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Normally, we would use a GNURadio "constellation modulator" to simulate QPSK as is done in the excellent [[Guided_Tutorial_PSK_Demodulation|Guided Tutorial on PSK Demodulation]].  
 
Normally, we would use a GNURadio "constellation modulator" to simulate QPSK as is done in the excellent [[Guided_Tutorial_PSK_Demodulation|Guided Tutorial on PSK Demodulation]].  
  
[[File:IQ_complex_tutorial_QPSK.png|thumb|600px|QPSK modulator]]
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[[File:IQ_tutorial_QPSK.png|thumb|600px|QPSK modulator]]
 
For the present tutorial we will simulate a QPSK without Nyquist filter in order to get phase states which can be simply displayed on a constellation sink. This is not possible with constellation modulator. Our QPSK modulator (complex representation) is build taking into account that the complex signal exhibit 4 different values, its obvious that both ''i(t)'' and ''q(t)'' have only 2 states so they are binary symmetric NRZ line codes:
 
For the present tutorial we will simulate a QPSK without Nyquist filter in order to get phase states which can be simply displayed on a constellation sink. This is not possible with constellation modulator. Our QPSK modulator (complex representation) is build taking into account that the complex signal exhibit 4 different values, its obvious that both ''i(t)'' and ''q(t)'' have only 2 states so they are binary symmetric NRZ line codes:
 
: <math>i(t), q(t) \in \{+1, -1\}</math>
 
: <math>i(t), q(t) \in \{+1, -1\}</math>
  
Flowgraph [[Media:IQ_tutorial_QPSK.grc|IQ_tutorial_QPSK.grc]] generates 2 sequences of bits, interpolates them to get 2 binary symmetric NRZ line codes. The NRZ signals are combined to create the complex equivalent baseband signal of the QPSK which can be transmitted to any SDR emitter.  
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Flowgraph [[Media:IQ_tutorial_QPSK.grc]] generates 2 sequences of bits, interpolates them to get 2 binary symmetric NRZ line codes. The NRZ signals are combined to create the complex equivalent baseband signal of the QPSK which can be transmitted to any SDR emitter.  
  
 
Simulate this flowgraph :  
 
Simulate this flowgraph :  

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