Re: [Discuss-gnuradio] OFDM results.

[Bob McGwier]

Matt Ettus wrote:
> 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.

To me there is no doubt about the nonlinear distortion as you can see 
curvature in the edges of the "square".  If you have amplitude 
components modulating the signal coming into the DBS-RX mixer,  it will 
not just spread them in angle.  I do take your point about the 
constellation points being symmetric rather than more elongated it might 
just be compression effects.  Back off will answer the question.
> 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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