Coded-Aided Phase Tracking for Coherent Fiber Channels

Abstract

In coherent optical systems laser phase noise can interact with digital equalization to cause equalization-enhanced impairments, which are a major obstacle for applying higher order modulation formats in coherent optical systems with digital chromatic dispersion compensation. In this paper a code-aided expectation maximization method to track phase noise in such systems is presented. A common measure of laser phase noise is the linewidth. It is shown that with ~ 11% redundancy, the laser linewidth tolerance for 975 km transmission distance can be increased by 50%, or the system reach for a laser linewidth of 5 MHz can be doubled. A phase-noise-robust 16-point 4-4-4-4 ring constellation was found to have better performance compared to 16QAM and a 2-6-8 ring constellations. Performance can be further improved with a lower code rate and fewer pilot symbols. It is also shown that algorithmic complexity can be reduced without significant reduction in the performance by reducing iterations and using low complexity codes.