| With the increase of deep space exploration activities,the traditional microwave communication technology can no longer meet the transmission requirements of large data volume in the future deep space exploration.Deep space optical communication technology is the key technology to realize large capacity and high-speed data transmission in the future deep space exploration,and is the development direction of deep space measurement and control communication.The development of deep space optical communication technology will greatly promote future deep space exploration activities,and will help mankind to further understand the mysteries of the universe,explore the origin of life and utilize space resources.Due to the long distance of deep space channel and high attenuation of communication link,high sensitivity single photon detection technology,high efficiency modulation and error correction coding technology are necessary.This paper first introduced the background and significance of deep space exploration,and then introduced deep space measurement and control communication technology.By comparing deep space microwave communication technology and deep space optical communication technology,the advantages of deep space optical communication technology are summarized.Then the characteristics of deep space channel are analyzed,the spatial geometric attenuation of deep space link is roughly estimated,and the key technologies of deep space optical communication are briefly introduced.By comparing the performance of RSPPM,SCPPM and LDPC-PPM,combined with the characteristics of deep space link,such as lack of photon number,low SNR and high BER,SCPPM coding scheme with strong error correction ability and high photon utilization efficiency is selected,and the photon counting detection method with high sensitivity is adopted at the receiver end.Secondly,the encoding and decoding method,SCPPM is introduced in detail.The encoder consists of external code(convolutional code of 1/2 coding rate),interweaver and internal code(accumulator and PPM modulation).In the decoder,Joint demodulation and decoding methods are adopted for internal code decoding to improve decoding performance.MAP decoding algorithm of convolutional code is adopted for external code,and external information is exchanged between internal code and external code through conventional Turbo iteration.Then,a complete codec algorithm based on MATLAB language was written,and the decoding performance under Poisson channel model was studied through simulation.The simulation results showed that when 4PPM modulation is used,the BER can decrease from 15%to less than 10-6 by SCPPM encoding.In addition,we also studied the influence of the modulation order,the number of decoding iterations,and the coding rate on the communication performance,and compared the two decoding methods:PPM independent soft demodulation decoding and joint demodulation decoding.Simulation results show that the joint demodulation decoding method has better error correction performance.,but when the modulation order is high,joint demodulation decoding algorithm computational complexity is very large.Then,the photon counting communication system based on Geiger APD single photon detector is studied.Because the detector does not have photon number resolution in a single detection event,the channel model is degraded from Poisson channel to binary asymmetric channel.We derived the expression of the BER under the BAC model and found that there is a BER lower bound which is only related to noise intensity.Then,the channel likelihood ratio is modified according to the BAC channel model,and the simulation analysis and experimental verification show that the modified channel likelihood ratio has better decoding performance.When 16PPM modulation is used and nb=0.1,the photon utilization efficiency is improved by about1.24d B.When 8PPM modulation is used and nb=0.2,the photon utilization efficiency is improved by about 4.23d B.Finally,in view of the disadvantages of higher BER and lower communication rate in non-photon number resolution system,a multi-channel detection method is proposed,which can reduce the effect of dead time on communication performance.The influences of receiving channels number and the slot widths on PPM demodulation BER were analyzed.Through simulation analysis and experimental verification,we found that the multi-channel receiving method has a photon number resolution ability,which can effectively reduce the BER and improve the communication rate.In addition,when the detector dead time covers multiple slot widths,the BER decreased at first and then increased with the increase of signal strength.This indicates that the signal intensity should be moderate when using this method,too large or too small signal intensity will all lead to the increase of BER.In addition,the arrival time of the photon in slot is uncertain at the receiving end,so the detector output pulse will not output at the same time,in view of this characteristic,this paper proposed a slot synchronization method based on the multi-channel detection,slot synchronization is accomplished by counting the number of rising edges and experimental results showed that this method had a good synchronization effect.The research in this paper has important ignificance for the system design,modulation coding and detection scheme selection in deep space optical communication.The multi-channel detection system proposed in this paper provides a reference method for the realization of high-speed optical communication system based on single photon detection. |
PDF Full Text Request