Four-dimensional coded modulation: 6PolSK-QPSK

In this contribution we compare the performance of polarization-division multiplexed quadrature phase-shift keying (PDM-QPSK) signals and 6-ary polarization-shift keying (6PolSK)-QPSK signals in wavelength-division multiplexing (WDM) experiments over up to 5280 km G.652 standard single-mode fiber (SSMF). Both modulation formats are compared at the same symbol rate of 28 GBd. Due to its higher spectral efficiency 6PolSK-QPSK then transmits 126 Gb/s compared to 112 Gb/s when using PDM-QPSK. This enables application of stronger forward error correction (FEC) codes with larger overhead. The achievable gain in maximum reach by application of an additional inner Reed-Solomon FEC code is quantified in transmission experiments. Furthermore, the interaction between FEC and digital compensation of intra-channel nonlinear effects is discussed.