What is constellation diagram? Draw constellation diagram for ASK, BPSK, QPSK.

Question : What is constellation diagram? Draw constellation diagram for ASK,
BPSK, QPSK.

Ans:   

Constellation diagrams are 2D graphical representations of digitally modulated signals.

A constellation diagram can help us define the amplitude and phase of a signal element,
particularly when we are using two carriers (one in-phase and one quadrature).

The diagram is useful when we are dealing with multilevel ASK, PSK, or QAM (see next

section). 

In a constellation diagram, a signal element type is represented as a dot. The bit or combination
of bits it can carry is often written next to it.

The diagram has two axes. The horizontal X axis is related to the in-phase carrier; the vertical
Y axis is related to the quadrature carrier. 

For each point on the diagram, four pieces of information can be deduced. 

The projection of a point on the X axis defines the peak amplitude of the in-phase
component; the projection of a point on the Y axis defines the peak amplitude of the
quadrature component. 

The length of the line (vector) that connects the point to the origin is the peak amplitude
of the signal element (combination of the X and Y components); 

The angle the line makes with the X axis is the phase of the signal element.

For ASK, we are using only an in-phase carrier. Therefore, the two points should be on the X axis.

Binary 0 has an amplitude of 0 V; binary 1 has an amplitude of 1 V. 

BPSK also uses only an in-phase carrier.

BPSK creates two different signal elements, one with amplitude 1 V and in phase and the other
with amplitude 1 V and 180° out of phase.

QPSK uses two carriers, one in-phase and quadrature.

11 is represented by (1,1) and phase of 45

01 is represented by (-1,1) and phase of 135

00 is represented by (-1,-1) and phase of -135

10 is represented by (1,-1) and phase of -45

The same way QAM can be represented by  different values as mentioned below.