| With the significant increase of communication data,the communication rate is facing great challenges and the radio frequency communication spectrum resources are tight and the rate is low,which can not meet the requirements of modern society for high-rate information transmission.However,laser communication has the advantages of unlimited frequency spectrum,high speed,high confidentiality and low cost,so using laser as the carrier of information transmission has obvious advantages.On the other hand,Mazo proposed the Faster-than-Nyquist(FTN)rate transmission theory in 1975,which pointed out that when the acceleration factor is greater than or equal to a certain threshold(Mazo limit),the transmission rate of the system can be effectively improved without changing the bandwidth and power of the communication system.Therefore,the combination of FTN technology and laser communication can greatly increase the transminssion rate of the system,and has a great application prospect.FTN is a non-orthogonal transmission technology.when the acceleration factor is less than the Mazo limit,the waveforms are superimposed tightly so that the interference value will be superimposed at the sampling point,which reduces the reliability of the communication system.In response to this problem,this paper mainly studies the transmission characteristics of FTN technology in the atmospheric laser communication system and analyzes the ISI introduced during FTN shaping.Meanwhile,studying and designing the corresponding interference elimination algorithm.Firstly,the 4PAM-FTN atmospheric laser communication system is constructed by combining 4PAM technology with FTN technology,the system model of transmitter and receiver is given,the bit error rate performance of the system under different acceleration factors is 0.8 is analyzed.Through the research of 4PAM-FTN atmospheric laser communication system,it is found that when the acceleration factor is 0.8,the reliability of the system will be greatly reduced,which seriously affects the normal communication function.To solve this problem,a block-transfer scheme based on matrix interference elimination is proposed.Simulation results show that the proposed scheme can effectively reduce the impact of ISI introduced during FTN shaping on the system BER performance under a fixed ISI length,and the technology of block-transfer can greatly reduce the system complexity.In order to further improve the reliability of the system,an adaptive pre-equalization algorithm based on point-by-point elimination is proposed.The theoretical bit error rateBER</sub>of the system under this algorithm is derived theoretically and the computational complexity of the system is calculated.Numerical analysis results show that the proposed algorithm can effectively eliminate the ISI caused by FTN forming under different acceleration factors,and its reliability is almost equal to that of the orthogonal transmission system.However,it can solve the ISI problem and increase the computational complexity of the system.Therefore,the system transmission rate and complexity can be balanced according to the actual needs,and select the best acceleration factor to optimize the system performance.To sum up,the two FTN atmospheric laser communication systems designed in this paper can effectively solve the problem of ISI generated during FTN forming,and achieve the expected effect of the algorithm proposed,and provide a reference for the further application of FTN technology in atmospheric laser communication system. |
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