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Re: [Discuss-gnuradio] OFDM Benchmark Tx Issue?

From: mleech
Subject: Re: [Discuss-gnuradio] OFDM Benchmark Tx Issue?
Date: Tue, 21 Feb 2012 13:13:21 -0500
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Although using the same device to check on said carrier as is transmitting it leads to compounding error in one direction or t'other.

Best to use another device (preferrably a lab spectrum analyser) to check the offset.




On Tue, 21 Feb 2012 12:56:30 -0500, Tom Rondeau wrote:

On Tue, Feb 21, 2012 at 11:35 AM, Florian Schlembach <address@hidden> wrote:
Are you seeing the 1 MHz offset when you use the uhd_siggen.py? Or is it
just with the OFDM transmitter?
What is this tool doing? Transmitting a sine and checking the offset. Sorry, for the moment I have no possibilitie to check that.
> It's because with the larger bandwidth, the subcarriers, too, have a
> larger bandwidth. The coarse frequency correction is only set to look at
> so large an offset based on a number of subcarriers (+/-5 or 10), so now
> with the same frequency offset, 5 (or 10) carriers is a larger frequency
> span to check.

does that mean that there is only a coarse frequency correction implemented? How could I overcome the offset in the easiest way? Checking it by usrp_siggen.py and adjust in manually or are there some smart correction algorithms implemented?
There is a coarse and a fine frequency offset correction. The fine correct makes sure that the subcarrier is centered in the bin; the coarse adjusts for an integer number of subcarriers off from the center frequency. By default, the OFDM receiver will correct for some number of subcarrier bins (it's either +/-5 or +/-10; I can't remember). If your radios have a frequency offset that is greater than the maximum number of subcarriers specified here, the receiver cannot receive the symbols correctly.
Yes, you can transmit a tone using uhd_siggen.py to fine out a rough estimate of the frequency offset and adjust either the transmitter or receiver based on this number.


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