Research On Constellation Shaping And Peak-average Power Ratio Reduction Methods In High-speed Optical Communication

In recent years,with the rapid development of the global communication industry,diversified new technologies including cloud computing,artificial intelligence,Internet of Things,big data and metaverse have been promoted and integrated.Increasingly rich emerging services and more flexible future applications put forward higher requirements for communication technology.Optical fiber communication systems face the transmission demand of higher speed and larger capacity.Constellation shaping technology obtains the shaping gain to approach the Shannon limit by designing the position distribution or probability distribution of constellation points,which is an effective means to improve the system capacity.Multi-carrier modulation(MCM)technology achieves high-bandwidth efficiency of parallel data transmission and capacity enhancement,which expands the number of channels by increasing the number of subcarriers.However,peak-to-average power ratio(PAPR)is an inherent disadvantage of MCM signals,which leads to signal nonlinearity and transmission performance degradation.Therefore,constellation shaping technology and PAPR reduction technology have become the research hotspot in high-speed and efficient optical communication systems.In this dissertation,based on the in-depth study of the constellation shaping technology and PAPR reduction methods in optical communication,in order to improve the nonlinear tolerance of high-order modulated signals,a geometric shaping scheme based on hexagonal lattice is proposed.In order to obtain greater constellation shaping gain,a hybrid shaping scheme of constellation subset based on hexagon lattice is proposed.In order to reduce the nonlinear effects caused by high PAPR,this dissertation proposes a low complexity PAPR reduction method based on partition-recombined selection mapping(SLM),and a combined PAPR reduction method based on low-density parity-check(LDPC)codes and mean-aid nonlinear compander transformation.The main research work and innovations of this dissertation are as follows:(1).To improve the nonlinear tolerance of high-order modulated signal in optical fiber communication systems,a geometric shaping(GS)scheme based on hexagonal lattice is proposed.The scheme takes the maximization of constellation figure of merits as the optimization objective,which achieves the increase of minimum Euclidean distance when the power is fixed.The scheme is simulated and verified by the coherent optical communication system.Simulation results show that,when the signal modulation order is 16/32/64/128,GS-MQAM can obtain 1.2 dB/1.0 dB/0.7 dB/0.4 dB OSNR gain compared with uniform-MQAM,respectively.Experimental results show that,under the soft decision BER threshold,GS-64QAM achieves OSNR gains of 0.8 dB and 3.5 dB compared with Square-uniform-64QAM in back-to-back and 375 km optical fiber transmission scenarios,respectively.(2).To obtain greater constellation shaping gain,a hybrid shaping(HS)scheme of constellation subset based on hexagonal lattice is proposed.Probabilistic shaping optimization is carried out on the basis of GS-MQAM constellation.All constellation points are divided into multiple subsets according to the concentric hexagon layer circle,which on the same layer circle are endowed with the same probability,so as to obtain HS-MQAM signals.The simulation and experimental verifications of the scheme are carried out through the coherent optical transmission system.Simulation results show that,at the soft decision BER threshold,compared with OSNRs of Square-64QAM,uniform GS-64QAM and PS-64QAM,the OSNR gains of HS-64QAM are 4.8dB,3.5dB and 1.1dB,respectively.Experimental results show that,compared with GS-64QAM and Square64QAM,HS-64QAM signal achieves OSNR gains of 4.1 dB and 7.6 dB,respectively,at the soft decision threshold.(3).To solve the problem of high computational complexity caused by increasing the number of candidate signals in traditional selection mapping algorithm,an orthogonal frequency division multiplexing PAPR reduction method based on low complexity partition-recombined SLM is proposed.In this method,the real and imaginary components of timedomain OFDM are partitioned at the transmitter,and then recombined according to certain rules.The time domain signal with minimum PAPR is selected for transmission,and then phase rotation sequence is removed from received frequency-domain signal to recover data at the receiver.Simulation verifies the complexity and PAPR reduction performance of the proposed scheme.Results show that when the number of candidates is 9 and the number of FFTs is 128,compared with traditional SLM,the proposed method can save 61.9%complex multiplication complexity and 60.3%complex addition computational complexity.The PAPR reduction performance of the proposed method is compared with that of the traditional SLM in different cases.In addition,an experimental system of coherent optical transmission is set up to verify the transmission performance.Experimental results show that,under the hard decision BER threshold,OFDM signal using the proposed method has a 1.15 dB OSNR gain and 1.04 dBm received optical power gain compared with the original OFDM signal.(4).To realize PAPR reduction and improve error performance synchronously,a generalized frequency division multiplexing(GFDM)PAPR reduction method combining LDPC-coded and mean-aid nonlinear compander transformation is proposed.In this method,LDPC-coded GFDM signals are compressed based on signal mean amplitude at the transmitter,and signal decompression and data recovery are realized at the receiver based on channel estimation module.The proposed improves transmission performance under nonlinear predistortion.In addition,the PAPR reduction performances under various conditions and transmission error performance of the proposed method are simulated.Results show that,when the compander factor is 2,the PAPR gain of the GFDM signal using this method is 5.27 dB compared with the original GFDM signal.At the soft decision and hard decision BER thresholds,GFDM signals using the proposed method,compared with original GFDM signals obtain 2.4 dB and 4.3 dB OSNR gains,respectively,which compared with LDPC-coded GFDM signals obtain 0.62 dBm and 0.08 dBm transmitted optical power gain,respectively.