Investigations And Realizations On Frequency Domain Equalizer For Few-mode Fiber Communication Systems Based On FPGA

With the further study of the single mode fiber's degree of freedom multiplexing technology,the channel capacity of single mode fiber is fast increasing.However,due to the limit of the nonlinear Shannon,the channel capacity of single mode fiber is about to reach the bottleneck.The technology of mode-division multiplexing in few mode fiber is one of the effective means to break through the limit of channel capacity and achieve a higher rate of optical communication.The new technology of mode-division multiplexing in few mode fiber is proposed,which is a new requirement for the traditional channel equalizer and equalization algorithm.The equalizer of mode-division multiplexing system from the traditional single input single output structure,gradually transformed into multiple input multiple output form.At present,there are a lot of equalization algorithms for mode-division multiplexing system,but these algorithms are based on the theory of simulation,and lack of practical test.In this paper,we research on the mode-division multiplexing system with the few mode fiber and establish mode-division multiplexing system model with transmission couple matrix.The simulation experiment of the two mode fiber transmission system is carried out.We research into adaptive equalization algorithm and analysis the complexity of time domain equalization algorithm and frequency domain equalization algorithm in mode-division multiplexing system,proving the superiority of the frequency domain equalization algorithm in the long-distance transmission system.The frequency domain LMS algorithm is extended to the equalization system with MIMO structure,and the frequency domain LMS equalization algorithm for the system with the mode-division multiplexing system is realized.Aiming at the inherent defects of the fixed step size LMS algorithm,the paper analyzes and compares several variable step size methods,and puts forward the variable step size function for the mode-division multiplexing system with few mode fiber.Multiple-input multiple-output equalizer with variable step-size frequency domain LMS algorithms is proposed to uncouple the 2×2 mode-division multiplexing system.To coordinate the conflicting requirement of convergence speed and convergence performance,frequency domain LMS adaptive algorithm is employed to update equalizer taps and modifies step-size factor with variable step-size function.In the 112 Gbit/s high-speed communication simulation system over 1000 km in few-mode fiber,signal Q2-factor increases 3.7 dB with same convergence speed.These studies demonstrate that the algorithm successfully uncouples the signals of mode-division multiplexing system,realizes fast convergence and low disorder.In this paper,we realize the hardware implementation of 256 point frequency domain LMS equalizer on Stratix IV board of Altera Company,and code each module with Verilog.The whole system is verified by the joint simulation on Modelsim and Matlab.Also the board level verification is carried out on the 250 MHz clock frequency,and the result is same as Modelsim simulation results,which shows that the method is feasible and has practical application value in the circuit design.