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