Research On Constellation Shaping In High Speed Coherent Optical Communication System

High-definition video,virtual reality and other digital demands are gradually breaking out.As the foundation of network data transmission,optical fiber communication carries more than 90%of the global data transmission,and is attracting more and more attention from society and academia.At present,optical core network is developing towards the direction of ultra-high speed,ultra-large capacity and ultra-long distance.Constellation shaping technology can change the distribution of signal constellation points through probabilistic shaping and geometric shaping methods,which can achieve a larger transmission capacity of optical fiber system and improve the ability of signal anti-nonlinear effect.Constellation shaping technology has become one of the hot research directions in the field of optical communication.However,in the actual deployment of constellation shaping technology,it is incompatible with other optical communication coding modulation technologies such as layered modulation,and the algorithm complexity is too high,which limits the application of constellation shaping technology.Therefore,it is of great significance to explore the coding modulation technology combined with constellation shaping in optical communication system.In this thesis,the key technologies of high-speed coherent optical communication and the key principles of constellation shaping technology are studied.The probabilistic shaping scheme based on layered modulation,the constellation adjustment scheme based on layered probability shaping and the probabilistic shaping scheme based on chaotic encryption are proposed,which solves the incompatible problem of probabilistic shaping and layered modulation,and realizes the enhancement of channel capacity.The complexity of probabilistic shaping algorithm is reduced.The main contents and innovations of this paper are as follows:(1)A probabilistic shaping scheme based on hierarchical modulation is proposed.The probabilistic shaping hierarchical mapping method is adopted to solve the conflict between hierarchical modulation algorithm and probabilistic shaping algorithm,and effectively improve the channel capacity of long-distance optical communication system based on hierarchical modulation.The bit error rate performance of the proposed scheme is simulated and the results show that the proposed scheme can effectively resist the nonlinear effect and channel noise in high-speed optical communication channels.When the sign rate is 60Gb/s and the signal-to-noise ratio is 14dB,the bit error rate of the probabilistic shaping scheme based on layered modulation is 10-2 in AWGN analog channel,which is significantly better than that of or-dinary QAM signal 10-1.In the SSMF channel,the bit error rate of the probabilistic shaping scheme based on layered modulation is 3*10-2,which is significantly better than that of the ordinary QAM signal 2*10-1.When the decision threshold is 10-2,the signal of probability-shaping scheme based on layered modulation can be transmitted correctly through 80kmSSMF channel,while the ordinary QAM signal cannot be decoded correctly under all SNR conditions.The results show that the proposed scheme can significantly improve the error performance of the system and thus improve the channel capacity.(2)An adaptive constellation position adjustment scheme based on hierarchical probabilistic shaping is proposed.Based on the application of layered probability shaping,the scheme adjusts the position of constellation points by changing the layered power ratio parameters and the layered phase angle parameters based on the channel characteristics to achieve geometric shaping and improve the anti-interference ability of information.The simulation verifies that the adjustment of parameters makes the position of constellation points change accordingly,further improves the geometric distance between constellation points,and realizes the improvement of channel capacity.By simulating the process of OLT terminal broadcasting 16QAM signals to three ONUs with optical fiber spans of 12km,24km and 37km,the results show that the coherent PON system based on layered modulation can effectively use the rich power of the system link and achieve a higher reachable information rate.(3)A probabilistic shaping scheme based on chaotic encryption is proposed.The scheme realizes probabilistic shaping through bit by bit arithmetic of information sequence and logical chaotic sequence,and changes the channel probability distribution by optimizing some operations in chaotic encryption algorithm.Compared with the mainstream probabilistic shaping algorithm based on CCDM,the probabilistic shaping algorithm complexity is reduced effectively.The simulation results show that the output rate of this scheme is significantly better than that of CCDM algorithm with short block length parameters.At the same time,the comparison simulation of 16-CAP,32-CAP and 64-CAP under different received optical power verifies that this scheme can significantly improve the channel capacity while ensuring the security of the physical layer.