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List:       gnuradio-discuss
Subject:    Re: [Discuss-gnuradio] USRP2 802.11 BBN Code TX - filter fix?
From:       Colby Boyer <csboyer () berkeley ! edu>
Date:       2009-11-23 20:55:14
Message-ID: 1bc830130911231255r6d73614bk7aa921337fffd007 () mail ! gmail ! com
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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 <doug.geiger@gmail.com> 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
>
> --
> Doug Geiger
> doug.geiger@bioradiation.net
> doug.geiger@ieee.org
>

[Attachment #3 (text/html)]

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.<br><br>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. <br>

<br><div class="gmail_quote">On Mon, Nov 23, 2009 at 4:34 AM, Doug Geiger <span \
dir="ltr">&lt;<a href="mailto:doug.geiger@gmail.com">doug.geiger@gmail.com</a>&gt;</span> \
wrote:<br><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, \
204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">

<div><div></div><div class="h5">George Nychis wrote:<br>
&gt; Hi all,<br>
&gt;<br>
&gt; I was taking a look with Brian at the 802.11 BBN code for the USRP2 in<br>
&gt; CGRAN (usrp2_version), in specific the transmission path.  I don&#39;t konw<br>
&gt; if anyone ever got to try out the TX code with the USRP2, but we found<br>
&gt; that the low pass filter after spreading was too small, causing a quick<br>
&gt; dropoff in frequency response.  This would likely lead to high BER at<br>
&gt; the receiver.<br>
&gt;<br>
&gt; I kind of based this off of the Simulink 802.11b model, as it seemed<br>
&gt; like the frequency response of the 802.11 BBN code was odd.<br>
&gt;<br>
&gt; If you look at matlab_spectrum.png and compare it to old_spectrum.png,<br>
&gt; the shape of the GR BBN old spectrum is very rounded in terms of the<br>
&gt; fall off and the fall off happens very quickly.<br>
&gt;<br>
&gt; So what we did next is take a look at the taps used in the filter, and<br>
&gt; confirmed from plotting them that the sampling frequency and the cutoff<br>
&gt; frequency was incorrect.  If you look at filter_orig_vs_new.png, you can<br>
&gt; see the difference between the two filters.<br>
&gt;<br>
&gt; In the end, we get new_spectrum.png which shows a much better waveform!<br>
&gt; Hopefully this helps BER of transmitting over the air.  I haven&#39;t gotten<br>
&gt; a chance to try this yet.<br>
&gt;<br>
&gt; - George<br>
<br>
</div></div>Interesting - I had only done a couple tests with TX on the USRP2:<br>
enough that I was able to see proper frames in Wireshark with a<br>
commercial card. I never hooked it up to a spectrum analyzer (or another<br>
USRP2 to do similar) - but from your figures it does look like it&#39;s<br>
rolling off pretty fast.<br>
 Doug<br>
<font color="#888888"><br>
--<br>
Doug Geiger<br>
<a href="mailto:doug.geiger@bioradiation.net">doug.geiger@bioradiation.net</a><br>
<a href="mailto:doug.geiger@ieee.org">doug.geiger@ieee.org</a><br>
</font></blockquote></div><br>



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