Research On Soft-Output Interference Cancellation Techniques For Intensity-Modulation And Direct-Detection Optical Fiber Transmission Systems

Driven by the rapid development of next generation mobile communication technology,industrial Internet and big data,as well as the national strategy of "New Infrastructure" and "Industrial Digital Transformation",the capacity of optical fiber network increases dramatically.Especially,the short-reach optical fiber transmission systems,such as data center optical interconnections and optical fiber access network,have the characteristics of high interface density,enormous device connections,and interfacing with the user terminal directly.Hence,the short-reach optical interconnections are most important for the capacity expansion of next-generation optical fiber network.Due to its costsensitive and power-consumption-sensitive characteristic,the intensity modulation/direct detection(IM/DD)scheme with low cost and low power consumption is favored.Therefore,the research of high-speed short-reach optical fiber transmission system based on IM/DD is of great significance to the continuous and rapid growth of optical fiber network capacity.However,as the increase in transmission capacity,the influence of channel distortions such as chromatic dispersion(CD)of optical fiber and system bandwidth limitation on signal becomes more and more severe,making the transmission performance degradation.Therefore,to further expand the capacity of short-reach optical fiber networks,two key scientific issues of "CD of optical fiber" and "system bandwidth limitation"are focused in IM/DD optical transmission systems.According to the mechanism of signal distortions caused by these two kinds of channel impairments,a series of soft-output interference cancellation techniques are proposed to compensate or resist the signal distortions based on singlecarrier modulation and multi-carrier modulation.And the soft-input information from soft-output interference cancellation techniques is provided to soft-decision forward error correction(FEC).Hence,softoutput interference cancellation techniques can support the highperformance transmission in the next-generation low-cost,high-speed short-reach optical interconnections.The main contents and core innovations are as follows:I.The Research on CD-Induced Frequency Selective Power FadingTo solve the severe inter-symbol interference(ISI)caused by CDinduced frequency selective power fading,the ISI cancellation techniques composed of adaptive equalizers and signal optimization detection are investigated.For the IM/DD systems with large CD,the signal optimization detection requires a long memory length to match the frequency selective power fading effect to realize the optimal signal detection.However,the traditional signal optimization detection based on maximum-likehood sequence estimation(MLSE)has the exponentially increasing computational complexity and storage space with the increase of memory length.Due to the limitation of the computing resources and storage resources of hardware,the memory length of MLSE cannot be indefinitely increasing,making the performance limitation of signal optimization detection.Therefore,to solve the memory length limitation of MLSE,the soft-output signal optimization detection and its simplified version are proposed,which are specifically introduced as follows:1)To solve the problem that the implementation complexity of MLSE increases exponentially with the memory length,an optimized detection based on Log-maximum a posterior estimation with the fixed number of surviving states(fixed-state Log-MAP)is proposed.The fixed-state Log-MAP can reduce the computational complexity and storage space from exponential order to linear order.Therefore,the fixed-state Log-MAP can support long enough memory length to eliminate the ISI for large-CD IM/DD systems.We experimentally verified the fixed-state LogMAP in a C-band 64-Gbit/s IM/DD on-off keying(OOK)optical system over a 100-km dispersion-uncompensated link.The experimental results show that compared with the traditional MLSE,the fixed-state Log-MAP detection not only improves the receiver sensitivity by 2 dB,but also reduces the computational complexity by three orders of magnitude and reduces the storage space to about 0.05%of MLSE.2)To further reduce the computational complexity of fixed-state Log-MAP,an optimized detection based on absolute-based fixed-state Max-Log-MAP(Abs-FS Max-Log-MAP)is proposed.The experimental results show that compared with fixed-state Log-MAP,Abs-FS Max-Log-MAP has little performance penalty without any multiplication during the update of survival path,which greatly reduces the implementation complexity.II.The Research on Limited-Bandwidth-Induced High-Frequency DistortionsTo solve the high-frequency distortions caused by the system bandwidth limitation,we investigate from two perspectives