400 Gbit/s 256 QAM-OFDM transmission over 720 km with a 14 bit/s/Hz spectral efficiency using an improved FDE technique

We demonstrate 400 Gbit/s PDM-256 QAM-OFDM transmission with 14 bit/s/Hz spectral efficiency by employing an improved FDE technique that extended the transmission distance to 720 km and achieved an SE-distance product of 10,080 km·bit/s/Hz.