Timing recovery in nyquist coherent optical systems

Timing recovery in coherent optical systems is a critical issue since residual chromatic dispersion (CD) and polarization mode dispersion (PMD) aggravate the clock tone extraction. The problem becomes more serious in case of a very dense channel packaging using Nyquist pulses. In this paper we discuss the timing problem in Nyquist optical systems and propose an algorithm that can work even with a roll-off-factor (ROF) equal to zero. The algorithm provides almost the same clock tone performance for all modulation formats that are considered to be used in coherent optical transmission systems.