of "how to eliminate ISI caused by bandwidth limitation" and "how to resist the influence of high-frequency fading on signal".The soft-output ISI iterative cancellation techniques for single-carrier modulation systems and intercarrier interference(ICI)iterative cancellation technique for bandwidthcompressed multi-carrier modulation systems are proposed,respectively.1.Soft-output ISI iterative cancellation techniques based on single-carrier bandwidth-limited systems.From the perspective of "how to eliminate ISI caused by high-frequency distortions",the ISI cancellation techniques consisted of adaptive equalizers and signal optimization detection are investigated.Different from the ISI cancellation techniques for CD-limited systems,the signal optimization detection for bandwidthlimited systems only requires 1-2 memory lengths to match the highfrequency fading effect.-Therefore,the memory length is no longer the research focus in bandwidth-limited systems,rather than becoming how to eliminate ISI caused by bandwidth limitation more effectively with the increase of transmission capacity,so as to transmit more information within limited bandwidth.A novel soft-output ISI iterative cancellation technique and its simplified version are proposed to improve the transmission performance for bandwidth-limited systems in this thesis.The specific contents are as follows:1)To improve the performance using conventional hard-output direct-detection faster-than-Nyquist(DD-FTN)interference cancellation technique,a soft-output direct-detection faster-thanNyquist(SO-DD-FTN)interference cancellation technique is proposed.The SO-DD-FTN combines soft-decision FEC with Turbo iteration technique to further improve the ISI cancellation performance.The experimental results show that SO-DD-FTN with three iterations has 5-dB receiver sensitivity improvement compared with DD-FTN in the bandwidth-limited system based on low-cost 10-G class optics.2)To solve the high computational complexity of SO-DD-FTN caused by multiple iterations,a threshold-assisted SO-DD-FTN is proposed to reduce the computational complexity.The research results show that compared to the conventional SO-DD-FTN,the computational complexity of thresholdassisted SO-DD-FTN reduces by 57%without performance degradation.2.Soft-output ICI iterative cancellation techniques based on multi-carrier bandwidth-limited systems.From the perspective of "how to resist the influence of high-frequency fading on signal",a novel bandwidth-compressed multi-carrier modulation scheme is proposed.Moreover,to eliminate ICI induced by bandwidth compression,an ICI iterative cancellation technique is proposed,which is applicable to the large number of subcarrier and high-order modulation.Hence,the proposed ICI cancellation technique can support the flexible application of multi-carrier modulation in optical networks.The specific contents are as follows:1)To solve the high-frequency fading,non-orthogonal discrete multitone(NO-DMT)is proposed,which combines the advantages of discrete multitone(DMT)and non-orthogonal frequency-division multiplexing(NOFDM).Moreover,the-superiority of NO-DMT in bandwidth-limited systems is proved theoretically for the first time.NO-DMT inherits the adaptive bit power allocation technology in DMT,which can make full use of the SNR on each subcarrier to enhance the tolerance of in-band spectral distortions.Moreover,NO-DMT also inherits the subcarrier spacing compression of NOFDM,which reduces the signal bandwidth and improve the robustness to the high-frequency distortions.2)To eliminate the ICI in NO-DMT,an ICI iterative cancellation technique based on soft-decision decoder is proposed.The NO-DMT with 1024 subcarriers and up to 64QAM modulation format is verified in the bandwidth-limited system based on 10-G class optics.The research results show that ICI can be eliminated effectively by the proposed ICI iterative cancellation technique.NO-DMT has 3-dB and 6-dB receiver sensitivity improvement compared with DMT and NOFDM,respectively.In conclusion,this thesis focuses on the two key scientific issues of"CD of optical fiber" and "system bandwidth limitation" in high-speed IM/DD optical fiber transmission systems.We investigate the schemes to eliminate the interference caused by channel distortions based on singlecarrier modulation systems,and investigate the schemes to resist the influence of channel distortions on signal based on multi-carrier modulation systems.A series of soft-output interference cancellation techniques are proposed to improve system performance and further reduce the computational complexity,which promotes the practicability of these techniques in low-cost and low-power-consumption short-reach optical interconnections.Moreover,a series of research results in this thesis have remarkable reference significance for the next-generation high-speed data center optical interconnects.