Non-uniform High-order Modulation Format In Optical Fiber Transmission System

With the rapid development of informatization and new business,fiber optic communication systems face increasing demands for transmission capacity,spectrum efficiency and flexibility.The high-order modulation format is a mature technology that improves the spectrum utilization of the system.However,there are nonlinear effects in fiber-optic communication systems.Therefore,the transmission power is limited,and a high optical signal-to-noise ratio cannot be obtained.In order to further improve the spectral efficiency without increasing the transmission power,it is necessary to shape the constellation.In recent years,the application of non-uniform high-order modulation in fiber-optic transmission systems has received extensive attention.This thesis studies the probabilistic shaping and geometric shaping techniques that are of great interest in nonuniform constellations.The main research work is as follows:1.Probabilistic shaping is realized by probabilistic amplitude shaping system model.This thesis proposed a probabilistic distribution and FEC joint implementation scheme for the distribution matching and forward error correction code modules in PAS scheme.The scheme is systematically designed according to the bit mapping of the constellation symbol and the characteristics of the LDPC check sequence,so that the distribution of the transmission constellation does not change after the addition of the check sequence,and the complex FEC decoding structure is not required at the receiver.2.At the receiver of PS transmission system,Thesis propose a simplified logarithmic ratio scheme considering the prior probability for the LDPC soft decoding.The simulation of PS-16 QAM indicates that the simplified LLR is an efficient approximation with negligible performance degradation.Then,simulations and experiments of PS-QAM is carried out at the same net transmission rate to fairly compare the post-BER gains.The coherent optical experiment of PS-16 QAM with simplified LLR indicates that there is a 1.17 dB gain of post-FEC BER compared with uniform 16 QAM.Besides,Thesis investigate the principle and implementation process of MAP detection for PS constellation.3.Thesis introduces the distribution characteristics of the geometrically shaped constellation by the iterative polarization quantization(IPQ)method,and evaluates the shaping gain of IPQ-QAM through simulation.Then,the author compares the performance of IPQ-QAM with PS-QAM of Maxwell-Boltzmann(MB)distribution.The simulation result of IPQ-64 QAM shows that the MI of the geometrically shaped IPQ-64 QAM is much better than PS-64 QAM in some intervals.However,geometrically shaped IPQ-QAM has shortcoming that GS technology greatly increases the complexity of the transceiver and IPQ-64 QAM is no longer Gray mapped.4.The realization principle and process of the pairwise optimal(PO)algorithm in geometric shaping.The geometric distribution of 8QAM and 32 QAM is optimized by generalized pairwise optimization algorithm,and the performance curves of PO-QAM are compared with traditional square QAM in simulation.The simulation results show that the optimized 8QAM has a GMI gain of 0.95 dB compared to the conventional 8PSK.The optimized 32 QAM has a GMI gain of 0.41 dB compared with a uniform 32 QAM.