Research On Frequency Offset Estimation DSP Algorithms In Optical Coherent Communication

As the rapid growth of capacity demand in optical networks, spectrally efficient quadrature amplitude modulation(QAM) formats combined with coherent receiver and digital signal processing(DSP) have attracted significant attention. The DSP algorithms to compensate distortions in coherent optical systems are also research focus area for their flexibility. And extensive studies are devoted to reduce the computational complexity and improve the performance of DSP algorithms.First, the recent development of coherent optical communication techniques are introduced in this thesis. Then, the principle of coherent detection and linear distortions of signal including chromatic dispersion(CD), polarization mode dispersion(PMD), IQ inbalance, frequency offset, and phase noise are discussed in detail. The corresponding compensation algorithms are also investigated. The key point of this thesis is the frequency offset estimation(FOE) algorithms in carrier recovery. Carrier recovery algorithms include FOE and carrier phase estimation(CPE). The frequency offset and phased noise are induced by the central frequency offset between the transmitter laser and local oscillator laser and the effect of “random walk” from the laser sources, respectively. The constellation will rotated because of the existence of frequency offset and phase noise, which will degrade the performance of system. Thus, it is important to design effective FOE and CPE algorithms. This thesis has investigated the present FOE algorithms, and simulations are conducted to analyze them. Then, a low complexity FFT-based FOE algorithm is proposed, which is applied in the second stage of two-stage FFT FOE. When compared with previous gradient descent(GD) and chirp-z transform(CZT) methods, the computational complexity of the proposed algorithms is lower by a factor of more than 4 with the same performance. The proposed algorithm has the fixed computation scheme and excellent compatibility, which are suitable for practical application.