| With the development of Internet technology,various high-speed and large-bandwidth services such as high-definition videos,VR experiences,cloud services,smart cities,and artificial intelligence are emerging in an endless stream,and the network traffic is showing a trend of continuous growth.To further improve the performance of speed,capacity,and spectrum utilization in optical fiber communication,it is necessary to further increase the overall network capacity.There are three natural or direct choices:one is to increase the symbol transmission rate,the other is to increase the number of carriers,and the third is to use higher spectral efficiency modulation formats.However,simply increasing the symbol transmission rate is not a good choice,because the system capacity is limited,and the spectrum efficiency has not changed.In addition,a faster symbol transmission rate raises the requirements of related optoelectronic devices.Given the target transmission rate and transmission distance conditions,the appropriate number of carriers can be selected according to the modulation format and adjustable parameters to optimize the overall network capacity.In the case of a certain number of carriers,how to maximize the transmission capacity of single channel carrier to make it as close as possible to the Shannon limit has become the focus of extensive research.Among them,the modulation format optimization technology optimizes the transmit power and modulation dimensions of the modulated signal,which can achieve faster transmission rate and higher spectral efficiency.It mainly includes constellation shaping technology and multidimensional modulation,etc.,and is one of the key technologies to achieve channel Shannon limit.Based on the research of modulation theory in optical communication systems,this thesis focuses on related modulation optimization technologies such as geometric shaping,probabilistic shaping,and multidimensional modulation.For some scientific issues,such as how to reduce the phenomenon of overlapping constellation points in the outer layer of the constellation caused by the nonlinear effect in the coherent optical communication system,how to further improve the transmission performance of the coherent optical communication system,and how to further improve the IM/DD with low complexity,adaptive weighted geometric shaping GS-MQAM coherent optical transmission scheme,geometric shaping optimization transmission scheme combined with KMeans clustering algorithm,geometric probability hybrid shaping optimizing coherent optical transmission scheme,two-dimensional CAP signal IM/DD transmission scheme based on geometric shaping and threedimensional CAP signal IM/DD transmission scheme based on geometric shaping are proposed to further improve the channel capacity of single carier channels.The main work and innovations of this thesis are as follows:1.Coherent optical transmission scheme of adaptive weighted geometric shaping GS-MQAM combined with K-Means algorithm.In order to alleviate the phenomenon that the constellation points in the outer layer of the constellation are overlapped and lead to bit errors caused by the nonlinear effect,an adaptive weighted geometric shaping GS-MQAM coherent optical transmission scheme is proposed.On the basis of researching the nonlinear effects and geometric shaping related theories in long-distance optical fiber communication channels,an adaptive weighted geometric shaping GS-MQAM constellation design method is proposed.By introducing an adaptive weighting factor related to Kerr effect and fiber transmission distance,the ovei-lapping problem of the outer constellation points induced by nonlinear damage is reduced,thereby the transmission performance of the system is improved.Moreover,in order to further improve the system tolerance of high optical power and nonlinearity,thereby increasing the fiber transmission distance to a certain extent,on the basis of adaptive geometric reshaping,combined with unsupervised K-Means clustering learning algorithm,a geometric reshaping optimization transmission scheme combined with K-Means clustering algorithm is proposed.The research results show that the proposed adaptive weighted geometric reshaping algorithm obtains greater gain when the modulation order is higher,so it is more suitable for higherorder modulation.When transmitting at a net rate of 120 Gbit/s and the error threshold of SD-FEC is 2×10-2,the proposed coherent optical transmission scheme of adaptive weighted geometric shaping GS-61QAM combined with K-Means algorithm compared with the adaptive weighted geometric shaping optimization scheme and the regular 64QAM,the optical fiber transmission distance has been increased by approximately 60 km and 450 km respectively.2.Geometric probability hybrid shaping optimizing coherent optical transmission scheme.Aiming at the problem of serious degradation of the transmission signal caused by the accumulation of nonlinear effects of high-order modulation signals in long-distance optical fiber transmission with the increase of optical fiber transmission distance,starting from the reason,which is transmitted signal with high-power,research the constellation shaping technology that can reduce the signal lauched power.On the basis of adaptive geometric shaping,a hybrid shaping optimization transmission scheme combined with probability amplitude shaping algorithm is proposed to further reduce the average energy of the constellation diagram and reduce the signal transmission power,thereby improving the transmission performance of the system and further approaching Shannon limit.The research results show that when the proposed hybrid shaping optimization transmission scheme transmits at an OSNR of 23 dB,the fiber length of 16×75m,the MI gains are about 0.3 bits/symbol and 0.7 bits/symbol compared compared with the geometric shaping optimization scheme and the conventional 64QAM respectively.3.Two-dimensional CAP signal IM/DD transmission scheme based on geometric shapingAiming at the special needs of the IM/DD system with low cost,low power consumption and high integration,based on the research of the principle of CAP modulation and geometric shaping,a low-complexity two-dimensional 16CAP constellation design scheme is proposed.By changing the number of constellation points of concentric rings with different amplitudes and the phase difference of the constellation points between adjacent rings in the star constellation,finding the constellation diagram with the smallest average energy when the minimum Euclidean distance is the largest,so as to increase the constellation figure of merit(CFM).In addition,a quasi-Gray coding mapping scheme based on error statistics is proposed to further improve the error performance of the system.The experimental results show that the proposed two-dimensional GS-16CAP with the quasi-Gray coding mapping can obtain about 0.6 dB and 1.4 dB receiver sensitivity gain compared with the two-dimensional GS-16CAP without quasi-Gray coding mapping and the traditional starshaped two-dimensional 16CAP with Gray coding mapping,respectively.4.Thr-ee-dimensional CAP signal IM/DD transmission scheme based on geometric shapingIn order to further improve the transmission performance of the IM/DD communication system,based on the research of multidimensional CAP modulation technology,a three-dimensional geometrically shaped CAP signal IM/DD transmission scheme based on a pyramid structure is proposed.This solution uses a pyramid structure with a fixed Euclidean distance between adjacent constellation points in a threedimensional space to give full play to the optimization potential of constellation geometry shaping in a high-dimensional space.Under the condition of a fixed minimum Euclidean distance,the average energy of the constellation is reduced as much as possible,and thereby increase the constellation figure of merit to further improve the transmission performance of the system.Experimental results show that the proposed 3D-GS-16CAP obtains a receiver sensitivity gain of about 0.4 dB compared with the traditional 3D-16CAP based on a regular hexahedron structure. |
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