Research On Channel Distortions And Digital Equalization Techniques For Low-cost Short-Reach Optical Fiber Transmission Systems

With the flourishing development of data services such as ultrahigh-definition(HD)video,virtual reality,cloud computing,as well as cloud storage,the global optical network data traffic has experienced an explosive growth.The short-reach application scenarios such as optical fiber access network and data center interconnection(DCI),have the characteristics of short distance,high interface density,and enormous connected devices.Thus,they are sensitive to cost and power consumption,and play critical roles in expanding the entire optical network capacity.Therefore,to meet the requirement of the continuous global data traffic growth,it is of great significance to investigate low-cost and high-capacity short-reach optical fiber transmission systems.Intensity modulation/direct detection(IM/DD)systems have low cost,low power consumption,compact size,and ease of integration,making them ideal for deployment in next-generation short-reach transmission applications.However,IM/DD system is extremely sensitive to system channel distortions brought by the rapid increase of transmission rate.To further expand the channel capacity in short-reach application scenarios,this thesis focuses on the channel distortions of high-speed IM/DD systems,such as bandwidth limitation,system nonlinear distortion and fiber chromatic dispersion.Moreover,advanced digital equalization techniques are innovatively proposed to deal with the channel distortions in two specific applications:optical access network and DCI.These techniques lay a good foundation for the next-generation short-reach optical transmission systems with low cost and high capacity.Due to the different key performance indicators in different application scenarios,the main research contents and core innovations are summarized from two aspects including optical fiber access network and DCI.I.Low-cost and high-speed PON systemOptical fiber access network is generally deployed with passive optical network(PON)system.Leveraging low-bandwidth devices can economize the system cost,but leads to a system bandwidth l imitation issue.Moreover,the system suffers from dispersion-induced power fading and nonlinear distortions.Hence,it’s important to investigate advanced digital equalization techniques to handle severe channel distortions in PON system.The characteristics of the multi-carrier modulation and single-carrier modulation have their own superiorities in point to multipoint PON system.Therefore,this thesis investigates both the multi-carrier PON and the single-carrier PON.1.Low-cost and high-speed I-SC-FDM PON system.Multi-carrier orthogonal frequency division multiplexing(OFDM)system facilitates flexible modulation and flexible subcarriers allocation,so it has been widely studied in optical fiber access network.Firstly,to solve the issues of high complexity and high PAPR using OFDM,this thesis investigates a real interleaved subcarrier frequency division multiplexing(I-SC-FDM)PON system.Compared with the conventional OFDM-PON,the I-SC-FDM system reduces the signal generation complexity from O(Nlog2N)to O(N).Besides,the I-SC-FDM system significantly reduces signal PAPR.Specifically,the I-SC-FDM system based on 8-QAM and 16-QAM reduces PAPR by 8 dB,and the I-SC-FDM system based on QPSK reduces PAPR by about 11 dB.Moreover,a novel frequency domain and time domain joint equalization algorithm is proposed in I-SC-FDM system.The results show that in a 40 Gbps PON system with 10-G class devices,compared with single frequency domain equalization and single time domain equalization,the frequency domain and time domain joint equalization improves the receiver sensitivity by 1 dB and 0.6 dB,respectively.Moreover,the complexity of frequency domain and time domain joint equalization is only 23.7%of that of single time domain equalization.2.Low-cost and high-speed PAM4-PON system.Single carrier PAM4 signal is of low complexity and low PAPR.Moreover,the spectral efficiency of PAM4 signal is twice that of conventional NRZ signal.Hence,PAM4 is of great potential in next generation single carrier PON system.Based on low cost 10-G class devices and PAM4 modulation,this thesis concentrates on the channel distortion,and conduct an in-depth analysis about the limitations of conventional equalizers in compensating channel distortions in PON systems.Moreover,this thesis innovatively proposes a series of novel equalization