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.