Study On Key Algorithms Of Digital Signal Processing In Coherent Optical Communication System

The coherent optical communication system has the advantages of high receiver sensitivity and high spectral efficiency. Recently, the coherent optical communication technology combined with advanced modulation formats has become one of the most promising technologies for next generation of high-speed optical communications technology. Furthermore the research and innovation in the digital signal processing in coherent optical receiver has attracted much attention from industry and research community.The thesis has discussed and analyzed three digital signal processing(DSP) algorithms at the receiving end in a typical single-carrier polarization division multiplexed(PDM) coherent optical fiber communication system. A simulation platform of a single-carrier high-speed PDM coherent optical communication system has been implemented. With the platform, n-order phase modulation(n-PSK) and n-order quadrature amplitude modulation(n-QAM) can be generated for the following DSP algorithms, including clock recovery, polarization demultiplexing and carrier phase recovery.In the module of clock recovery, the feedback digital clock synchronization scheme using the Gardner algorithm to estimate the timing error has been analyzed. The convergence curves and performance of the Gardner algorithm on different OSNRs in the communication system have been simulated. In the module of polarization demultiplexing, the algorithms including constant modulus algorithm(CMA), multi modulus algorithm(MMA) and cascaded multi modulus algorithm(CMMA) have been studied for comparing their system performance. Specifically, the advantages and disadvantages of these algorithms have been compared. Meanwhile, the performance of the convergence curves with key parameters has been plotted for analysis. Based on these results, how to select the appropriate parameters in the communication system for optimizing the algorithms’ performance has been provided. Finally, the BER curves of these algorithms have been simulated to evaluate their performance in the whole system. In the module of optical carrier phase recovery, three algorithms including Viterbi-Viterbi algorithm, QPSK partitioning algorithm and blind phase searching(BPS) algorithm have been investigated. The performance of these algorithms in the system has been tested. To evaluate the algorithms, the effects of phase noise on the optimal average window length of the Viterbi-Viterbi algorithm and the QPSK partitioning algorithm have been analyzed. Then, the performance of BPS algorithm under the influence of phase noise in 16 QAM modulation format system has been studied. At last, the OSNR penalties of BPS algorithm and QPSK partitioning algorithm in 16 QAM system have been calculated to evaluate their linewidth torlerance.