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Re: [Discuss-gnuradio] OFDM results.

From: Bob McGwier
Subject: Re: [Discuss-gnuradio] OFDM results.
Date: Fri, 06 Jun 2008 14:55:22 -0400
User-agent: Thunderbird (Windows/20080421)

Matt Ettus wrote:
Jeff Brower wrote:

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.


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