What is described by the constellation diagram of a QAM or QPSK signal?

A constellation diagram is a two-dimensional graph used to visualize digital modulation schemes like QAM and QPSK. The coordinate plane plots the In-phase (I) and Quadrature (Q) components of a signal, allowing engineers to see all the valid states of the signal at a glance. Each dot on this diagram represents a specific symbol. The distance from the center of the graph dictates the amplitude of the signal, and the angle from the horizontal axis dictates the phase. It does not display frequency domain information, multiple carriers, or error correction data.