Research On WDM-PON System And Channel Equalization Based On PAM4

With the rapid development of information society,5G networks have emerged with a large number of new communication application scenarios,such as enhanced mobile broadband（e MBB）,high reliability and low latency connectivity（u RLLC）,and massive Io T（m MTC）.Terrestrial mobile 5G and its access networks are facing huge challenges.In order to meet the needs of future communication development,it is particularly important to study advanced 5G forwarding network bearer technology for broadband converged access.In the 5G forwarding network,it is necessary to meet the requirements of high speed,large capacity and large number of AAU accesses between DU and AAU.Wavelength division multiplexing（WDM）-based passive optical network（PON）is one of the preferred solutions to meet the needs of 5G forwarding network.In this thesis,we study the 5G forwarding network bearing scheme and its key technologies,propose the 5G forwarding network structure based on IM/DD’s PAM4WDM-PON,analyze its transmission performance,and study the digital equalization technology for this system.The main research contents and contributions of this thesis are as follows.（1）In order to meet the forward transmission requirements of different application scenarios of 5G,PAM4 WDM-PON is used as the forward transmission network bearer scheme,and the IM/DD-based PAM4 WDM-PON transmission system is built and its transmission performance is analyzed using joint simulation of VPI and MATLAB software.Short-distance optical transmission systems usually use IM-DD technology combined with some advanced multi-level modulation techniques to achieve high transmission rates when considering cost and complexity.In this thesis,Mach Zendel modulator（MZM）is used for optical modulated transmission,and the PAM4modulation format has higher transmission rate,better spectral efficiency,lower power consumption,and lower device bandwidth compared with conventional NRZ.The effects of different bias voltages in the system on the MZM modulation performance,the effects of different transmission distances and dispersion on the transmission performance,and the effects of different fiber incident powers on the transmission performance are further analyzed,and the feasibility of the 400Gbps PAM4 WDM-PON as a 5G forwarding network bearer scheme is verified by simulation experiments.（2）The equalization algorithm based on the least mean square（LMS）algorithm,the improved normalized least mean square（NLMS）algorithm for equalization,and finally the improved fractional normalized minimum mean square error（FNLMS）algorithm are proposed.Since the LMS algorithm introduces the noise term of the tapped coefficients,which leads to a larger amount of steady-state misalignment and slower convergence,making the algorithm less capable of tracking channel changes.Therefore,the NLMS algorithm is proposed to be used for equalization.Since the tap power vector of the filter is normalized by the squared Euclidean parametrization of input signal,the gradient noise of the NLMS algorithm is not overly amplified,and the convergence speed of NLMS is faster and the tracking ability of the channel change is better.In contrast,the fractional-order variant of FNLMS refines the scaling of the estimated parameters in the adaptive filtering framework,which can extract more details in the signal and improve the steady-state and convergence performance.The simulation compares the BER result plots of LMS,NLMS and FNLMS.The FNLMS equalization algorithm achieves the minimum BER of 2.2×10^-5 at 2 d Bm optical received power,which better than the minimum BER of LMS and NLMS.