Constellation Diagram - Interpretation

Interpretation

Upon reception of the signal, the demodulator examines the received symbol, which may have been corrupted by the channel or the receiver (e.g. additive white Gaussian noise, distortion, phase noise or interference). It selects, as its estimate of what was actually transmitted, that point on the constellation diagram which is closest (in a Euclidean distance sense) to that of the received symbol. Thus it will demodulate incorrectly if the corruption has caused the received symbol to move closer to another constellation point than the one transmitted.

This is maximum likelihood detection. The constellation diagram allows a straightforward visualization of this process — imagine the received symbol as an arbitrary point in the I-Q plane and then decide that the transmitted symbol is whichever constellation point is closest to it.

For the purpose of analyzing received signal quality, some types of corruption are very evident in the constellation diagram. For example:

• Gaussian noise shows as fuzzy constellation points
• Non-coherent single frequency interference shows as circular constellation points
• Phase noise shows as rotationally spreading constellation points
• Attenuation causes the corner points to move towards the center

A constellation diagram visualises phenomena similar to those an eye pattern does for one-dimensional signals. The eye pattern can be used to see timing jitter in one dimension of modulation.