Research On Key Techniques Of Digital Signal Processing In Kramers-Kronig Coherent Optical Transmission Systems

With the rapid development of new generation mobile communication,cloud computing,the Internet of things(IoT)and other emerging technologies,high-speed and high-capacity optical transmission technology continues to innovate.However,the explosive growth of data traffic increases the cost and power consumption of the physical layer of the optical transmission network.To further increase the optical transmission distance and rate,as well as meet the needs of low cost,low power consumption and high-performance,the complex modulation direct detection self-coherent optical transmission technology represented by Kramers-Kronig(KK)receiver has been proposed in recent years.KK receiver is able to extract the phase information from the detected intensity digitally for the complex full-field reconstruction,which breaks through the limitation that the traditional direct detection technology can only obtain the intensity information.KK receiver has become an attractive receiving technonlgy because it allows the use of advanced digital signal processing(DSP)techniques,and has the characteristics of high spectral efficiency and low receiver complexity.This dissertation focuses on the KK coherent optical transmission system and the advanced DSP techniques,theoretical investigation and experimental demonstration have been performed.The main work is as follows:1.Research on the transmitted and received theory in the KramersKronig coherent optical transmission systemAiming at the Kramers-Kronig(KK)coherent optical transmission system,the advanced digital signal processing techniques,numerous signal distortions as well as the mechanism and implementation of the various optoelectronic devices in the transmitter-side and receiver-side are elaborated and discussed in detail.Meanwhile,the technical problem of the extra digital upsampling operation caused by the nonlinear functionsinduced spectrum broadening in the KK complex field reconstruction is investigated.Besides,the principle and implementation of the without digital upsampling KK(WDU-KK)receiver is introduced and analyzed,the performance of the above KK receivers is investigated and compared.2.Research on the parallelized Kramers-Kronig reception technique based on the joint overlap approach and bandwidth compensationTo investigate the practical engineering feasibility of the KK coherent optical transmission system,the parallelization of the KK receiver is validated.Firstly,the hardware implementation and computational complexity of the KK receiver and WDU-KK receiver are introduced.Then,in parallelized KK receiver,the time-domain edge effect caused by the convolution operation and the frequency-domain Gibbs phenomenon induced by the limited tap number Hilbert transform finite impulse response(HT-FIR)filter.Based on the problems mentioned above,a joint overlap approach and bandwidth compensation(OLA-BC)architecture is proposed,and the spectrum and the waveform of the signal under the edge effect and the Gibbs phenomenon are analyzed and demonstrated through numerical simulation.Moreover,a high-rate optical transmission system with a fiber length up to 1440-km is conducted to experimentally investigated the performance of the proposed OLA-BC scheme.The transmission result shows that the presented OLA-BC architecture can effectively eliminate the edge effect and the Gibbs phenomenon,and successfully transmit the 16-QAM signal over 960-km standard singlemode fiber(SSMF)with a BER lower than the 7%hard-decision forward error correction(HD-FEC)threshold and 1440-km SSMF with a BER lower than 20%soft-decision FEC(SD-FEC)threshold.It is the first time that the KK receiver is parallelized implemented and achieves the longest transmission distance of KK reception-based 100-G data rate.This work can be treated as a significant reference for the engineering of the KK receiver in the future.3.Research on the performance of the Hilbert phase retrieval FIR filter in the parallelized Kramers-Kronig receiverIn the KK receiver,the performance of the practical implementation of the Hilbert FIR filter is extremely important,as the in-band flatness,the ripple,the group delay,the Gibbs phenomenon,and the edge effect,which indeed impair the phase retrieval.For the first time,the performance of four different linear-phase HT-FIR filter schemes used for KK phase retrieval that are in the form of the type Ⅲ and the type IV based on the frequency-domain(FD)sampling approach and the time-domain(TD)windowing function approach are investigated and compared.By analyzing the performance of four HT-FIR filters under different upsampling rate scenarios,it is concluded for the first time that a trade-off between the reduced inter-symbol-interference(ISI)and the Gibbs phenomenon is essential to obtain an optimal sampling rate.Then,the performance of four HT-FIR filter schemes is investigated in a singlesideband(SSB)16-QAM optical transmission system experiment.The experimental result shows that a 1440-km standard single-mode fiber transmission can be realized when the frequency-domain sampling approach-based Type Ⅲ HT-FIR filter with only 3 samples per symbol(3SPS)upsampling rate and 8 non-integers tap coefficients is used,and the BER can be lower than the SD-FEC threshold.4.Research on the performance of the practical Kramers-Kronig phase retrieval FIR filter with the Gibbs phenomenonWhen the HT-FIR filter is used for KK phase retrieval,the performance of the KK system is strongly impaired due to the Gibbs phenomenon,by the roll-off factor of the shaping filter,frequency gap between the direct-current(DC)component and one edge of the signal spectrum,carrier-to-signal power ratio(CSPR),tap number of the HT-FIR filter,and the digital upsampling rate.In the numerical simulation and fiber transmission experiment,we have systematically investigated the practical KK phase retrieval performance considering the abovementioned five aspects under the Gibbs phenomenon,and the optimal digital upsampling rate is performed under different scenarios.Based on the above research results,an important conclusion is obtained for the first time,when the HTFIR filter is used for the KK phase retrieval,the performance of the KK system cannot be further enhanced only by employing a higher digital upsampling rate,but there is an optimal digital upsampling rate.This part provides a sufficient and effective theoretical basis and experimental reference to the subsequent real-time KK optical transmission system.