Re: [Discuss-gnuradio] Schmidl and Cox method in OFDM blocks

[Martin Braun (CEL)]

On Sun, Aug 25, 2013 at 07:45:56PM +0200, Shashank Gaur wrote:
> I have been trying to understand OFDM blocks, especially equalization and
> estimation part of the code. In the file ofdm_chanest_vcvc_impl.cc, schmidl 
> and
> cox method is used.
> 
> I am unable to understand use of this method for channel estimation and
> equalization. Also there are some ToDo left out in the code, some light on 
> that
> would also be useful.

Hi Shashank,

have you read the paper on Schmidl & Cox? It's referenced in the code.

Also, make sure you've read these:
http://gnuradio.org/doc/doxygen/page_packet_data.html
http://gnuradio.org/doc/doxygen/page_ofdm.html

Once you've understood the theory, here's the practical side:

The algorithm is split up into two parts, the fine frequency + timing
estimation (== packet detection) and the coarse frequency + channel
coefficients estimation.

The first is done in ofdm_sync_sc_cfb. The two outputs are the frequency
offset and a trigger signal to mark the beginning of the packet. We use
the first to immediately correct the fine frequency offset.

After packetization, there are two separate signal processing paths for
header and payload. When the header is modulated, the coarse freq.
offset is not yet known, so that needs to be estimated along with the
channel coefficient.
There is only one TODO left in these blocks: There is a method to
improve the estimation quality of the channel coefficients by applying
knowledge of the maximum channel delay, which is not yet implemented,
but this way, it can be added without any API changes.

MB

-- 
Karlsruhe Institute of Technology (KIT)
Communications Engineering Lab (CEL)

Dipl.-Ing. Martin Braun
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