Research On MIMO Detection Technology In Fiber Mode Division Multiplexing System

As cloud computing,mobile networks,big data and other emerging broadband-consuming services have continuously achieved breakthrough development in recent years,the demand for global user data traffic is increasing at an alarming rate.However,the options for further enhancing the capacity of single-mode fiber(SMF)transmission sys-tems are limited and have reached the point of diminishing returns.Now the need for high spectral efficiency through fiber transmission is imminent.One way to improve the spectral efficiency is to take advantage of the spatial freedom in the fiber.The current mode-division multiplexing(MDM)technology using few-mode fiber(FMF)or multi-mode fiber(MMF)has attracted widespread attention.The MDM system uses multiple modes in FMF or MMF to transmit many independent parallel data streams.The electric field in the MDM system can be represented by a set of orthogonal spatial and polarization modes.When all the degrees of freedom utilized in SMF are used in each spatial mode of the MDM system,the total bit rate increases in proportion to the number of spatial modes.Although the long-distance transmission of the MDM system inevitably leads to the cou-pling between different modes,which leads to crosstalk and interference,the multi-input multi-output(MIMO)digital signal processing(DSP)technology can separate parallel data streams.This thesis focuses on the impact of linear transmission impairments in fiber of MDM transmission links,focusing on MIMO detection technology,and mainly completes the following tasks:(1)According to the matrix transmission theory,a simulation model of an MDM system with D polarization modes is established.Some basic end-to-end properties of linear MIMO systems are introduced,and linear propagation impairment such as modal dispersion,mode coupling,and mode-dependent loss(MDL)are studied.(2)First,considering the influence of accumulated MDL and environmental inter-ference for the mode coupling matrix in the fiber of MDM link,a general optical fiber dynamic MDM transmission system was established by simulation.Then analyzed the influence of accumulated MDL on the MDM transmission system,and at the same time imposes different intensities of environmental interference to affect the mode coupling of the MDM system,and then introduces the adaptive MIMO equalization algorithm - least mean square(LMS)and the recursive least squares(RLS),and used for MDM channel dynamic compensation,finally using mean square error(MSE)as the index to evalu-ate the performance of these two kinds of adaptive MIMO equalization algorithm.The simulation results show that the performance of MDM transmission link is impaired by accumulated MDL.The two channel compensation algorithms can well compensate the performance impair of the system caused by environmental interference,but the existence of accumulated MDL will make the performance of the dynamic MDM transmission link change greatly after compensation,that is,the variance of the MSE after maximum like-lihood(ML)demodulation changes greatly.(3)Based on the characteristics of the signal-to-interference plus noise ratio(SINR)in the MDM system,that is,the probability distribution of SINR is approximated by the generalized gamma distribution,and an improved reduced-search algorithm(IRS)based on multilevel SINR thresholding(IRS-MST)is proposed.IRS-MST dynamically deter-mines the size of the search space for each mode based on multiple SINR thresholds.Furthermore,a compounding function is used to balance the size of the search space and performance.Simulation results show that IRS-MST can achieve near ML performance for different accumulated MDLs.Compared to the conventional IRS algorithm,the com-plexity of IRS-MST is approximately 5.4 times smaller than that of the IRS.