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

From: George Nychis
Subject: Re: [Discuss-gnuradio] implementing TX 802.11 short preamble, needs debugging
Date: Mon, 30 Nov 2009 17:02:13 -0500

Hi Daniel,

Any help would be greatly appreciated :)  Maybe I can poke at the time synchronization code also.

It seems as though the decoder's inability to detect some of the short preambles might be due to the scrambler.  If I make my own "preamble" that uses 7 bytes of scrambled 1s instead of 7 bytes of scrambled 0s (as specified), the receiver is able to detect all 50 preambles.  If I use 7 bytes of 0s, it can only detect 44 preambles.  Digging...

- George

On Mon, Nov 30, 2009 at 4:43 PM, Daniel Sumorok <address@hidden> wrote:
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.


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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