AT&T Research Labs is claiming a new distance record for 400Gbit/s signals and will present its results in an OFC/NFOEC paper next week.
AT&T says it managed to send 400Gbit/s signals across 12,000 km on a 100GHz-spaced grid. The company's own previous record, using a 50GHz grid, was more like 3,000 km, using a 50GHz grid.
The release doesn't give full details, because they'd like you to attend the paper presentation at 3 p.m. Tuesday, March 19, in Anaheim, Calif. But it does mention that the 400Gbit/s signals were built from subcarriers and that AT&T's technique involves tuning the modulation to match the available spectrum and/or to trade off spectral efficiency for reach.
Tunable modulation has been a persistent theme as the 100Gbit/s generation has matured. It's probably going to be commonplace in 400Gbit/s deployments.