The Research Of Bit Synchronization Based On Quadrature Phase Shift Keying Coherent Optical Transmission System

Digital optical coherent detection is a hot topic in high-speed opticaltransmission. With the help of digital signal processing, it has a higher receivingsensitivity and a better spectral efficiency. After photoelectric conversion, a clocksignal can be extracted from the received data signal, which always has a certainfrequency offset. We note that transmission signal distortion caused by clock skewand chromatic dispersion (CD) in coherent optical transmission systems. After CDcompensation filter, there is still a certain degree of residual CD. The inaccurate BitSynchronization induced by CD and clock noise reduces the performance of thedigital receiver system. To resist CD and time jitter, bit synchronization should bework properly with a certain degree of CD and clock noise, and should be veryaccurate to find the beginning and end of each symbol in digital communicationsystems.Through study of bit synchronization technology in QPSK coherent opticalcommunication systems, the main achievements of the paper conclude as follows:1、A modified Gardner phase detector for coherent optical QPSK signal. Theproposed scheme, utilizing the features of the signal and Inter-Symbol interference(ISI), also combining the information both time and frequency domain of the signal,relieves the impairments to the timing recovery caused by CD and noise. A40Gb/spolarization division multiplexed DQPSK (PM-DQPSK) signal transmission andcoherent detection digital communication system is experimentally demonstrated. Theresult shows that the scheme is efficient in resisting CD and has a good performanceon time jitter.2、A joint scheme which embeds an adaptive equalizer into the all-digital timingrecovery loop is proposed to a high dispersion tolerance system. The proposed schemeutilizes the low CD and interference features of the signal after CD compensationfilter and combine the information both the QPSK signal and frequency domain of thesignal phase. The scheme combines the adaptive equalizer with the full digitalsynchronization effectively. It uses the advantages of the adaptive equalizereffectively reduce the fiber dispersion effect to make up for the deficiency of lowdispersion tolerance in bit synchronization. The result shows that the scheme isefficient in resisting CD and has a good performance on signal transport and high dispersion tolerance.