Research On Synchronization Techniques Of Coherent Optical Orthogonal Frequency-Division Multiplexing System

As the backbone of the modern-day information infrastructure, high-speed optical fiber communication system is evolving to a higher stage with larger capacity and longer transmission distance. In recent years, the orthogonal frequency-division multiplexing (OFDM) has gained increasing concern in the optical fiber communication domain, since it offers the advantages of high electrical and optical spectral-efficiency, dispersion-insensitivity, and computation-efficiency. And there have been various research and experiments on OFDM in optical fiber transmission. Especially for the coherent optical OFDM, it implements the coherent detection in the receiver end, and performs superior in optical signal-to-noise ratio (OSNR) sensitivity. However, the local laser added in the receiver end for coherent detection makes CO-OFDM system more sensitive to the frequency offset to the transmit laser, and the phase noise caused by the laser Iinewidth. Meanwhile, due to the multicarrier nature, OFDM signals show extremely high peak-to average-power (PAPR). High PAPR characteristic results severe optical fiber nonlinearity impact on OFDM signals in the long distance transmission, and requires high resolution of the digital/analog converters. Thus, the frequency offset/phase noise sensitivity and high PAPR characteristic are two main disadvantages for CO-OFDM system. And how to implement more correct signal synchronizations and effective PAPR reduction become critical challenges. The main research work in this thesis is described as follows:1. OFDM principles are reviewed, including the introduction to OFDM basics and analysis of the PAPR characteristic and frequency offset/phase noise sensitivity for OFDM signals. In addition, principles and construction of the coherent detection are reviewed, and an overall scheme of CO-OFDM system is presented.2. Based on the principles of OFDM and coherent detection, a20Gbit/s QPSK CO-OFDM simulation system is constructed. And the end-to-end OFDM signal processing procedure is described.3. Repeated clipping and frequency domain filtering scheme for PAPR reduction is deeply thoroughly analyzed, with the measurements including the margin of PAPR reduction, power spectral density (PSD), signal-clipping noise ratio (SCNR), bit error rate (BER). Performance analysis prove that repeated clipping and filtering operation is an effective and simple way to reduce the PAPR of OFDM signals.4. Modified timing and frequency synchronization methods for CO-OFDM system are proposed. The modified timing synchronization method solves the ambiguity problem in Park’s method, and the modified frequency synchronization method only uses single training symbol but maintains equivalent accuracy compared to Schmidl’s method. The modified scheme is tested in the20Gb/s QPSK CO-OFDM simulation system, and the results show that it gives a more accurate timing estimation, and a more economical frequency offset estimation in wider frequency range. In addition, with the aid of CD compensation before the synchronization, the subcarrier recovery achieves better performance, making the OSNR tolerance3dB higher at the BER of10-3. Thus, the modified scheme is proved effective and economical.