Research On Modulation And Demodulation Technology Of Multi-pulse Pulse Position Modulation

Deep space exploration is of great significance for humans to explore the mysteries of the universe,expose the origin of life,and develop space resources.At present,laser communication has become an important communication method in deep space exploration.Multi-Pulse Pulse Position Modulation(MPPM)and photon detection technology have become key technologies in the field of deep space optical communication due to their advantages such as efficient bandwidth utilization and sensitive detection efficiency.This thesis focuses on the modulation and demodulation techniques of MPPM in deep space optical communication.1.Aiming at the shortcomings of the existing MPPM iterative soft demodulation algorithms that require complex publicity derivation due to changes in modulation order or constellation mapping scheme,an MPPM iterative soft demodulation algorithm based on a lookup table is proposed.According to the MPPM constellation mapping scheme,a mapping table of corresponding bit group to MPPM symbol is obtained,and then a new MPPM iterative soft demodulation algorithm is used for demodulation and decoding.The advantage of this MPPM demodulation algorithm is that it can be directly converted from MPPM constellation mapping tables,bit log likelihood ratios and slot log likelihood ratios to MPPM symbol log likelihood ratios,so as to avoid the complexity caused by the need to re-derive the formulas due to the change in the MPPM modulation scheme.2.Due to the unreasonable selection of the position of the redundant point of the existing MPPM constellation point scheme,it causes the problem of large inter-symbol and inter-slot interference.Aiming at this problem,a new MPPM constellation point selection scheme optimization criterion is proposed.The optimization criterion is that the larger the average Hamming distance between the constellation points and the smaller the probability that the signal pulses are located in adjacent time slots,the better the decoding performance at the receiving end.Based on this criterion,a partial search algorithm is used to obtain an MPPM constellation point selection scheme with stronger anti-delay jitter capability.The simulation results show that under the Poisson channel,the MPPM constellation point selection scheme in this thesis can effectively improvethe decoding performance of the receiving end,especially the performance gain is more obvious under large delay jitter,large pulse duty cycle and unprotected time slot.3.In view of the shortcomings of the existing bit group to MPPM constellation point mapping scheme with weak anti-interference ability,an MPPM bit group mapping scheme with better performance is proposed.When 2-5MPPM is adopted,the optimal scheme obtained by full search is selected as the scheme in this thesis;when the MPPM modulation order is higher,the sub-optimal scheme is selected as the scheme in this thesis.Compared with the existing scheme,the scheme in this thesis has a larger average Hamming distance between constellation points and a smaller probability of signal pulses in adjacent time slots,so it has stronger anti-interference ability.The simulation results show that under different MPPM order conditions,the MPPM bit group mapping schemes in this thesis are better than the existing schemes.