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

[Daniel Sumorok]

I remember I had the same problem with the crc error on the short preamble when 
I wrote the code, but I never figured out what the problem was.  However, I did 
receive an email from someone who claimed to have fixed the problem (I think) a 
couple of years ago.  If you want I can email him.  Also, I don't know how much 
the code has changed. but there is a bug in the time synchronization code that 
will cause some packets to be dropped.

 -Dan

On Nov 30, 2009, at 4:14 PM, George Nychis 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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