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

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.

 

-Marcus

 

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

On Tue, Feb 21, 2012 at 11:35 AM, Florian Schlembach <florian.schlembach@tu-ilmenau.de> 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.

Yes.

 

> 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.

@Tom
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.

Tom