock caused by it.The experiments verify the effect of different system parameters on the average bit error rate(BER).The experimental results show that,with an average of only 10 photons in a time-slot,underwater photon-counting communication with a BER of 3.0×10-4,a baud rate of IMbps,and a maximum theoretical communication distance of 362m can be achieved.2.Considering the effect of the number of photons in the time-slot on the system performance,an adaptive modulation technique using a feedback channel to adjust the system baud rate is proposed and verified.The experimental results show that changing the system baud rate according to the change of the number of photons in the time-slot can effectively reduce the system BER.When the number of photons in a time-slot is less than 8,the link adaptive modulation technology can automatically reduce the baud rate from 500Kbps to 250Kbps,and the system BER is reduced from more than 1.44×10-2 to less than 5.02×10-4.3.We proposed and verified a video transmission link protocol for high BER environment.At the transmitting terminal,each pixel of the video frame has a sequence number indicating its position,and the sequence number information and the pixel information are encapsulated into independent data frames.After the receiving terminal distinguishes between the sequence number frame and the pixel frame according to the flag bit,the pixel and the sequence number are correspondingly placed into the storage unit.The experimental results show that when the BER is more than 0.6,the contour of the video frame can still be recovered,which effectively solves the problem of pixel dislocation of video frame caused by bit error.