Research And FPGA Implementation Of Dispersion Equalization Technology In Coherent Optical Communication

With the advent of 5g era,emerging technologies such as cloud virtual reality,Internet of vehicles and cloud computing put forward higher requirements for data throughput.The coherent optical communication system with ultra long transmission distance and ultra large capacity has become a research hotspot in the field of optical communication.Because the optical signal generated by the laser contains different frequency components,the transmission speed of these different frequency components in the optical fiber is different,resulting in dispersion effect.The inter symbol crosstalk caused by dispersion effect will affect the coherent demodulation and threshold decision at the receiver.Nowadays,optical communication networks are developing towards high-speed and long-distance,and the influence of dispersion effect on the quality of optical communication is more serious.Therefore,balancing the influence of dispersion effect is of great significance in the field of optical communication.How to make rational use of existing hardware resources and realize low-cost dispersion equalizer is one of the important problems in the application of digital coherent optical communication system.Focusing on coherent optical communication system,this paper briefly introduces the structure and basic principle of transmitter,optical fiber link and receiver,mainly studies the dispersion formation mechanism and equalization method,designs a time-domain dispersion equalizer,and makes theoretical analysis and hardware verification.The main contents include:1.A dispersion equalizer cmgd-fir based on momentum gradient descent optimization algorithm is designed,which can improve the convergence speed and ensure the dispersion equalization effect.The algorithm uses momentum gradient descent(MGD)to optimize the gradient exponentially weighted average and correct the deviation in each update iteration.In addition,a single carrier coherent optical transmission system is built based on optisystem simulation platform,and the performance of the algorithm is verified by joint simulation with MATLAB.The simulation results show that the momentum gradient descent algorithm can update the tap weight to the minimum value faster,and the higher the learning rate,the faster the learning rate;When the learning rate is 0.1 and the momentum coefficient is 0.98,the convergence value of the proposed optimization algorithm reaches 0.015461;Under different SNR conditions,the equalization effect of the equalizer designed by the optimization algorithm is better than that of gd-fir and classical time-domain fir equalizer;At the same time,the pulse shaping filter is used to limit the baseband signal bandwidth,and the dispersion equalization is carried out in the narrow band.The number of taps is reduced by about 1 / 3,which still has a good equalization effect and reduces the complexity of hardware implementation;2.Based on FPGA hardware development platform,a cmgd-fir dispersion equalization system is designed and verified by experiments.The core part of the dispersion equalization module is designed with direct serial structure.The number of data quantization bits is 12 bit and the clock frequency is 50 MHz.The module cooperates with the memory module,equalizer coefficient control module and data flow control module to calculate the data.After the functional simulation of all modules is passed,the experimental verification is carried out.The equalization results are transmitted back to the host computer at 115200 baud rate through the serial communication module,and the joint analysis is carried out by optisystem and MATLAB.The results show that the CMGD-FIR designed in this paper uses less hardware resources after synthesis,and the equalization system has good effect and low bit error rate,which is close to the system simulation results,and verifies the feasibility of the dispersion equalization system.