Thursday 8 March 2012

Constellation diagram interpretation


The constellation diagram provides graphical representation of the complex envelope of each possible symbol state.

The X-axis of the diagram is called in-phase component and the y-axis represents the quadrature component.

 The distance between signals on constellation diagram relates to how different the modulation waveforms are and how well the receiver can differentiate between all possible symbols when random noise is present.
 
Some of properties of the modulation scheme can be inferred from the constellation diagram:

BW occupied by the modulation signals decreases as no. of points increases : i.e. if modulation scheme has a densely packed constellation it would be more bandwidth efficient.

Pe is proportional to the distance between the closest points in constellation : densely packed modulation scheme is less energy efficient than the modulation scheme that has sparse constellation

Comparison between constellation diagram interpretation on power and BW efficiencies. 
Probability of error and constellation diagram:
The constellation diagram can also be employed to find the upper bound for symbol error rate in AWGN channel with PSD=No
Is



And dij is Euclidean distance between ith and the jth points.


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