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Re: [Discuss-gnuradio] About the simple_mac.py in bbn80211

From: Douglas Geiger
Subject: Re: [Discuss-gnuradio] About the simple_mac.py in bbn80211
Date: Fri, 16 Oct 2009 16:49:01 -0400

One thing to be aware of: the USRP1 does *not* have sufficient
bandwidth to the host PC to do real DSSS 802.11b. On the receive side
they were able to get it 'sort of' working by simply accepting the
aliasing that results from under-sampling. For transmitting/receiving
between two USRP1's - I believe the original BBN code simply took the
DPSK-modulated sequence, and ran it through a RRC filter, without any
spreading taking place - so the bandwidth of the USB link is no longer
an issue. This mode allows the USRP1 to do 802.11-like networking -
but it won't inter-operate with regular 802.11 gear.

I will also note that the original BBN code had a bug in the matched
filter design (in bbn_firdes_barker.cc), it did not reverse the barker
sequence (which makes the FIR filter/convolution operation the same as
multiplying by the [forward] sequence). I have not checked in the fix
to my branch, but the usrp2_version branch achieves the sample thing,
with the addition of a resampling block to make the FIR operation work
on a lower data rate. Since it worked, I assume the error simply made
the effective SNR worse - as it was with the USRP1 it could only
receive very high SNR 1 and 2Mbps packets from commerical 802.11 gear.

On Fri, Oct 16, 2009 at 10:30 AM, Chen Chen <address@hidden> wrote:
> Since I have made changed in the bbn_80211b_rx.py and bbn_80211b_tx.py (also
> some related script such as bbn_80211b.py and bbn_80211b_pkts.py) as you
> mention: blks-blks2, hier-block->hier_block2, and the __init__ call.
> (modifiy theĀ original version since it is for usrp1). It can run but the
> receiver can not show anything.
> receiver side:
> ./bbn_80211b_rx.py -f 2412M -d 24 -S 8 -g 35
> transmitter side:
> ./bbn_80211b_tx.py -f 2412M -i 24 -S 8
> So I am wondering if I modified the simple_mac.py, it will cause the same
> problem?
> Does anyone make the bbn_80211b_rx.py and bbn_80211b_tx.py work on the
> usrp1? and also for simple_mac.py?
> Thanks
> Chen

Doug Geiger

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