| The development of modern technology has led to the rise of new services that require huge traffic resources,such as the Internet of Things,big data and smart cities.At the same time,the demand of individual users for data traffic is also growing substantially,which all put higher requirements on communication capacity.As the "last kilometer" in the transmission process,short-range optical communication technology has been under great pressure in this wave of capacity crisis.Now it has set its eyes on the field of spatial dimension multiplexing that has never been involved before,and space division multiplexing technology based on multi-core multiplexing has gradually matured.Moreover,the capacity improvement of single-mode single-core fiber does not stop.According to theoretical studies,there is a certain gap between the transmission capacity of the system using the traditional uniform high-order QAM modulation and the nonlinear Shannon limit,and constellation shaping technology has become a popular research direction by effectively improving the spectrum efficiency under the premise of low complexity.By changing the geometric coordinates and distribution probability of signal mapping,constellation shaping technology can effectively improve the transmission capacity and anti-interference ability of the system,which is a promising scheme in coded modulation.On the basis of short-range optical communication system,starting from the transmission environment of multi-core multiplexing,this paper focuses on the study of geometric shaping and probabilistic shaping technology in coded modulation.Aiming at the problems faced by constellation shaping,this paper makes research and breakthroughs,and puts forward corresponding improvement schemes to achieve further increase of transmission capacity.The main research contents and innovations of this paper are as follows:(1)The joint geometric/probabilistic shaping of even order three-dimensional constellations is studied,and a hybrid geometric and probabilistic shaping scheme based on three-dimensional constellation mapping is proposed.Based on the traditional three-dimensional constellation,the structure of the scheme is improved,and the fixed minimum European distance is used to reduce the average energy method to optimize,and combined with probabilistic shaping to further improve the anti-interference ability of the constellation.The simulation system based on CAP transmission is used for verification.The simulation results show that the improved constellation structure improves the SNR performance by 0.9d B compared with the traditional constellation structure in the corresponding uniformly distributed system,and the maximum SNR performance is improved by 2.3d B after the addition of probabilistic shaping technology,which effectively proves that the scheme has a good effect.(2)In view of the poor effect of direct three-dimensional geometric shaping,the reasons affecting constellation performance were analyzed and the corresponding rules were summarized,and the layered combined three-dimensional CAP-16 signal transmission scheme was proposed.This scheme is developed from two aspects of basic unit combination and layered symmetry,and fills constellation points in each position step by step.Under the premise of keeping the minimum Euclidean distance unchanged,the average energy of constellation points is reduced as far as possible,and the performance index of constellation is greatly improved.At the same time,the mapping rules of each constellation point were re-planned according to the adjusted structure to minimize the bit error rate of constellation point misjudgment.The correlation simulation results proved the effect of geometric shaping and mapping rule adjustment.The experimental results show that compared with traditional threedimensional CAP-16,the layered combined scheme achieves an average optical power gain of1.4d B at a bit error rate of 1×10-3,using a 2km seven-core weakly coupled optical fiber transmission platform. |
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