I am trying to implement an eye diagram for an application, where the input signal is QPSK. However, I feel that there is some fundamental concept concerning these plots that I am missing. Several definitions and descriptions for these diagrams that I have seen are all along the same line:
The eye diagram repeatedly overlays the time width of n symbols
Which sounds straight forward enough (though there are possibly variations?), but I'm not sure that is all there is to it.
The simulated input signal that I am testing with:
- modulated QPSK, generated from random symbols
- No raised cosine, RRC, or any filtering
- No noise added
Eventually, I will modify the signal (such as adding filtering) to see the effects on the system.
This is the image I get when overlaying the symbols in time (showing 1 symbol per trace):
Since I've never seen any example that looks like this, I tried looking at other variations. Using the eyediagram function in Octave, it produces the barn door (breaking the signal into the real and complex):
The points on the "door" are just the constellation points received (not the samples). So when I see examples like this, with a noisy signal:
I don't have enough reputation to post another image here, though it would help to explain my question. Similar to the above image: for a noisy signal, the lines of the diagram are fuzzy. It would indicate that there are more analog points used to fill in the plot
Where are the other points coming from to create the noise, if not the received waveform? Then there are the horizontal components of the image. How is it possible to get the horizontal components from a QPSK waveform? Even if separated into I and Q representations? Again, it makes sense when connecting the received constellation point at T intervals, but I do not see how to get this when plotting the signal itself.
What am I missing or not understanding here?
I updated the diagram to plot only the received symbols. Previously I was plotting the received waveform, which was a modulated signal (that is how the first image was produced). Below are two diagrams showing just the in-phase plot. The second one has noise added:
Without noise, the lines are straight as PSK mentioned in the comments. With noise, the lines are still straight. Which is the other part of the question. Looking at an example here:
(this is from Matlab example at https://www.mathworks.com/help/comm/gs/scatter-plot-and-eye-diagram-with-matlab-functions.html)
Where do the smooth transitions come from? The lines are not straight. There are other in-between points that are filling in the diagram. Where are they coming from?