Contents 1. Introduction
The three-dimensional (3D) simplex modulation format is one of the promising formats for use in ultralong-haul transmission systems because it offers better receiver sensitivity than that with dual-polarization binary phase-shift keying (DP-BPSK) at a given bit rate and spectral bandwidth [1], [2]. In this 2-bit/symbol format, four symbol points are placed at each of the four apexes of a 3D simplex (tetrahedron) in the 3D space spanned by three out of four orthogonal axes of optical modulation, namely, in-phase (I) and quadrature (Q) axes of the two orthogonal polarizations. Since squared inter-symbol Euclidian distance divided by the average optical power in the 3D simplex format is 4/3 times larger than that of DP-BPSK, this format offers an asymptotic sensitivity gain of 1.2 dB over DP-BPSK, although this gain is reduced to ~0.7 dB at a high bit error rate (BER) of 10−3 because Gray coding is not possible. Experimental results with 3D simplex modulation have been reported in [2], where the signal was generated by driving three out of four Mach-Zehnder sub-modulators (sub-MZMs) in a standard dual-polarization quadrature phase-shift keying (DP-QPSK) modulator with three interdependent binary data signals.
