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Digital Modulation: Quadrature Phase-Shift Keying (QPSK) Signal Constellation and Eye Diagrams

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raised cosineroll-off factor
0.5
IQ amplitudeimbalance
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IQ phase error
0
trajectory
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A digitally modulated signal is represented by its equivalent complex low-pass or base-band signal. The real and imaginary parts of this signal are called the quadrature components
x
i
(t)
and
x
q
(t)
. The figure shows the signal constellation in the complex plane and the eye diagrams of the quadrature components for QPSK (quadrature phase-shift keying). The eye diagram shows how the waveforms used to send multiple bits of data can potentially lead to errors in interpretation. You can change the roll-off factor of the base-band pulse-shaping filter to see the effect of an amplitude or phase imbalance of the modulator.
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