Re: [Discuss-gnuradio] OFDM results.

[Matt Ettus]

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