Research On Frequency-domain Equalization Technology For Few-Mode Fiber Mode-division Multiplexing System

With the rapid development of digital communication services,the multiplexing of existing optical fiber communication system expansion technologies in time,frequency,polarization,and high-order modulation technologies can no longer meet the demand for future information.Therefore,how to improve transmission capacity of optical fiber communication system is a key issue to be solved urgently.Under this background,mode-division multiplexing technology based on few-mode fiber utilizes limited number of modes in the few-mode fiber as independent channels to transport multi-signals,thus achieving a dramatic improvement in capacity,which has become one of the key technologies of high-speed and large-capacity optical transmission system.However,mode coupling and differential mode group delay in few-mode fiber mode-division multiplexing technology limit the transmission distance and rate severely.Therefore,the demultiplexing technology of output signal is the key to ensure the reliable transmission of signals.This thesis focuses on frequency-domain equalization technology for few-mode fiber mode-division multiplexing system,and the mainly contributions of this thesis are as follows:(1)Based on the transmission characteristics of few-mode fiber,the effects of differential mode group delay and mode coupling in few-mode fiber on signal transmission are studied in detail.Considering the factors such as chromatic dispersion,differential mode group delay and mode coupling in few-mode fiber,simulation model of few-mode fiber transmission link is established on the MATLAB software according to matrix trans-mission model theory.(2)Frequency-domain equalization technology of mode-division multiplexing system is studied.Firstly,the characteristics of time-domain equalization and frequency-domain equalization are introduced.Considering the need of lower complexity of equalization in the practical communication system,the necessity of frequency-domain equalization is illustrated.Then,the basic principles and computational processes of frequency-domain least mean square algorithm and frequency-domain constant modulus algorithm(FD-CMA)are introduced in detail.And computational complexity of two algorithms is compared.Aiming at the poor performance of FD-CMA algorithm for high-order modulation format signal,frequency-domain multimodulus algorithm(FD-MMA)is used for equalization and the expression of MIMO equalization algorithm is derived.(3)A 3 modes mode-division multiplexing system based on few-mode fiber is built.The effects of iterative step size on the performance of frequency-domain equalization algorithm are studied on MATLAB software in the aspects of constellation,convergence speed and steady-state error.In addition,the performance of FD-CMA algorithm and FD-MMA algorithm are simulated and compared in the 56Gb/s 3 modes QPSK system and 112Gb/s 3 modes 16QAM system.The simulation results show that FD-MMA has a FD-MMA has a 1 dB improvement of optical signal-to-noise ratio tolerance compared with FD-CMA at the bit error rate of 10-3 for constant modulus QPSK signal.For non-constant modulus 16QAM signal,FD-MMA has a 2.5 dB improvement of optical signal-to-noise ratio tolerance over FD-CMA at the bit error rate of 10-3.