One more thing, it only catches 44 out of 50 short preambles. - George
On Mon, Nov 30, 2009 at 4:14 PM, George Nychis <[email protected]> wrote: > Hi all, > > I'm in the progress of implementing the 802.11 short preamble on the TX > side. There is code on the RX side (based on yesterday's discussion) to > handle a short preamble, but I am positive there is no code on the TX side > to transmit using a short preamble. Of course, I'd like to implement it as > an option. > > The main crux of it is ensuring that only the preamble is modulated at > 1Mbps, and that the PLCP header is actually modulated at 2Mbps. This is > unlike the long preamble where both are modulated at 1Mbps. > > Right now, I have the loopback detecting the short preamble, but the CRCs > fail. If you want to try it: > svn co https://www.cgran.org/svn/projects/bbn_80211/branches/usrp2_version > > Build, etc, and then go to gr-bbn/src/examples and run > ./bbn_80211b_sptest.py ... you will see this, for example: > Recieved (short) header! > signal: 0xCA > service: 0xA0 > length: 0x52FA > crc: 0xB86C > Calculated crc: 0xE491 > *** BAD CRC *** > > To explain my changes a bit, I implemented a small option (which we can add > to the command line) here: > > https://www.cgran.org/browser/projects/bbn_80211/branches/usrp2_version/gr-bbn/src/examples/bbn_80211b_pkt.py?rev=383#L166 > > If using a short preamble, we use 7 bytes of scrambled 0's and the reversed > SFD: > > https://www.cgran.org/browser/projects/bbn_80211/branches/usrp2_version/gr-bbn/src/examples/bbn_80211b_pkt.py?rev=383#L175 > > During modulation, I make sure to split the preamble and the PLCP header, > and modulate them according to the preamble option: > > https://www.cgran.org/browser/projects/bbn_80211/branches/usrp2_version/gr-bbn/src/examples/bbn_80211b_pkt.py?rev=383#L257 > > xpsk_mod_header_1 is to modulate the header at 1Mbps and xpsk_mod_header_2 > at 2Mbps: > > https://www.cgran.org/browser/projects/bbn_80211/branches/usrp2_version/gr-bbn/src/examples/bbn_80211b_pkt.py?rev=383#L103 > > Again, when encoding, ensure we modulate based on the preamble option: > > https://www.cgran.org/browser/projects/bbn_80211/branches/usrp2_version/gr-bbn/src/examples/bbn_80211b_pkt.py?rev=383#L272 > > One last time, for prepending to the payload: > > https://www.cgran.org/browser/projects/bbn_80211/branches/usrp2_version/gr-bbn/src/examples/bbn_80211b_pkt.py?rev=383#L331 > > ... and that's it. If anyone pokes at it to pass the CRC with any success, > let me know... I'll keep working on it. > > - George > > > >
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