Research On Pulse Position Modulation And Demodulation In Deep Space Laser Communication System

Deep space laser communication is a way of communication that uses laser as a carrier to transmit images,data,video and voice between deep space probes and ground stations or earth-orbit vehicles.The deep space laser link has both the characteristics of a near-Earth laser link and unique characteristics due to the particularity of its application environment,such as high signal transmission attenuation,severe atmospheric influence,strong background light interference,and large Doppler frequency shift,etc.The slot duty cycle of PPM pulse is low so that it is easy to obtain higher peak power using PPM/direct detection system.Meanwhile,single photon detection can be realized by using a superconducting nanowire single photon detector.Comprehensively considering the above two factors,PPM/single photon detection system is more suitable for the link of lunar,earth,and deep space laser links.PPM modulation and demodulation technology of the deep space laser communication has been studied in this paper,and a kind of symbol synchronization program has been proposed.Firstly,PPM and the single-photon detection are briefly introduced,and then the PPM modulation system in deep space laser communication is studied in detail.The complete system architecture and the principles and characteristics of SCPPM coding and decoding,semiconductor laser,Mach-Zehnder modulator,optical amplifier,transmitting lens and receiving lens,spatial channel,detector array,electrical pulse shaping method,and traditional demodulation method are given in chapter 2,which provide corresponding theoretical support for the design of symbol synchronization program in the following chapter.Then,the article focuses on the tailing phenomenon caused by the photon detectors and completes the simulation analysis of the error performance of PPM modulation system based on SPD array receiver.According to the paper^[1],the BER model of the SPD recovery time affecting only the current slot and multiple PPM slots are proposed.Based on these two models,the BER model of the SPD recovery time for multiple PPM symbols is derived.Combining the three models,it can be concluded that the longer the SPD recovery time is,the higher the BER of the system.In practical engineering application,the communication system can be optimized by increasing the number of SPD to meet the BER index.Finally,a scheme of symbol synchronization module in PPM demodulation are proposed,special coherent frame headers and a match filter and loop filter for PPM modulation are designed,and then the whole MATLAB simulation model of symbol synchronization module is built,which verified the feasibility of the scheme.Then according to the scheme,the FPGA code of each module of the modulation and demodulation system is completed and the simulation results by Model Sim are given.At last,the preliminary hardware construction was completed on the PPM integrated signal processing board.More work needs to be done to verify the correctness and practicability of the PPM symbol synchronization method proposed in this paper,and to make full preparation for the realization of high-speed PPM modulation and demodulation system.