Constellation shaping is a digital signal modulation energy-efficiency enhancement scheme that improves on 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 the combinations are equally used. In the real world the transmission media (channel) is not uniformly distorting the combinations: some require lower energy and resist channel noise more than others.