How exactly do I plot a constellation Diagram? I am relatively new to signal processing and I would like to correctly draw a constellation diagram to check the results of a study. I need my signal processing to be as close as them to make sure I am doing it correctly.
I am trying to use constellation diagrams to fingerprint ADS-B signals which use a PPM (pulse position modulation) scheme. Considering I got some IQ data, it is sufficient to do a scatter plot of the IQ data to call it a constellation diagram ? Or do I need to collect only samples from IQ data that are in special positions(like the position of the pulses)? I am also asking this question because I do not get exactly the plot that the study shows for constellation diagram. I would be very thankful if someone can point why there is a difference or something I missed/misunderstood.
Here is the link of the study
You can find the plots on page 4, FIGURE 8. (What they call contour stellar image is just a constellation diagram but with colors based on density of points nearby from what I understood (FIGURE 7)).
And Using the following python code:
from scipy.stats import gaussian_kde
import matplotlib.pyplot as plt


fig = plt.figure()
ax = fig.add_subplot(111)


xy = np.vstack([in_phase,quadrature])
z = gaussian_kde(xy)(xy) #This is just to get colors to see better

ax.set_xlabel(&quot;in_phase&quot;)
ax.set_ylabel(&quot;quadrature&quot;)

ax.scatter(in_phase, quadrature, c=z)

I get the following images (2 exemple):
Typically a constellation diagram is not just the raw RF values because that would be extremely disorganized. You would end up with a bunch of points in a gaussian distribution about the origin (thermal noise) and a &quot;ring&quot; around it that represents your signal at any particular phase during the sampling.
What you typically want to plot is the baseband signal, the RF after it is filtered, mixed against the carrier wave, and coherent (phase-locked). This will indicate which constellation values are being received with some noise (hence why in Figure 7 they are scattered about the data positions). I have no idea what is going on in Figure 8, but it's not a constellation diagram. It looks like something of the author's own invention to better analyze the given data.