Research On Signal Equalization Technology For Mode Division Multiplexing System Based On Lattice Reduction

With the widespread use of Internet businesses,the demand for global data traffic has grown exponentially,making the capacity of single mode fiber communication systems increasingly close to the nonlinear Shannon limit.Mode-division multiplexing(MDM)technology based on few-mode fibers(FMF)utilizes the mutually orthogonal modes of FMF as independent channels for information transmission,which can multiply the capacity of fiber communication system.However,in the practical FMF,the special impairments such as mode coupling(MC),differential mode group delay(DMGD),and mode dependent loss(MDL)will make the transmission signal in MDM system suffer from channel crosstalk and intersymbol interference,which seriously affects the performance of MDM system.Among them,MC and DMGD can be compensated by traditional signal equalization techniques.However,MDL will directly degrade the orthogonality of the channel matrix,especially in the long-distance MDM system the MDL is large,which results in serious deterioration of the orthogonality of the channel matrix.At this time,the impairment compensation performance of the traditional equalization techniques drop sharply,which limits the transmission capacity and the transmission distance of MDM system.Therefore,the search for a new signal equalization technique to realize the impairment compensation of MDM system in the presence of MDL is one of the key issues in the long-distance optical fiber system transmission.Around the above question,this paper first set up the MDM simulation system based on the special impairment in FMF.Then,we explore the impairment characteristic of MDL,analyze the influence of MDL on the orthogonality of the channel matrix,and verify that traditional signal equalization algorithms are difficult to achieve signal impairment compensation in MDM system with MDL.Finally,a signal equalization technology based on lattice reduction is proposed for signal impairment compensation in MDM system with MDL.The specific works of this paper are as follows:Firstly,the basic conception of FMF is introduced,and the orthogonality of mode of FMF is verified.The generation and the effect on the transmitted signal of maintransmission impairments are studied,including: MC,DMGD and MDL.Secondly,according to the matrix transmission model,the MDM simulation system is modeled.On this basis,the 6×6 dual polarization MDM simulation system is set up with the VPI simulation platform and Matlab programming software.And then the accuracy of the simulation system is verified by estimating the channel implulse response.Thirdly,the impairment characteristic of MDL is analyzed.The changing relationship between MDL and the coupling strength or transmission distance is verified.The impairment compensation performances of the frequency-domain least mean square algorithm(FD-LMS),the frequency-domain multimode blind equalization algorithm(FD-MMA)and minimum mean square error(MMSE)algorithm for MDM system in the presence of MDL are studied,and the bit error rates of the output signals of the three equalization algorithms in different MDL situations are compared.The simulation results show that the MDL increases with the increase of the transmission distance,and traditional signal equalization algorithms are difficult to achieve signal impairment compensation of MDM system.At last,a signal equalization algorithm based on lattice reduction is proposed for impairment compensation of MDM system with MDL.The principles of the lattice reduction and the signal equalization algorithm based on lattice reduction are introduced.The performance of the proposed algorithm is simulation verified.We compared the condition numbers of the channel matrix before and after the lattice reduction,verified that the lattice reduction can improve the orthogonal of the channel matrix,and studied the demultiplexing performance of the proposed signal equalization algorithm based on lattice reduction under the different system transmission distance.It is proved that the proposed algorithm can effectively achieve the signal impairment compensation in the presence of MDL,and it has low computational complexity and is easy to implement.