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Re: [Discuss-gnuradio] implementing TX 802.11 short preamble, needs debu
From: |
Daniel Sumorok |
Subject: |
Re: [Discuss-gnuradio] implementing TX 802.11 short preamble, needs debugging |
Date: |
Mon, 30 Nov 2009 16:43:00 -0500 |
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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