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.gei...@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.gei...@bioradiation.net > doug.gei...@ieee.org >
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