Re: [Discuss-gnuradio] Question about ofdm_sync_long block of gr-ieee802-11 module

[Jeon]

Dear Bastian,

I missed one another thing.

`if(((d_count + o) % 64) == 0)` statement in https://github.com/bastibl/gr-ieee802-11/blob/master/lib/ofdm_sync_long.cc#L141

Is this because of a length of an OFDM signal being a multiple of 64, which is the number of subcarriers? (Except cyclic prefix)

Regards,

Jeon.

2015-05-09 22:32 GMT+09:00 Jeon <address@hidden>:

Dear Marcus and Bastian,

Thank you for your detailed answers.

Also, thank you for useful usage of gist. :)

Regards,

Jeon.

2015-05-09 21:42 GMT+09:00 Bastian Bloessl <address@hidden>:

Hi,


> On 09 May 2015, at 13:23, Marcus Müller <address@hidden> wrote:
>
> 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:
>
> oooooooo|xx¦oooooo|xx¦oooooo|...
>    128  |16¦128-16|16¦128-16|…
>

Exactly, it’s to remove the cyclic prefix (remove the first 16 samples of every 80 samples). Unfortunately, the first 128 samples (the long training sequence) don’t fit into the same scheme. Therefore the special case here.

Best,
Bastian


>
> 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.
>
> Greetings,
> Marcus
>
> [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.
>>
>> Regards,
>> Jeon.
>>
>>
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--
Dipl.-Inform. Bastian Bloessl
Distributed Embedded Systems Group
University of Paderborn, Germany
http://www.ccs-labs.org/~bloessl/

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