Constellation shaping is an energy efficiency enhancement scheme for digital signal modulation that improves upon the Amplitude and phase-shift keying (APSK) and the conventional Quadrature amplitude modulation (QAM)) modulation schemes by transmitting low-energy signals more frequently than high-energy ones.
"Constellation" is a pattern of the possible signal combinations; in a static constellation, all of the combinations are used equally. In the real world, however, the transmission media (channel) distorts the signal unevenly; some combinations require lower energy and resist channel noise more than others.
A "shaped constellation" transmission sends some signal combinations more often and others less frequently to optimize the signal quality at the destination or to maintain the same quality using less transmission energy.
Probabilistic constellation shaping
Probabilistic (and adaptive) constellation shaping changes the shaping parameters based on predefined terms. This allows a significant capacity increase (e.g. 15-43% on 16-QAM channel). This technique gathered more interest in September 2016 when Nokia Bell Labs demonstrated working 1 Tbit/s data transmission channels between German cities. In October 2016, Alcatel-Lucent and Nokia Bell Labs claimed to have achieved 65 Tbit/s transmission over a 6,600 km (4,100 mile) single mode fiber in laboratory trials.