Research On Demultiplexing Technology Based On Unconstrained Frequency Domain Equalization In Mode Division Multiplexing System

At present,the Internet and traditional industries are deeply integrated.As a result,Information technology has penetrated almost every corner of the society and the demand for network bandwidth has increased explosively.At the same time,since the capacity of the traditional single-mode optical fiber communication system approaches the Shannon limit,one of the main media of data transmission-the optical fiber communication network faces unprecedented challenges.In order to deal with the capacity limit of the single mode fiber,experts and scholars have put into the research of the capacity expanding technology of the optical fiber communication network.On the condition that time,wavelength,polarization state,frequency and other degree of freedom has been applied,the degree of freedom of mode is arousing scholars' attention.The technology of mode division multiplexing(MDM)over few mode fibers uses mutually orthogonal modes to transmit signals,which greatly expands the system capacity of single fiber.However,in the actual MDM system,the differential mode group delay(DMGD)and mode coupling severely affects the quality of the signal.Therefore,it is an urgent problem to effectively compensate the obstacles in the mode division multiplexing system to improve the quality of signal transmission.In this dissertation,we study the affect mechanism of above obstacles in MDM system,and build up a complete simulation system based on it.We propose a low-complexity algorithm,the unconstrained frequency-domain least mean square(FD-LMS)algorithm,to compensate those impairments.In addition,a variable step size unconstrained FD-LMS algorithm is proposed on the basis of the unconstrained FD-LMS algorithm.They both perform well in mode demultiplexing.The main work of this dissertation is as follows:Firstly,according to the theory of Maxwell's equations,mode theory of the modes supported by the few mode fiber is analyzed in detail.Light field distribution of some low order modes are given,and the mode orthogonality of those modes is verified.Secondly,we thoroughly explore the influence of DMGD,mode coupling and other obstacles on signals in mode division multiplexing system.The transmissionmodel of the model division multiplexing system is constructed according to the characteristics of those impairments.And we set up a 6×6 simulation setup based on the model.In addition,considering that the computational complexity of the traditional frequency domain least mean square algorithm is high,a demultiplexing method based on the unconstrained FD-LMS algorithm is proposed.And the 6 × 6 MDM simulation system is used to verify the demultiplexing performance of the proposed algorithm.The simulation results show that bit error ratio(BER)performance of the unconstrained FD-LMS algorithm is similar with the traditional constrained FD-LMS algorithm.However,its computational complexity is much lower than that of the traditional FD-LMS algorithm.Finally,considering that convergence speed of the traditional algorithm is slow,the proposed unconstrained FD-LMS algorithm is improved,and we propose a variable step size unconstrained FD-LMS algorithm which has high convergence speed.The optimal step-size of each frequency bin is derived by canceling the posteriori error.At the same time,the convergence property of the algorithm is verified theoretically,and the great advantage of the algorithm in the convergence speed is proved by the simulation system.The simulation results show that,in the transmission link of 80 km,the convergence speed of the algorithm is increased by45.1% compared with the traditional constrained FD-LMS algorithm.Besides,with the transmission distance increases,the convergence speed of the proposed algorithm is further enhanced.At the end of this dissertation,all contents are summarized.The problems that need to be solved in the system are pointed out,and the follow-up work is prospected.