schemes,which can be summarized as:1)To solve the high complexity issue of Volterra nonlinear equalizer(VNLE),this thesis proposes a memory polynomial equalizer(MPE)to replace VNLE in PON system.Besides,both MPE and VNLE are combined with feed-forward and feed-back filtering structure,namely FFE-DFE-Polynomial and FFE-DFE-Volterra.The experimental results show that in a 40 Gbps PON system,FFE-DFE-Volterra and FFE-DFE-Polynomial achieve similar equalization performance,while the complexity of FFE-DFE-Polynomial is only 16.67%of-that of FFE-DFE-Volterra.2)To solve the high complexity issue of feed-forward equalizer with many taps,in this thesis,the circular convolution is used to replace linear convolution in FFE to realize fast frequency domain equalization(FDE).The results show that when the number of FFE taps is greater than 20,the fast FDE is more effective.In addition,this thesis investigates post-filter and simplified maximum likelihood detection(MLSD)to solve the issue of the system noise enhancement by FFE.The experimental results show that the receiver sensitivity in a 50 Gbps PON system is improved by more than 4 dB using the proposed fast FDE+post-filter+simplified MLSD.3)To improve the performance of conventional decision feedback equalizer(DFE),this thesis proposes a hybrid decision scheme(HDS).The results show that in a 50 Gbps PON system,compared with single symbol decision scheme,the optimal HDS improves the receiver sensitivity by 0.7 dB and 1.3 dB at the forward error correction(FEC)thresholds of 10-and 10-3 respectively.Ⅱ.Low-cost and ultrahigh-speed DCI system with high dispersion toleranceThe transmission rate of next-generation DCI system is projected to be much higher than that of PON system,so it is more sensitive to fiber dispersion.Note that when the fiber dispersion leads to null points in signal spectral,the channel equalization is completely different from that without null points.This thesis focuses on the how to effectively compensate for the signal spectral null points caused by the fiber dispersion.We find that DFE based on autoregressive filter can compensate the spectral null points.Moreover,this thesis proposes a series of novel equalization schemes with high dispersion tolerance,and these schemes achieve some leading results in the experimental demonstration.The results can be summarized in detail as follows:1)To improve the effective SNR to enhance DFE equalization performance,this thesis proposes to reduce the carrier signal power ratio(CSPR)so that the effective signal SNR can be enhanced,and the enhanced signal-signal beating interference with MPE can be simultaneously suppressed.Then,DFE is effective to compensate for the spectral null points caused by fiber dispersion.The experimental results show that,thanks to the joint compensation of nonlinear distortion and fiber dispersion with MPE-DFE,we successfully achieved a C-band 56 Gbps PAM4 over 80 km standard single-mode fiber(SSMF)transmission with maintaining the bit error rate(BER)below the 7%FEC threshold.2)To solve the long error propagation issue using the conventional DFE decision scheme when the channel quality is poor,this thesis proposes a novel decision scheme based on error erasure.The experimental results show that the proposed decision scheme achieves C-band 112 Gbps PAM4 transmission over 20 km SSMF transmission.Moreover,the error erasure decision scheme reduces the average error length from 8.7 to 2.3.3)To solve the error propagation issue of the conventional DFE,the thesis proposes a multi-symbol joint decision scheme.Moreover,in the proposed multi-symbol joint decision scheme,we study three tree-search algorithms,namely maximum likelihood detection(MLD)algorithm,M algorithm and SD algorithm.Experimental results show that in a C-band 112 Gbps PAM4 system over 20 km SSMF transmission,the multi-symbol decision scheme reduces the BER from 1.2×10-2 to 6.0×10-4.Besides,compared with MLD algorithm,both M algorithm and SD algorithm reduce the search complexity by three orders of magnitude.In conclusion,the main focus of this thesis is on how to compensate for the channel distortions in short-reach transmission application scenarios including both the optical fiber access network and DCI.Moreover,this thesis innovatively proposed a series of equalization techniques based on digital signal processing,and conducted experimental demonstrations and verifications,which have laid a solid foundation for the next-generation short-reach optical fiber transmission system.