Carrierless amplitude phase modulation
Carrierless amplitude phase modulation (CAP) is a variant of quadrature amplitude modulation (QAM). Instead of modulating the amplitude of two carrier waves, CAP generates a QAM signal by combining two PAM signals filtered through two filters designed so that their impulse responses form a Hilbert pair.
Source: Wikipedia — Carrierless amplitude phase modulation (CC BY-SA 4.0)