Research On Digital Signal Processing(DSP) Techniques In Single Carrier High Speed Optical Communication Systems

With the rapid development of Internet of things(IoT),cloud computing,virtual reality(VR),ultra-high definition(UHD)multi-media and mobile data communication,the demand for network bandwidth is growing explosively.As the basic physical layer of communication network,high speed optical communication system is developing in the direction of high capacity and high rate.Specially,short reach transmission networks such as data center interconnection(DCI)and optical access network are facing multiple pressures such as rapid growth of capacity and deployment cost,and the next generation short reach optical communication system has become one of the current research hotspots.In this thesis,we focus on single carrier high speed optical communications and study digital signal processing(DSP)techniques in short-reach,high-speed and low-cost PAM transmission,including the clock recovery,equalization,FEC coding and probabilistic shaping modulation modules.The main contents and innovations of this dissertation are as follows.1.The overall architecture of PAM transmission system is firstly studied,and the different schemes of transmitter and receiver are compared Based on the architecture,we analyze the common signal impairments including clock offset,bandwidth limitation,dispersion,nonlinear effect and power fading effect.To degrade the impairments,the application status of DSP modules in PAM system including clock recovery,equalization,FEC encoding and probabilistic shaping modulation is investigated.The application scenarios of different clock recovery algorithms are analyzed,and the performance and complexity of various equalization technologies are compared.Meanwhile,the development trend of FEC coding schemes is summarized and the application of probabilitisic shaping modulation in PAM transmission is studied.Additionally,the problems of the existing modules are put forward.2.Based on tap weights in equalizer,an all digital joint clock recovery and channel equalization scheme(named CR-FFE)is proposed.A 10GHz directly modulated laser(DML)based 50-Gbit/s PAM4 transmission experiment is implemented.Experimental results show that the proposed CR-FFE algorithm can resist SCO up to 1000 ppm after 40-km standard single-mode fiber(SSMF)transmission,which is 50 times higher than the traditional schme.The algorithm simultaneously eliminate the inter-symbol interference(ISI)and track large sampling clock offset(SCO),thus solving the problem of incompatible prerequisites between clock recovery and channel equalization in PAM4 transmission.3.Based on multi-dimensional basis expansion,an enhanced hard decision method is proposed,and a 50-Gbit/s PAM4 system based on 10 GHz DML is experimentally demonstrated to investigate the proposed scheme.After 30-km transmission,with FFE equalizer,the enhanced hard decision can achieve reliable transmission with BER below 3.8×10-3 hard decision threshold compared to traditional hard decision;and with VNLE equalizer,the enhanced hard decision can realize the optical power budget gain of 0.6 dB compared to traditional hard decision.Meanwhile,compared with the traditional MLSE algorithm with memory depth of 5,the received optical power gain of 2.5 dB can be achieved by the proposed method under two-dimensional expansion.4.We analyze the performance of polar code in PAM transmission for the first time by demonstrating the transmission systems of 28-Gbaud PAM4 and PAM8 based on commercial 10 GHz DML over 10-km SSMF in C-band without dispersion compensation.By studying the probability density function of PAM signal after nonlinear transmission,an improved polar soft decision decoding via non-identical Gaussian distribution based log-likelihood ratio(LLR)estimation is proposed.The experimental results show that,compared with the traditional polar decoder based on identical Gaussian distributions,the improved decoder can achieve the additional optical power budgets of 0.7 dB and 1 dB for PAM4 and PAM8 systems,respectively.5.The CCDM based probabilistic amplitude shaping scheme and the many-to-one mapping based BICM-ID scheme are studied and analyzed,respectively.A low complexity BICM scheme based on many-to-one mapping without iteration is proposed for probability shaping coded modulation and a polar coded probabilistic shaping PAM8 system is demonstrated experimentally.An optimal clock recovery method for PS signal is also proposed to degrade the impact of severe eye skew effect.The experimental results show that shaping gains of 1.2 dB,0.8 dB and 0.4 dB are achieved after BTB,2-km and 10-km transmission,respectively.Finally,the BER performance and complexity of LDPC codes and polar codes with different code lengths are compared.When the code length is 1024,512,256,compared with LDPC code,the polar code can achieve sensitivity gain of 1 dB,1.4 dB and 2.2 dB and reduce the number of required multipliers by 35%,27%and 20%,respectively.The results prove the superiority of the proposed scheme in performance and computational complexity.