| Underwater wireless optical communication has the advantages of low latency,high bandwidth,and compact and flexible transceivers,and is expected to play an important role in ocean exploration.High-sensitivity single-photon detectors are used to detect optical signals to increase the communication distance of underwater wireless optical communication.When using a single-photon detector to detect optical signals,binary modulation methods such as On-Off Keying（OOK）and Pulse Phase Modulation（PPM）are commonly used for communication.However,binary modulation has low communication efficiency and cannot meet the needs of high-speed transmission.In contrast,multi-level modulation has the advantages of high bandwidth utilization and fast bit rate,which can effectively solve the above problems.There have been a few studies on applying multi-level modulation to Single Photon Avalanche Diode（SPAD）,but the schemes are not perfect and the technical details are not clear.Therefore,this paper focuses on the research of underwater photon counting wireless communication system based on Quadrature Amplitude Modulation（QAM）,and the main research contents and achievements are as follows:1)A model of underwater photon counting wireless communication system based on QAM was built.To improve the accuracy of the model,the Poisson random process of sending and receiving photons and the attenuation characteristics of signals in underwater channels were simulated,taking into account the dead time characteristics of single-photon detectors for modeling.The effects of different light intensities,symbol recover intervals,and dark counts on system performance were studied.The results showed that with the increase of light intensity,the recovered signal gradually approached the original signal.When the light intensity exceeded a certain range,there was a problem of incomplete recovery of the signal.However,even if the recovered signal was only half of the original signal,the signal could still be correctly demodulated by observing the constellation diagram and setting the decision threshold reasonably.In addition,the theoretical communication distance formula of the system was derived.2)To address the signal recovery problem in photon counting communication,a modulation signal recovery scheme based on symbol recover interval counting was designed.At the receiving end,the modulated signal with Poisson noise was recovered by symbol recover interval counting of the pulse signal output by the single-photon detector,followed by smoothing the waveform by mean filtering,and finally subtracting a fixed value to complete the signal recovery.To solve the symbol recovery problem during signal recovery,a symbol compensation scheme was designed.At the receiving end,by observing the constellation diagram and on the basis of carrier synchronization,adding or subtracting sine or cosine signals to its output two signals,it was experimentally verified that this scheme can effectively reduce the errors caused by inaccurate symbol recovery.3)A QAM modulation-based underwater photon counting wireless optical communication system was established.Tap water was selected as the experimental water quality,and the effects of different light intensities,symbol recover intervals,and background light on system performance were investigated.Experimental results demonstrate that even when the restored signal is only half of the original signal,the system can still communicate normally by adjusting the decision threshold according to the constellation diagram,consistent with the modeling conclusion.When the average photon count rate is 9 Mcps,the system’s error symbol rate is8×10-4.The theoretical communication distance of the system was calculated as 104 m using the distance formula,which verifies the correctness of the model and proves the feasibility of applying QAM modulation to SPAD for underwater wireless optical communication. |
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