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 WDM transmission experiments. First, for the linear AWGN 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.
@article{arxiv.1803.02206,
title = {Exploring and Experimenting with Shaping Designs for Next-Generation Optical Communications},
author = {Fanny Jardel and Tobias A. Eriksson and Cyril Méasson and Amirhossein Ghazisaeidi and Fred Buchali and Wilfried Idler and Joseph J. Boutros},
journal= {arXiv preprint arXiv:1803.02206},
year = {2018}
}