Exploring and Experimenting With Shaping Designs for Next-Generation Optical Communications

A class of circular 64-QAM that combines “geometric” and “probabilistic” shaping aspects is presented. It is compared to square 64-QAM in back-to-back, single-channel, and wavelength division multiplexing (WDM) transmission experiments. First, for the linear additive white Gaussian noise channel model, it permits to operate close to the Shannon limits for a wide range of signal-to-noise ratios. Second, WDM simulations over several hundreds of kilometers show that the obtained signal-to-noise ratios are equivalent to–or slightly exceed–those of probabilistic shaped 64-QAM. Third, for real-life validation purpose, an experimental comparison with unshaped 64-QAM is performed where 28% distance gains are recorded when using 19 channels at 54.2 GBd. This again is in line–or slightly exceeds–the gains generally obtained with probabilistic shaping. Depending upon implementation requirements (core forward-error correcting scheme for example), the investigated modulation schemes may be key alternatives for next-generation optical systems.