Research On Constellation Shaping Technology Under Optical Fiber Nonlinear Effect

With the development of optical fiber communication technology,the demand for high-speed,large-capacity,and long-distance transmission in coherent optical communication has been increasingly studied.The transmission performance of coherent optical communication systems is mainly limited by Gaussian white noise and fiber nonlinear interference noise,so constellation shaping technology has been proposed to improve the spectrum efficiency and make the transmission capacity close to the Shannon limit.Constellation shaping techniques are generally divided into probability shaping and geometric shaping,which have gained great attention in recent years.This paper also studies the constellation shaping technology on the basis of the existing ones.This paper mainly focuses on simplifying and improving the implementation process of MPDM in PS and the optimization and performance comparison of APSK modulation format in GS.The main research work is as follows:1.Firstly,the realization process of the probability amplitude shaping scheme is introduced.Due to its rate adaptability,multiple LDPC coded modulated PS-16 QAM signals with different source entropy and the same net bit rate are realized,and they are compared with a uniform 16 QAM signal with the same bit rate The performance under AWGN dominance and fiber nonlinearity is compared to verify the shaping gain of the PS signals.2.It is explained that the current ideal CCDM has the problem of too high complexity,and then we propose a simplified model based on pairs of probability on the basis of MPDM.This division method first selects a fixed block length n,and only considers the pairwise partitioning with probabilistic complementary characteristics at this time.As the block length in MPDM increases,the search for composition pairs is reduced.This implementation method eliminates the constant component in CCDM.Under the same block length,the rate loss of the simplified MPDM is reduced,and the implementation complexity is greatly reduced.We analyzed the performance of the two distribution matching methods under the dominance of AWGN and fiber nonlinearity through simulation and experiment.The results show that the simplified MPDM can also obtain shaping gain and lower rate loss.3.The construction method and bit mapping of the geometrically shaped APSK constellation are introduced.We propose an improved 64-APSK,and compare the transmission performance of two 64-APSK coded and modulated signals and square64 QAM signals in optical fiber communication systems through simulation and experiments.First,we calculated the MED,MPI,and PAPR of various constellation diagrams through enumeration,and selected the best APSK constellation.Then,it was verified under the leading role of AWGN and fiber nonlinearity.The simulation results show that in the AWGN channel,the mutual information of the proposed APSK based on Gray mapping is better than 64 QAM based on Gray mapping by about 0.8 dB.It is verified by experiments that compared with the uniform square 64 QAM,the proposed64-APSK can still show better tolerance when the nonlinear effect is strong.