QPSK Modulation

[RickWill]

Hello,

I ran the example "MT niFGen PSK Signal Generation.vi" in the ../examples/modulation/FGEN examples with my PXI-5422 AWG, however the waveform that was generated and loaded to the AWG does not look like what I would expect for a phase modulated signal (please see attached).

Does anyone know why?

Thanks,

[RickWill]

Hello,

It seems that increasing the symbol rate helps with the shape of the modulated signal.  It now looks more like  a QPSK modulated signal. The default value that came with the example was 100.0K, I changed it to 20M and it does look better I think, I might not have enough symbols per unit time in the previous picture. However I am not sure at which rate I should set. I am very new to this and just started learning QPSK mod/demod technique.

Thanks

[CanadaGuy]

Be careful with "textbook" definitions. QPSK is also known as 4-QAM...quadrature amplitude modulation. Both shapes you've shown are accurate, but it depends on the specification of the signal, as you have discovered. What you choose is dependent on what you are trying to achieve.

What is it you are trying to do?

[RickWill]

Hi Raceybe,

Thanks for replying.  One of my tasks is to send a digital pattern to a board that expects a QPSK signal.  The board will demodulate the QPSK signal and recover my digital pattern.  The expected QPSK signal should look like the modulated QPSK signal that I posted in the previous post.  I am trying to modify the example "MT niFGen PSK Signal Generation.vi"  that came with the Modulation Toolkit however as I have said the shape of the modulated QPSK signal keeps changing as I play with the symbol rate.  So are you saying that if I expect a QPSK signal that has only phase changes, then I need to play with the symbol rate until I got what I expected? Using the example "MT niFGen PSK Signal Generation.vi" with QPSK option, how does NI Modulation software decide when to do amplitude modulation, phase modulation or both?

Thank you for your help.

[CanadaGuy · Accepted solution]

Your "desired" text book signal has infinite bandwidth. You will never see a signal like that in the real world. Once a signal is filtered to finite bandwidth so that a real instrument can process it, you will no longer have a constant amplitude or envelope of your signal. Put another way, a QPSK signal (or 4 QAM) would ideally have a constant amplitude on the unit circle, but to transition between the four quadrants after filtering, you will have excursions and samples less than and greater than the unit circle.

That said, as you increase the symbol rate from a low value to a high value, the carrier spends less time per symbol (symbol rate up means symbol period down). That is what produces the different appearance, with harsher transitions and such. I would say that the VI is doing exactly as it should.

Take a look through here https://en.wikipedia.org/wiki/Phase-shift_keying and try to get more comfortable with the different terminologies. The key thing in my mind to take away is that the text book and reality differ greatly due to the difference in text book BW (often infinite) and real BW (very much finite).

[RickWill]

Hi Raceybe,

Thank you for your reply, I have another question:

I have a bit pattern that I need to send, when I use the NI "MT QPSK Generation" example and "MT Scope Demod" example, I was able to send the bit pattern and detect the same bit pattern in the captured waveform.

However, the requirement for our project is that I have to send a preamble signal with the same frequency as the IF frequency for the QPSK. So I use the AWG to play out the preamble and the QPSK signals together as a sequence and capture the whole thing with the digitizer. I then consider the whole captured signal as the modulated QPSK signal and demodulate it to detect the sent bit pattern.  However, I am not able to detect the bit pattern that I sent.  In theory, would this work? Should I be able to detect my bit pattern by this method?

Thank you,

Rick.