Research On Compensation And Mitigation Of Nonlinear Impairments In Fiber Transmission Systems

Optical fiber transmission systems have been widely used in the fields including fiber-guided weapons,communications,industrial control,radar,light-controlled flight,and remote monitoring.However,as the capacity and distance increases,Kerr nonlinearity becomes more and more serious,and currently being the major limiting factor of transmission performance.Therefore,research on compensation and mitigation of the nonlinear impairments induced by Kerr effect,is a very important topic in next generation high-speed,large-capacity and flexible-grid fiber transmission system.Fiber transmission system includes single-carrier and multi-carrier system.The two most widely used single-carrier modulations are quadrature phase shift keying(QPSK)and 16quadrature amplitude modulation(16QAM).QPSK is adopted in 100 Gb/s commercial transmission systems.Due to the high spectral efficiency(SE),16QAM is applied to the next generation 200/400 Gb/s systems,while suffering a more serious nonlinearity than QPSK.Orthogonal frequency division multiplexing modulation(OFDM)can configure each sub-carrier and provide flexible data rate for different application scenarios,thus is considered in next agile transmission systems.However,OFDM suffers high Peak to Average Power Ratio(PAPR)and severely nonlinear effects,also its transmission performance is limited by the fading effect induced by the bandwidth-limited components.This dissertation mainly focuses on the compensation and mitigation technologies for nonlinear impairments in high-speed,large-capacity and flexible-grid transmission systems.And the following three aspects were investigated:First,for single-carrier QPSK system,traditional phase conjugate twinwave(PCTW)can mitigate the nonlinearity effect well,but sacrificing half SE,this paper will investigate how to improve the SE;Second,for single-carrier16QAM system,this paper will investigate how to improve the nonlinear tolerance to mitigate nonlinearity.Third,for multi-carrier OFDM system,this paper will investigate how to improve the PAPR performance to reduce nonlinear impairment.The main contributions of this thesis are listed as follows:1.Research on the issue of half SE reduction in conventional PCTW1)Modified PCTW(M-PCTW)was proposed.It was shown that by modulating one of the conjugated signals E_x~*with additional bits E_a,E_x can still mitigate the nonlinear impariments in a similar manner as in the conventional PCTW method,while the additional bits E_a,representing the improved SE,is the dominant source limiting the system performance.Then Forward Error Correction coding was applied to E_a to overcome this performance bottleneck.The overall system performance can thus approach that of the conventional PCTW method while the SE is significantly enhanced.Simulations of a 25-Gbaud PDM-QPSK system over15,200 km show that the proposed scheme can increase the normalized SE from 50%to 80%for a similar performance as the conventional PCTW method.The proposed concept can also be applied to higher-level formats.2)We improve the encoding/decoding of PS-QPSK to enhance the performance and proposed Improved PS-QPSK.We show that QPSK-based modified phase-conjugated twin wave is similar to the improved PS-QPSK but provides additional gain via fiber nonlinearity mitigation.2.Research on fiber nonlinearity mitigation for single-carrier 16QAM systems:1)A constellation selection strategy for multidimensional modulation format was proposed.It improves nonlinearity tolerance by optimizing the probability of constellation distribution.Based on the selection strategy,combined with the four dimension 128-amplitude modulation format(4D-128QAM),a 4 dimension 64-amplitude modulation format(4D-64QAM)with a SE of 3 bit/s/Hz and a 8 dimension 8192-amplitude modulation format(8D-8192QAM)with a SE of 3.25 bit/s/Hz are proposed.4D-64QAM shows a much better nonlinearity transmission performance than PDM-8QAM and thus is a very competitive alternative modulation format for PDM-8QAM.8D-8192QAM balances SE and nonlinear tolerance well.In point to point transmission system,this format has a similar transmission performance with PDM-8QAM;in the ROADM-enabled WDM system,compared to 4D-64QAM?PDM-8QAM and 4D-128QAM,this format can pass through more add/drop nodes and shows better transmission performance.2)We verify sysmmetric dispersion map can benefit the ROADM-enabled transmission and mitigation the nonlinear impariments.3.Research on the bandwidth limitation effect and high PAPR of multi-carrier OFDM system1)We propose a DFnT-S-OFDM scheme which spreads information over both temporal and frequency dimensions.Simulations show that DFnT-S-OFDM is more resilient against the frequency fading than OFDM and against the residual ISI than Discrete Fourier Transform spread Orthogonal Frequency Division Multiplexing modulation(DFT-S-OFDM).Therefore,it maximizes the net data rate by reducing the CP length or increasing the useful signal bandwidth.2)We introduce a new Sparse Orthogonal Cyclic Transform(S-OCT)matrix,and based on this matrix,we proposed a low complexity Sparse Orthogonal Cyclic Transform Multiplexing modulation technique(S-OCTM),It is theoretically proved that the technique can equalize the frequency selective fading.The technique reduces the superposition of the subcarrier due to the sparse characteristic of the S-OCT matrix,and thus it shows a greatly reduced PAPR and higher tolerance to fiber nonlinearity than OFDM and OCDM.Compared the DFT-S-OFDM,S-OCTM shows a better dispersion tolerance under insufficient length of cyclic prefix and is more tolerable to strong optical filtering.Therefore,S-OCTM is robust to both linear(the bandwidth limitation effect and ISI)and nonlinear impairment(PAPR and fiber nonlinearity),and thus is a promising multicarrier solution for optical transmission.