[Discuss-gnuradio] implementing TX 802.11 short preamble, needs debugging

[George Nychis]

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