What is constellation diagram? Show the constellation diagram of QPSK.

Constellation diagram: A constellation diagram is a representation of a signal modulated by a digital modulation scheme such as quadrature amplitude modulator or phase-shift keying.

Constellation diagram are basically graphical representation of the complex envelope of each possible signal. The x-axis represents the quadrature component of the complex envelope. The distance between signals on a constellation diagram relates to how different the modulation wave forms are and how well a receiver can differentiate between them when random noise is present.

Greater the distance, greater would be the ability of a particular modulation scheme (demodulator on the receiver side) to distinguish between the signals in case the signal gets contaminated due to different noise present in the channel.

Figure: Constellation diagram