Impact of optical channel distortions to digital timing recovery in digital coherent transmission systems

We investigate the systematic influence of optical polarization effects and chromatic dispersion (CD) on digital timing phase estimation in receivers with coherent demodulation. It is shown by analysis and by simulation that half-Baud differential group delay (DGD) in combination with certain states of polarization (SOP) attenuates the clock tone such that the timing phase can not be estimated. Furthermore, the fundamental limitation of CD to digital timing recovery is analyzed. Proposals for mitigation schemes with low complexity and robust timing phase estimation are discussed.