1
$\begingroup$

I was wondering how a diagram like this is generated.

enter image description here

I am generating a QPSK signal using MATLAB by taking a cos and sin wave, multiplying each one by either 1 or -1 and adding them together to form a real valued QPSK signal. I then simulate an AWGN channel and send it to my demodulator. I mix it down using another cos and sin wave plus Low Pass Filter to get my recovered I and Q data. At this point I am left with the baseband which I can recover my data from by taking the angle of the I and Q components.

I am wondering at what point in that chain I could generate a the plot I post above.

Thank you

| improve this question | |
$\endgroup$
  • 1
    $\begingroup$ It looks like a QPSK signal that is still at ~2 samples per symbol. If you have a part of your pipeline where you are sampled at around that rate, plot the imaginary part versus the real part and you should get a diagram like the above. $\endgroup$ – Jason R May 20 '16 at 18:07
0
$\begingroup$

This is what I do when making such a plot for a QPSK modulation:

First I upsample the I and Q data so that I can see more of the trajectory between samples given (assuming you start with at least 2 samples per symbol otherwise, due to Nyquist, the information on the trajectory between symbols is not available, but as long as you have 2 or more samples, all the information of what happens in the first Nyquist zone is preserved):

Is = resample(I, 10, 1);

Qs = resample(Q, 10, 1);

and then I make a plot of I vs Q 

plot(Is,Qs)

I used a resampling increase of 10, but you could make it higher if desired. The result is the interpolated samples for the full trajectory during symbol transitions.

| improve this answer | |
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.