Nonlinear Signal Processing Technique For High-speed Optical Direct Detection Multi-carrier Systems

In recent years,with the development of broadband datacom and telecom services,users' data traffic has grown rapidly,and high-speed optical transmission systems are playing an important role.At the same time,the high-bandwidth,large-capacity transmission demand is sinking towards the short-reach transmission system side,making it the key direction for the next step of the development of the broadband optical fiber transmission network.Among the existing solutions,the high-speed optical multicarrier transmission system based on direct detection technology is a strong competitor of short-reach transmission network because of its low cost,high spectral efficiency and flexible resource scheduling abilibty.However,in the context of broadband fiberoptic transmission systems,nonlinear interference from the key optoelectronic devices has become a major factor limiting system capacity and performance.Studying nonlinear interference and corresponding nonlinear signal processing methods in highspeed optical direct-detection multi-carrier transmission systems have important scientific significance.Based on the aforementioned background,this thesis studies the nonlinear signal processing technology of high-speed optical direct detection multi-carrier system,mainly involving nonlinear interference modeling of high-speed optical direct detection multi-carrier system,non-linear interference compensation and nonlinear quantization based nonlinear signal processing techniques and their verification in high-speed optical direct inspection multi-carrier experimental transmission systems,the main research contents are as follows: 1)Nonlinear interference characteristics of high-speed optical direct detection multi-carrier systemHigh-speed optical direct detection multi-carrier systems have been extensively studied,but nonlinear interference modeling still lacks systematic research and discussion.Based on the theoretical analysis of the basic principle of multi-carrier modulation,this thesis theoretically models the transmission characteristics and nonlinear interference characteristics of the key components in the high-speed optical direct detection multi-carrier system.High-order models for direct detection of nonlinear interference in multi-carrier systems and their effects on system performance:The multi-carrier modulation characteristics and nonlinear interference of lasers and optical modulators are studied,including the multi-carrier modulation characteristics of electro-absorption modulation laser(EML),vertical cavity surface emitting laser(VCSEL),Mach-Zehnder modulator(MZM).The influence of chromatic dispersion of short-reach fiber transmission on optical multi-carrier modulated signal transmission and the nonlinear interference caused by the combination of nonlinear effects of optical transmitters and optical chromatic dispersion are studied.The nonlinear beating noise interference caused by the square-law detection characteristics of photodetectors is studied.2)Nonlinear interference compensation based nonlinear signal processing technology for optical direct detection multi-carrier system with Volterra filteringAt present,the signal processing research of optical direct detection multi-carrier system mainly focuses on linear systems,and there lacks system-level and effective processing method for nonlinear interference compensation.Combining the nonlinear interference modeling and theoretical analysis of optical direct detection multi-carrier system,this thesis studies the Volterra filtering-based nonlinear signal processing technique for compensating nonlinear interference,which is used to process and compensate the nonlinear interference of high-speed optical direct detection multicarrier system:The time domain Volterra filtering technique applied to optical direct detection multi-carrier system is studied.The nonlinear interference compensation based nonlinear signal processing process of optical direct detection multi-carrier system based on time domain Volterra filtering technology is designed.The computational structure optimization is performed in the time domain for low complexity Volterra filtering technique.The nonlinear interference compensation technology based on Volterra filtering proposed in this thesis is experimentally verified in EML-based high-speed optical direct detection multi-carrier system.Finally,using Volterra filtering technology,this thesis successfully verified single-wavelength 200 Gbps line rate C-band 0.8km/1.6km standard single-mode fiber-optic direct detection multi-carrier system,the SNR gain for all subcarriers are in the range of 0dB~3.5dB,which helps the system reach the bit error rate threshold of SD-FEC.3)Nonlinear interference compensation based nonlinear signal processing technology for optical direct detection multi-carrier system with kernel filteringThe aforementioned nonlinear interference compensation technology based on Volterra filtering can effectively improve the performance of high-speed optical direct detection multi-carrier system.In order to further reduce the complexity of nonlinear interference compensation technology,this thesis proposes a nonlinear interference compensation technology based on kernel filtering for optical direct detection multicarrier system.The nonlinear interference of the multi-carrier modulated system is processed and compensated in a low complexity manner:A nonlinear interference compensation algorithm based on kernel filtering is proposed.The feasibility and completeness of the reproducing kernel Hilbert space for nonlinear interference compensation are theoretically analyzed.The kernel least mean square filter(KLMS)and the kernel recursive least squares(KRLS)algorithms are proposed.The nonlinear interference compensation based nonlinear signal process of optical direct detection multi-carrier system based on kernel filtering technology is designed,and its computational complexity is compared with Volterra filtering based nonlinear interference compensation technique.The results show that compared with the high exponential computational complexity of the Volterra filtering,the kernel filtering not only reduces it to the linear(KLMS)or squared(KRLS)level operation complexity,but also eliminates the computational complexity of the multi-carrier signal re-modulation process.The nonlinear interference compensation technology based on kernel filtering proposed in this thesis is experimentally verified in VCSEL-based high-speed optical direct detection multi-carrier system.Finally,this thesis successfully verified the Cband 10 km single-mode seven-core single-wavelength 500 Gbps line rate direct detection multi-carrier system,the SNR gain for all subcarriers are in the range of 0dB~6dB,which helps the system reach the bit error rate threshold of HD-FEC.4)Nonlinear quantization based nonlinear signal processing technology optical direct detection multi-carrier system based on with compression codingThe aforementioned nonlinear interference compensation nonlinear signal processing technology can effectively support the high-performance transmission of high-speed optical direct detection multi-carrier system,in order to further support the high-fidelity transmission of higher-order QAM mapped multi-carrier signals,this thesis proposes a nonlinear quantization technology based on compression coding for optical direct detection multi-carrier systems:This thesis proposes to quantize the analog multi-carrier modulated signal by means of compression coding,and then transmit it in the optical direct detection multicarrier system by digital signal.The receiver recovers the analog multi-carrier modulated signal by decoding,to achieve high-order modulation and high fidelity optical direct detection multi-carrier systems.The differential lossy coding algorithm included in scalar space,the clustering lossy coding algorithm included in vector space and the lossless coding in entropy space are analyzed.Besides,the lossless coding algorithm is combined with the above-described lossy coding in the cascaded coding scheme.The nonlinear signal process of optical direct detection multi-carrier system based on compression coding is designed and experimentally verified in the MZM-based 20 km standard single-mode fiber high-speed optical direct detection multi-carrier system.The lossy coding,hybrid coding and other coding techniques are used for parameter optimization and transmission performance analysis.Finally,the high-speed direct detection multi-carrier system based on hybrid compression coding technology can support QPSK to 4096 QAM 5G-NR-OFDM signal carrier aggregation,common public radio interface(CPRI)equivalent rates are in the range of 50 Gbps to 167 Gbps.The application of nonlinear quantization technology and nonlinear interference compensation technology in high-speed optical direct detection multi-carrier system and their network application scenarios is discussed.The research shows that nonlinear interference compensation technology is more suitable for high-speed optical transmission scenarios such as data center networks and passive optical networks,and nonlinear quantization technology is more suitable for high-speed optical transmission scenarios such as wireless fronthaul networks.