Research On The Key Technology Of Single Photon Detection In Underwater Visible Light Communications

With the continuous consumption of land resources,human beings are paying more and more attentions to the oceanic resources because of the two thirds of the earth's surface area.As a result,many countries have put forward the corresponding marine strategy.At the same time,the underwater wireless communication becomes an important research direction.As the transmission rate of the underwater acoustic communication is low and the communication distance of the traditional radio frequency communication in the water is shorter,the visible light communications?VLC?for underwater condition bacome an effective auxiliary technology.For the VLC system,the light-emitting diode?LED?is often used as the light source and the photoelectric diode?PD?or avalanche diode?APD?is used as the receiver.However,due to the high detection threshold of PD/APD devices,the communication distance of the PD-based/APD-based underwater VLC?UVLC?system is limited.To realize the long-distance underwater communication,some scholars have adopt the single photon avalanche diode?SPAD?device with high sensitivity as the receiver.However,due to the nonnegative characteristics of the signal and channel of the UVLC-SPAD system,the complexity of the underwater channel probability distribution and the approximate Poisson output distribution of the SPAD device,the transmission and detection method in traditional PD-based VLC system cannot be directly applied in the UVLC-SPAD system.Therefore,in this paper,we foucs on the theme of the long distance information transmission for the UVLC-SPAD system.To the design of transimitter signal,the union detection of the receiver,the union detection and experimental system for the actual SPAD device under the dead time effect and photon-counting pulse waveform,this paper will follow the several aspects:Part 1:Design the underwater long-distance UVLC-SPAD link system.At present,due to the constraints of the absorption and scattering effect,the majority of the underwater model is based on the close distance channel.To meet the demand of the realistic requirement of long distance underwater communications,we put forward the long distance underwater channel model based on the large-scale monte carlo numerical simulation?MCNS?.The proposed model includes the LED optical system model,the long distance dual exponential channel model and the receiver optical system model.Then we present the maximum likelihood detection algorithm based on Poisson channel.The simulation environment is based on the actual underwater environment,devices and open experimental data.And then,the performance of long distance UVLC system under different conditions are studied.The system model proposed in this chapter provides the theoretical and experimental basis for the subsequent signal design and the union detection of receiver signals.Part 2:The union detection design of the UVLC-SPAD system and the single symbol training likelihood detection under the absorption,scattering and weak turbulence effects.This chapter is mainly divided into three main parts:?1?Underwater integrated channel.We focus on the absorption and scattering effects in the first part.However,the underwater turbulence can also cause the problem of channel fading.Hence,we introduce the lognormal model of underwater weak turbulence effect.As a result,the comprehensive underwater channel is based on the dual exponential model of the absorption,scattering and the lgnormal model of weak turbulence.Meanwhile,this is the theoretical foundation for the later research.?2?The union detection design of receiver.This subsection aims to the digital type SPAD?pure Poisson noise?and analog type SPAD?Poisson+Gaussian?P+G?mixed noise?.There is the characteristic of time domain correlation among the received multiple symbols as the underwater turbulence channel within a certain time is a constant.Hence,we put forward the maximum likelihood union detection,the generalized maximum likelihood union detection?GMLUD?and the corresponding fast search algorithm under the pure Poisson noise.Then we propose the GMLUD under the P+G under the mixed noise model.The simulation results verify the performance of the proposed algorithm is close to the performance of the symbol by symbol maximum likelihood detection with channel state information.?3?The single symbol training maximum likelihood detection receiver.If all the transmitted datas in the frame structure are?0?,the channel blurring effect will occur.As a result,the GMLUD algorithm will show the inevitable errors flat layer phenomenon even though the signal-to-noise or the transmitted power increases.Hence,we peopose the sechme that add a single?1?bit before the data frame structure and the single symbol maximum likelihood detection receiver.The receiver can not only eliminate the error level caused by the fuzzy effect of the channel,but also can estimate the channel information.And then,the training maximum likelihood detection?TMLD?receiver and Training-modify?TM?quasi maximum likelihood detection receiver based on the Anscombe root transformation are proposed.The simulation results also show that the performances of the proposed two kinds of receivers are better than the training maximum likelihood receiver under Poisson model.Part 3:Research on UVLC-SPAD system transmission technology of multi-wick.The multi-wick high-speed transmission technology with parallel drive scheme is proposed because the LED lamp consists of multi-wick.For the parallel drive scheme,different data can be transmitted through different LED wick,thus the spectrum efficiency and multiplexing gain can be acheved.To distinguish the photon-counting signals from the single SPAD receiver,the different wicks of the transimitter utilize the user-specific interleaver which is adopted in the interleave-division multiple-access technology.In the receiver,for the Poisson statistical distribution characteristics of SPAD,the serial successive interference cancellation?SIC?algorithm based on the ideal Poisson photon-counting channel is proposed.Compared with the direct current-biased optical orthogonal frequency division multiplexing?DCO-OFDM?technology,the error performance and the anti-LED electro-optical conversion performance of the proposed scheme are better than the DCO-OFDM scheme that based on SPAD receiver.Part 4:The union detection based on the photon-counting pulse waveform and the dead time effect of the SPAD device.The experimental study shows that the output of photon-counting waveform by SPAD is not the ideal pulse structure.The output waveform will last 10³0 ns in the time domain,which is namely as the dead time effect of SPAD.This also makes the output model of SPAD not a standard Poisson model.Therefore,according to the photon-counting pulse waveform,the synchronization scheme and GMLUD algorithm based on dead time effect of SPAD are proposed.As the weak turbulence model under the water surface/outdoor free space optical?FSO?channel and the weak turbulence model under water are lognormal models.To verify the proposed algorithm and synchronization scheme,we designed a long distance FSO weak light VLC-SPAD system undet the turbulence fading channel.The transmitter is the LED indicator which is widely used in the field programmable gate array and the receiver is the actual SPAD device.The experimental results show that the performance of the GMLUD based on the dead time limit is better than the ideal Poisson channel model.Based on the proposed scheme,the communication distance is over 320 m when the data rate of the weak light VLC-SPAD system is 100 kbps.In addition,based on the normal LED lamps?the power is nearly 10 W?,the proposed receiver optical system,the SPAD receiver and the proposed detection algorithm,we achieve the performance of 1244m/1Mbps/BER<10^-7,real-time transmission voice and TXT text transmission.The 1244 m remote real-time visible light communication system has passed the third-party testing of the China information and communication research institute.Finally,to carry out the test of the actual underwater performance,we design the corresponding UVLC experimental system based on the actual SPAD receiver.