Error Correcting Coding For Photon Counting Wireless Communication Based On Weak Links

In the traditional wireless optical communication system,the detector converts the optical signal into a current signal when it works in an analog mode.Due to the shortcoming of low receiving sensitivity,the long-distance wireless optical communication has the characteristics that the link is easily interrupted,the bit error rate is high,and the burst error is large.In recent years,to achieve long-distance optical wireless communication,a single photon detector with single photon limit sensitivity is used to detect the optical signal at the receiver.When the modulated light signal is incident on the single photon detector,the detector outputs a discrete sequence of single photon pulses.Due to the quantum effect of photons and the random process in which photons are detected,the most of errors are symbol deletion caused by the randomness of discrete single photon pulses rather than the symbol errors caused by dark count pulses and background light pulses.This paper focus on the characteristics of the erasure channel in the long-distance photon counting wireless communication,and research on the method of extracting the synchronization signal and the bit information from the discrete single-photon pulse sequence.Based on this,the error correction coding method suitable for the weak link photon counting wireless optical communication is studied.The main research contents and results are as follows:1.Based on the established weak link photon counting wireless optical communication system,the photon Poisson statistical distribution,underwater propagation characteristics and single photon detection Poisson stochastic process are integrated to establish a communication model based on dual random Poisson distribution.The effects of photon number,noise photon and signal demodulation parameters on the bit error rate are derived theoretically,and Monte Carlo method is used to verify the simulation.The simulation results show that the signal demodulation method based on photon counting in time slot is better than the demodulation method based on adjacent photon time measurement.2.The paper proposes a signal synchronization and signal extraction method based on intra-slot photon counting for photon counting wireless communication,and performs experimental verification based on FPGA.The extraction methods include gate control signal generation,photon pulse acquisition,frame head detection,synchronous signal extraction and photon count demodulation.The effects of gate control signal,period number and threshold on demodulation results is studied.The experimental results show that the number of gate control signal is 80,the cycle time is 20us,the average count rate of detector is 2Mcps,and the optimal synchronization threshold is 20 photons,the bit error rate is 10^-2 order of magnitude.3.LT coding and decoding is designed on the communication system.By adjusting the optical power and demodulation threshold,the number of receiving code packets at the receiver and the influence of degree distribution parameters?and?on the decoding error correction performance of LT are verified.The experimental results show that the enough number of source packages recovered by LT coding when there are 640 source packets on the sender.The error rate of the communication system is in the order of 10^-1 to 10^-2.In order to improve the decoding success rate,after the CRC-16 test of the encoded packet,the decoding success rate can reach 100%,but the delay is increased,and the data transmission rate is reduced.For the purpose of improving the decoding performance of LT code,it is necessary to cascade QC_LDPC codes with good sparsity.4.The LT code and QC_LDPC code cascade design are carried out on the communication system.It is proposed to encode the inner code with the switch degree distribution.The LT code is used as the inner code and the QC_LDPC code is used as the cascading scheme of the outer code.The experimental results show that the communication error rate is 10^-4 order of magnitude when the switching coefficient?is 0.1 and the synchronization threshold is 22.