Research Of Channel Model And DSP Algorithm In Mode-division Multiplexing Transmission Systems

With the improvement of the extent of the information society,demand for network capacity grows rapidly.According to the current demand trends,in 2020,the traditional single-fiber transmission system will not meet the needs of network capacity.Therefore,the research and development of ground-breaking high-capacity optical fiber communication system and its key technologies is of practical urgency.Since 2009,the concept of mode division multiplexing has been proposed,which can greatly improve the capacity and spectral efficiency of optical fiber communication systems.It's an effective solution to transmission capacity and network capacity.National research institutions carried out relevant research work rapidly.As a new research direction,mode division multiplexing system still has more critical scientific problems to study and solve.For example,the impact of random mode coupling in the few-mode fiber transmission link on transmitted signals and the performance of the system,and the internal mechanism and design of a variety of key modules such as mode coupler,mode multiplexer/demultiplexer,DSP and so on.Based on "The National Key Basic Research Development Plan(973 Plan)”project "Multi-dimension multiplexing optical fiber communication basic research",this dissertation studies mode-division multiplexing transmission system and its key technologies,and the main content of this research is the transmission channel model of mode-division multiplexing transmission system and the MIMO DSP algorithm which is used for compensating mode coupling in the receiver.The main research work and innovations of this dissertation are as follows.1.Establish the linear transmission channel model of few-mode fiber with random mode coupling.In order to verify the performance of the algorithm for demultiplexing in the receiver,we should establish the transmission channel model of few-mode fiber at first.In establishing the transmission channel model,this dissertation mainly considers the impact of random mode coupling.Depending on considering modal field properties or energy properties,Mode coupling can be described by field coupling models,which account for complex-valued modal electric fields,or by power coupling models,a simplified description that accounts only for real-valued modal powers.Depending on the different coupling strength,transmission channel model of few-mode fiber can be divided into multi-segment and single-segment matrix transmission model.2.Propose and simulate the general model of mode multiplexing transmission system.By reading and studying a large number of latest research results of the world abolt mode-division multiplexing and referencing to the general model of mode-division multiplexing transmission systems in VPI simulation platform,the author proposes the general system model used in the simulation is this dissertation,which consists of transmitter,mode converter,mode multiplexer,few-mode fiber,mode demultiplexer,coherent receiver,digital signal processing module and so on.The author has deep studied the principles to achieve the above devices in the current.On this basis,the author sets up a general mode-division multiplexing transmission systems simulation platform which can transmit a few modes simultaneously.3.Using multi-input multi-output time-domain equalization algorithm and constant modulus algorithm to decouple mode coupling.And performances of the two algorithms are compared.The dissertation first sets up a 4×4 mode-division multiplexing transmission system using single-segment matrix transmission model.To compare performances of CMA and DA-TDE in different coupling strength,the results show that CMA performs better than DA-TDE in strong coupling strength.Furthermore,the author uses multi-segment matrix transmission model to simulate a 6×6 mode-division multiplexing transmission system in all strong coupling strength.The results show that CMA performs also very well.