Journal
JOURNAL OF LIGHTWAVE TECHNOLOGY
Volume 27, Issue 22, Pages 5115-5126Publisher
IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
DOI: 10.1109/JLT.2009.2029064
Keywords
AWGN; dual polarization quadrature phase shift keying (QPSK); four-dimensional (4-D) constellations; lattices; digital modulation; optical communications; polarization-switched QPSK; power efficiency; sensitivity; simplex; spectral efficiency; sphere packing
Funding
- Swedish Governmental Agency for Innovation Systems (VINNOVA)
- Swedish Foundation for Strategic Research
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Coherent optical transmission systems have a four-dimensional (4-D) 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 phase shift keying/quadrature 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 format. Sphere packing 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 lattice-theoretical 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.
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