USRP2 802.11 BBN Code TX - filter fix?

Hi all,

I was taking a look with Brian at the 802.11 BBN code for the USRP2 in
(usrp2_version), in specific the transmission path. I don’t konw if
ever got to try out the TX code with the USRP2, but we found that the
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
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,
shape of the GR BBN old spectrum is very rounded in terms of the fall
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
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
chance to try this yet.

  • George

One more thing… if I take a look at the constellations between the
the old filter and the new filter, they’re different. I’m not much of a
communications person… so maybe someone can enlighten me here :slight_smile:

But, this is DPSK or DBPSK… I imagined there would be two definitive
constellation points, but instead the symbols are spread across a
In the case of the new filter, the complex samples seem to exceed pi/4
5pi/4 and the data is much more spread. Is this due to the spreading

  • George

If those are the post-spread samples, then I think that makes sense.
barker spreading code “randomizes”(appears) everything.

Have you tried decorrelating with the matched filter? If that results in
tight constellations then all is good.

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
this change.

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