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Re: [Discuss-gnuradio] USRP2 802.11 BBN Code TX - filter fix?

From: Colby Boyer
Subject: Re: [Discuss-gnuradio] USRP2 802.11 BBN Code TX - filter fix?
Date: Mon, 23 Nov 2009 12:55:14 -0800

If someone writes a Fast Walsh Transform block, then 5.5 and 11 Mbit is possible. Only a few changes need to be made to the MAC block to accommodate this change.

Getting G (OFDM) to work seamless with B will be be a bit difficult I think. You are then switching rates, 20 MHz vs 11 MHz if I remember correctly.

On Mon, Nov 23, 2009 at 4:34 AM, Doug Geiger <address@hidden> wrote:
George Nychis wrote:
> Hi all,
> I was taking a look with Brian at the 802.11 BBN code for the USRP2 in
> CGRAN (usrp2_version), in specific the transmission path.  I don't konw
> if anyone ever got to try out the TX code with the USRP2, but we found
> that the low pass filter after spreading was too small, causing a quick
> dropoff in frequency response.  This would likely lead to high BER at
> the receiver.
> I kind of based this off of the Simulink 802.11b model, as it seemed
> like the frequency response of the 802.11 BBN code was odd.
> If you look at matlab_spectrum.png and compare it to old_spectrum.png,
> the shape of the GR BBN old spectrum is very rounded in terms of the
> fall off and the fall off happens very quickly.
> So what we did next is take a look at the taps used in the filter, and
> confirmed from plotting them that the sampling frequency and the cutoff
> frequency was incorrect.  If you look at filter_orig_vs_new.png, you can
> see the difference between the two filters.
> In the end, we get new_spectrum.png which shows a much better waveform!
> Hopefully this helps BER of transmitting over the air.  I haven't gotten
> a chance to try this yet.
> - George

Interesting - I had only done a couple tests with TX on the USRP2:
enough that I was able to see proper frames in Wireshark with a
commercial card. I never hooked it up to a spectrum analyzer (or another
USRP2 to do similar) - but from your figures it does look like it's
rolling off pretty fast.

Doug Geiger

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