Realtime digital signal processing in coherent optical PDM-QPSK and PDM-16-QAM transmission

Coherent fiberoptic transmission with synchronous detection of 4, 8 or more bit/symbol enhances spectral efficiency. It relies on polarization-division multiplexing (PDM) and quadrature phase shift keying (QPSK) or higher-order quadrature amplitude modulation (QAM). A coherent polarization diversity, in-phase and quadrature receiver detects the optical field information. Its most important tasks are to recover the carrier in a laser phase noise tolerant manner and to control optical polarization electronically. We present suitable digital signal processing designs and their usage in realtime coherent transmission. We likewise discuss chromatic dispersion (CD) and polarization mode dispersion (PMD) equalization, needed for longer fibers.