Hi,
On 07/08/2017 05:03 AM, Tellrell White wrote:
Hello Guys
I'm currently using the IEEE 802.15.4 PHY. I've added a random source to
the flow graph and also a qt gui time and constellation sink. I have two
questions, why does the constellation plot attached show more than the
usual 4 constellation points seen in O-QPSK constellation?? Is it due to
the sampling rate I'm using which is 8M in this case.
The default sampling rate is 4M, I don't know what you changed when you
tried to make it 8M. If you add a constellation sink to the unmodified
flowgraph you will see constellation points on the unit circle (your
second screenshot), which is what you would expect, since IEEE 802.15.4
uses O-QPSK with a half-sine pulse shape (see the standard).
Also, I was curious as to if the vector source added which implements
the half sine pulse shaping was creating a problem so I disabled it and
I was able to see the expected 4 constellation points. So, is the source
of extra constellation points I'm seeing?? I've attached both the .grc
file and 2 screen shots, one representing both constellation plots
discussed.
With this you removed the pulse shaping, so you see only the QPSK
constellations, but it's also not 15.4 anymore. Most standards use some
kind of pulse shaping, so you don't see only the constellation points
when you look at the final IQ signal. I don't understand why this is a
problem for you.
Best,
Bastian
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