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Re: [Discuss-gnuradio] Question about ofdm_sync_long block of gr-ieee802

From: Marcus Müller
Subject: Re: [Discuss-gnuradio] Question about ofdm_sync_long block of gr-ieee802-11 module
Date: Sat, 09 May 2015 13:23:23 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0

Hi Jeon,

in such a case, linking to the original file [1] including line number might be more helpful, as it gives more context.

So, looking at this, it's taking the delayed input samples and copies them (offset-corrected) to the output, considering parts of the samples,
x = ignore, o = copy, starting at the frame start:

   128  |16¦128-16|16¦128-16|...

I'm not too familiar with 802.11p frames, but that looks like Basti's leaving out some pilots in between the data, probably only copying the preamble and data payload. But I could be wrong.


[1] https://github.com/bastibl/gr-ieee802-11/blob/master/lib/ofdm_sync_long.cc#L128

On 05/09/2015 12:06 PM, Jeon wrote:
I am looking gr-ieee802-11 module.

a part of the source code is pasted here (https://gist.github.com/gsongsong/3df4553fe96e2d62b83e)

I have a question on the following statement:

`if(rel >= 0 && (rel < 128 || ((rel - 128) % 80) > 15))`  in `case COPY`
`if(((d_count + o) % 64) == 0)` in `case RESET`

I am wonder what do these numbers(128, 80, 15, 64) and modulo operation have logical meaning in IEEE 802.11p PHY frame?

I've read 802.11 OFDM specification, thus I have knowledge about it,
but I have no idea what are roles of these things in implementation.


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