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Quadrature phase-shift keying (QPSK) modulator varies the signal's phase
to transmit information.
At first let us appoint the in-phase component of the carrier as phase reference.
In-phase component of the carrier has phase 0°.
Each QPSK symbol assigns a specific phase shift to the carrier wave.
Phase shifts of the four constellation points: 45°, 135°, 225°, and 315°
Start
Now let's generate a QPSK signal from a bit sequence.
- Two bits are mapped to a symbol.
- The symbol specifies a phase shift that is imposed on the carrier. For the symbol duration the phase shifted carrier signal is transmitted.
QPSK signal generation
Exercise
Now it's your turn! Work out the transmit signal for this bit sequence.
 Bit sequence 1101001110
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Transmit signal for this bit sequence.
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Next steps
- Switch to random bit sequence and see the generated QPSK signal.
- Note that for the signals shown above the carrier frequency has been selected in a way that it fits nicely to the symbol duration. Now change the carrier frequency to any arbitrary value.
 Bit sequence 0010001010
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The QPSK signal is generated by phase-shifting the carrier.
This applies for any carrier frequency.
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All symbol transitions are possible.
I/Q Signals of quadrature modulation.
Adding in-phase (blue) and quadrature (red) components yields the transmit-signal (black).
1. The bit sequence on the left shall be QPSK modulated. Assign the corresponding transmission signal on the right per drag & drop!
The figure below is the in-phase component of the carrier.
