| Driven by the new internet of things,5G,and mobile Internet traffic,as the mainstream technology of optical access network,the next generation passive optical network(NG-PON)has become an important solution for broadband access network.The NG-PON system based on intensity modulation direct detection is suitable for practical deployment in the short term due to the simpleness and low cost.However,the physical layer impairment has become a key bottleneck of restricting the development of NG-PON.In this thesis,the equalization/compensation schemes based on Volterra theory are proposed to solve the physical layer impairment issue in NG-PON.In addition,considering the peer-to-peer traffic characteristics and the different signal noise ratios(SNRs)between the optical line terminal(OLT)and optical network units(ONUs),the hierarchical modulation technology is used to optimize the system traffic carrying efficiency and the power budget.The main research works and innovation points of this thesis are shown as follows.(1)In order to extend the system bandwidth,focusing on the nonlinear physical layer impairment issue introduced by the use of electroabsorption modulator and fiber's self-phase modulation negative chirp effect in orthogonal frequency division multiplexing PON(OFDM-PON),this thesis proposes a nonlinear impairment equalization method based on sparse Volterra equalizer,which adopts the orthogonal search algorithm to reduce the complexity of Volterra equalizer.The simulation results show that under the optimal configuration of system parameters,50Gbps OFDM-16-quadrature amplitude modulation(16-QAM)signal is transmitted through 55km standard single-mode fiber(SSMF)and equalized by the sparse Volterra equalizer.The system power budget realized is approximately 30dB,which can support the optical branch ratio of 1:64.The system performance of the sparse Volterra equalizer is similar to that of the full Volterra equalizer.The offline experimental is conducted for 20Gbps OFDM signal through 95km SSMF.The results show that the total tap coefficients are reduced by about 28%compared with the full three-order Volterra equalizer after the use of sparse three-order Volterra equalizer in the system.(2)Focusing on reducing the system cost in PON with "one master and many slave" structure,this thesis proposes a pre-equalization method based on the Volterra inverse equalization theory in frequency domain to equalize the physical layer nonlinear impairment caused by the direct detect.Simulation results show that when the signal transmission distance of 40Gbps OFDM-16-QAM format is less than 20km,the equalization of pre-Volterra equalizer has the similar effect with the post Volterra equalizer.Compared with the post two-order Volterra equalizer,the design of the two-order pre-Volterra equalizer reduces the DSP complexity of the whole system by 96.7%.(3)According to the traffic characteristics of NG-PON and the link difference between OLT and ONUs,this thesis proposes a improved traffic carrying efficiency scheme based on hierarchical modulation and network coding.In this scheme,the difference of SNRs between OLT-ONUs is caused by different branches and transmission distances.Thus,the modulation efficiency is optimized by hierarchical modulation technology.Meanwhile,the network coding technology is used to reduce the forwarding volume of peer to peer traffic required by the OLT.In the end,the overall traffic carrying efficiency of the system is improved.The simulation results show that the 40Gbps hierarchical modulation-OFDM-16-QAM format signal can be flexibly adapted to different channel parameters with different hierarchical power ratios under the setted optimal system parameters.When the longest SSMF transmission is 30km and the proportion of peer-to-peer traffic in the network is 100%,the traffic carrying efficiency of the system can be increased up to 1.37 times of the original.(4)Focusing on the existing waste of channel power margin in NG-PON,this thesis proposes a power budget improvement scheme based on the hierarchical modulation technology.In order to improve the power budget and spectral efficiency of the system link,the hierarchical modulation pulse amplitude modulation(PAM-4)format and digital Nyquist filter are adopted at the transmitter end through power resource reconstruction.In order to solve the issues of power fading effect caused by channel CD,linear and nonlinear inter-symbol interference,the link residual sideband filter,overlapping frequency domain equalization and iterative equalization algorithm are used to further improve the system's power budget.The simulation results show that the spectral efficiency of the system with Nyquist shaping is two times that of the system without shape.After the optical fiber transmission,the simulation results verify the effectiveness of the equalization algorithm with the different transmission distances.Under the condition of 6dB SNR difference caused by the transmission distance difference of 30km SSMF between two ONU groups,the optimal hierarchical power ratio is 0.3 and the optimal power budget is increased by about 2.4dB. |
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