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

[George Nychis]

One more thing... if I take a look at the constellations between the using 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 :)

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

- George

On Mon, Nov 23, 2009 at 1:01 AM, George Nychis <address@hidden> 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

gr_new_complex.png
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gr_old_complex.png
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