Power-Efficient Modulation Formats in Coherent Transmission Systems (original) (raw)

2000, Journal of Lightwave Technology

Coherent optical transmission systems have a fourdimensional signal space (two quadratures in two polarizations). These four dimensions can be used to create modulation formats that have a better power efficiency (higher sensitivity) than the conventional binary and quaternary phase-shift keying (BPSK/QPSK) signals. Several examples are given, with some emphasis on a 24-level format and an 8-level format, including descriptions of how they can be realized and expressions for their symbol and bit error probabilities. These formats are, respectively, an extension and a subset of the commonly used 16-level dual-polarization QPSK (DP-QPSK) format. Spherepacking simulations in 2, 3, and 4 dimensions, up to 32 levels, are used to verify their optimality. The numerical results as the number of levels increases are shown to agree with latticetheoretical results. Finally we point out that the use of these constellations will lead to improved fundamental sensitivity limits for optical communication systems, and they may also be relevant as a way of reducing power demands and/or nonlinear influence.