Design of APSK Constellations for Coherent Optical Channels with Nonlinear Phase Noise

TL;DR

This paper explores the design and optimization of APSK constellations for coherent optical communication systems affected by nonlinear phase noise, demonstrating significant performance improvements over traditional modulation formats.

Contribution

It introduces optimized APSK constellations tailored for nonlinear phase noise conditions and proposes a labeling method for rectangular APSK to enhance bit error rates.

Findings

APSK can outperform 16-QAM by 3.2 dB at SEP of 10^-2.

Optimized APSK constellations improve robustness against nonlinear phase noise.

A near-optimal labeling method for rectangular APSK is proposed.

Abstract

We study the design of amplitude phase-shift keying (APSK) constellations for a coherent fiber-optical communication system where nonlinear phase noise (NLPN) is the main system impairment. APSK constellations can be regarded as a union of phase-shift keying (PSK) signal sets with different amplitude levels. A practical two-stage (TS) detection scheme is analyzed, which performs close to optimal detection for high enough input power. We optimize APSK constellations with 4, 8, and 16 points in terms of symbol error probability (SEP) under TS detection for several combinations of input power and fiber length. Our results show that APSK is a promising modulation format in order to cope with NLPN. As an example, for 16 points, performance gains of 3.2 dB can be achieved at a SEP of 10^-2 compared to 16-QAM by choosing an optimized APSK constellation. We also demonstrate that in the presence of severe nonlinear distortions, it may become beneficial to sacrifice a constellation point or an entire constellation ring to reduce the average SEP. Finally, we discuss the problem of selecting a good binary labeling for the found constellations. For the class of rectangular APSK a labeling design method is proposed, resulting in near-optimal bit error probability.