| With the large-scale commercialization of 5G networks,Internet traffic will continue to increase explosively.Optical communication systems using intensity modulation and direct detection are already overwhelmed.High-order modulation formats combined with digital signal processing technologies have become a new development direction.This solution can greatly improve spectrum efficiency and transmission distance,so that it can meet increasing traffic demand.In practical power-constrained communication systems,probabilistic constellation shaping technology can increase system capacity,optimize signal tolerance to non-linear effects without affecting code modulation performance and non-spread spectrum occupancy,and further improve the error performance of the receiver,reducing optical signal-to-noise ratio(OSNR)tolerance requirements,increasing the signal transmission distance.On this basis,this paper focuses on the theoretical and experimental research around the beyond-100 G advanced modulation and demodulation technology,digital signal processing technology in high-speed optical communication,and probabilistic constellation shaping technology in high-order quadrature amplitude modulation(QAM)modulation.The main research work and the results are as follows:(1)The basic principles of reception modulation and demodulation are studied.The modulation and demodulation principles of higher-order modulation such as quadrature amplitude modulation are analyzed,and the principle of optical coherent detection and the working mechanism of optical coherent receiver are analyzed in depth.In view of the modulation scheme of 400Gbit/s optical transmission system,the fourcarrier PM-QPSK,dual-carrier PM-QPSK,dual-carrier PM-16 QAM and single-carrier PM-16 QAM transmission schemes based on Nyquist WDM system are studied for technical comparison,and the transmission performance and application scenarios of different modulation schemes are analyzed.(2)The basic principles of link loss such as fiber loss,chromatic dispersion,polarization mode dispersion,phase noise,and nonlinear effects are analyzed theoretically.Aiming at the corresponding link impairment,the digital signal processing(DSP)compensation technology based on coherent reception,such as the constant mode algorithm(CMA)and multi-mode algorithm in the Bussgang-like blind equalization algorithm,is analyzed and studied.A 400 Gbit / s polarization-multiplexed coherent optical communication simulation system was constructed using VPI transmission and simulation experiments were conducted.The effects of link loss such as fiber loss,dispersion,and polarization mode dispersion(PMD)on high-speed optical signals were analyzed.And DSP compensation algorithms such as CMA and blind phase equalization algorithm(BPS)are used to recover the damaged signal.The simulation results show the compensation effect of the DSP recovery algorithm on high-speed optical signals in actual lines.(3)The demonstration illustrates the necessity of research on constellation shaping techniques and probabilistic constellation shaping.Then,the relationship between the mutual information in the channel and the channel capacity is explained by the knowledge of information theory,and the technical principle of probabilistic constellation shaping is explained.It is proved that the maximum value of the shaping gain is 1.53 dB and that in a power-limited optical communication system,when the input signal distribution is a Gaussian distribution,the mutual information(ie,the channel capacity)of the system is the largest.The advantages and disadvantages of different probabilistic constellation shaping schemes are compared.(4)Aiming at the problem of channel limitation in beyond 100 G optical networks,a rate-adjustable quadrature amplitude modulation probabilistic constellation modulation scheme based on arithmetic coding is studied.The optical network transmission capacity is closer to the Shannon limit,and the noise recovery ability of the signal is improved.In this scheme,the sending end is composed of a distribution matcher and an FEC encoder,which are responsible for the tasks of probabilistic shaping and channel coding of the signal,respectively.The system adjusts the rate by changing the probabilistic distribution of the distribution matcher;at the receiving end the signal is decoded by a forward error correction(FEC)decoder and the noise signal is recovered by a reverse distribution matcher.In order to study the transmission performance of the shaping scheme,two comparison experiments are performed.The results of experiment 1 show that the gain of OSNR is 0.382 dB and the mutual information gain is 0.09bit/symbol for probabilistic shaping(PS)16QAM versus 16 QAM.The results of experiment 2 show that at the same spectral efficiency(SE),when PS-64 QAM is compared with 16 QAM,the OSNR gain is 0.8dB and the mutual information gain is 0.153bit/symbol.At the same time,the modulation format of probabilistic constellation is added in the two experiments,and the bit error rate performance is significantly better than that of the standard 16 QAM. |
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