Study On Techniques Of Mitigating Signal Impairments In Coherent Optical Transmission System

In recent years,with the advent of services such as cloud services,cloud computing,Internet of things and big data,and so on,traditional intensity-modulation/direct-detection(IM/DD)optical fiber communication systems have not been able to meet such a large amount of traffic demand.Coherent optical communication system based on digital signal processing(DSP)enables to employ high-order modulation formats with high spectral efficiency such as M-ary phase-shift keying(M-PSK)and M-ary quadrature-amplitude modulation(M-QAM),which can improve single channel transmission rate and achieve long unrepeated transmission,so it can meet the increasing traffic demand.In coherent optical communication systems,the bandwidth limitations of optoelectronic devices,the I/Q phase skew of modulator and the fiber nonlinearity effects,and so on,can cause serious signal impairments,especially high-order modulation signals,the received signal is distorted.Based on this,this paper mainly compensates for these signal impairments by DSP algorithm to improve the performance of coherent optical communication system.This paper mainly focus on two aspects to compensate these signal impairments.First,we utilized transmitter DSP algorithms to preprocess the signal for compensating the bandwidth limitation of optoelectronic devices.Second,at the DSP unit of receiver,SVM algorithm is utilized to find the optimal decision boundary for improving the system bit error rate(BER)performance.Due to the bandwidth limitation of optoelectronic devices,the received signal will be distorted,which will lead to system performance degradation.This paper simply introduced the transmitter DSP algorithms(including root-raised cosine filter and S21 parameter compensation method)are employed to compensate the signal impairments caused by the bandwidth limitation of the arbitrary waveform generator(AWG)and the modulator.The performance of single-polarization 25 Gb/s and 50 Gb/s QPSK signals before and after compensation was experimentally tested,respectively.By observing the constellation diagram,Q value and error vector magnitude(EVM),the signal performance was greatly improved.Then,the paper introduced the clock recovery and carrier phase recovery algorithms of the receiver.Clock recovery including Gardner timing recovery and the improved Gardner timing recovery algorithm,carrier phase estimation including Mth-power carrier phase recovery algorithm,a cascaded FIR filter phase recovery algorithm and data-aided maximum likelihood estimation algorithm.In addition,because higher-order modulation signals are more vulnerable to system noise,this paper finally focus on signal impairments mitigation in 64-QAM coherent optical communication systems using SVM algorithm to create optimal decision boundary.The effects of various impairments such as modulator nonlinearity,I/Q phase skew of modulator,optical fiber Kerr nonlinearity,and amplified spontaneous emission(ASE)noise on 64-QAM coherent optical communication systems have been experimentally studied.The BER of 75-Gb/s 64-QAM signal in back-to-back(BTB)and 50-km single-mode fiber(SMF)transmissions was measured,respectively.By using the SVM algorithm to optimize symbol decision boundaries,signal impairments are greatly mitigated and the system BER performance is improved.