What is the signal constellation used to represent?

The constellation depends on the quality of digital modulation. Different debugging methods and constellations are different, which are usually represented by points in similar and rectangular coordinate systems.

For example, BPSK is a rectangular coordinate system with two points in quadrant 1-3. QPSK is a point in four quadrants. QAM is more.

It is more direct and clear to use constellation diagram to represent modulation mode than mathematical expression. Many instruments also support constellation testing and display.