Jeff Brower wrote:
Bob-
In your sixteen QAM and other figures I see two effects.
Notice just the slightest hint that arcs through the top four
constellation points in the 16 QAM is not straight. This curvature is
caused by nonlinearity.
Your result almost surely can NOT be clock jitter. If you had a lot of
clock jitter, the pictures would look much worse. Notice the
dispersion gets larger as your proceed away from the origin. This is
almost surely phase noise in some oscillator.
"Phase noise" of an oscillator to me means jitter. Can you clarify? Do you
mean a
sinusoid oscillator that has some issue with shape; i.e. non-linearity? Thanks.
I have to disagree with Bob here. I think that you are seeing some form
of distortion.
In a QAM pattern, gaussian noise will look like uniformly-sized round
balls. Phase noise will look like ovals which are longer in the
"rotation direction" than they are along the line to the center of the
constellation, proportionally so for the outer points than the inner
ones. ISI, frequency-selective fading and ICI will, over time, tend to
look round as well. Distortion like nonlinearities in the transmitter
will mostly affect the outer constellation points. This is a little
more complicated in an OFDM system rather than straight single-carrier
QAM, but that still looks like what you are seeing. Also, 31
subcarriers is a very small number for OFDM, so the affect is more like
on plain old QAM.
I would suggest backing the transmit power off a bit further.
Matt
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