Digital Analysis (QAM View OPT4): Constellation Mode
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9
Common Impairment Types
Figures 9-12 through 9-14 show constellation displays depicting three
common impairments – thermal (system) noise, phase noise, and coherent
interference. Gain compression and I/Q imbalance (not shown here) are
examples of two other impairment types with specific graphic signatures.
Thermal noise impairments cause the dots on the graph to spread out
toward the grid lines (decision boundaries), indicating significant errors (see
Fig. 9-12). The zoom feature would show more detail about the position of
these dots in relation to the grid lines.
Fig. 9-12 Thermal Noise Impairment
Phase noise impairments cause the dots on the graph to form a circular
pattern (see Fig. 9-13). Phase noise is best seen on the grid without the use
of the zoom feature.
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