Research On Joint Laser Phase Noise And Frequency Offset Mitigation Method In Coherent Optical Communication System

With the wide deployment of high bandwidth applications such as cloud computation, mobile internet etc., the network data traffic and bandwidth requirements are increasing rapidly, brings new challenges for fiber optical communication systems. Currently, adopting the advanced modulation and multiplexing techniques, as well as the digital signal processing based coherent optical detection have been the very important method for increasing capacity and bitrate of the fiber optical communication systems. Coherent Optical Orthogonal Frequency Division Multiplexing (CO-OFDM) has becoming one of the hot topics in the optical transmission areas. However, as the CO-OFDM systems are very sensitive to laser phase noise and frequency offsets between local oscillator and transmitter laser, which highly impacts the correctness of the received high speed signals. Therefore, it is of a great importance to improve the CO-OFDM performance by developing an effective laser phase noise and frequency offsets compensation method. This thesis focuses on the study of laser phase noise and frequency offsets compensation for coherent optical systems, the main accomplished work and innovations are as follows:(1) The impacts of laser frequency offset and phase noise in both single-carrier and OFDM based coherent optical receivers are analyzed, as well as their compensation mechanisms;(2) Targeting on the problem of phase noise of CO-OFDM systems introduced by laser linewidth, a Multi-Carrier Differential Detection (MCDD) based phase noise compensation scheme is proposed. Simulation results show that comparing to traditional Common Phase Error (CPE) based compensation method, the MCDD based scheme can achieve better performance in the laser phase noise conditions;(3) A joint laser phase noise and frequency offset compensation scheme based on the cooperation of MCDD and Modulus-Preserving Differential Precode (MP-DP) is proposed for CO-OFDM systems. Compared to the traditional schemes based on training symbol and pilots, the proposed scheme has the advantage of low overhead and complexity. Simulation results prove the effectiveness of simultaneous laser frequency offset and phase noise compensation for CO-OFDM systems.