Re: [Discuss-gnuradio] What are coase freq. estimation and fine freq. estimation in OFDM? Did the implementation in gnuradio follow the 802.11 standard?

[Martin Braun (CEL)]

On Sun, Apr 14, 2013 at 03:57:31PM +0800, Yingjie Chen wrote:
> Hi guys,
> 
> I have check the 802.11 standard before. It said the coase freq. estimation is
> done in short preamble, and the fine freq. estimation is done in long 
> premable.
> However, to my best knowledge, gnuradio only uses one preamble for
> synchronization. So how are these two freq. estimation works done in one
> preamble. BTW, can anyone tell me the exact different between coase one and
> fine one? Are they both related to the FFT bin aligning?


The OFDM stuff in GNU Radio has *nothing* to do with 802.11x. It's just
a more-or-less generic OFDM implementation.

I very much so recommend reading the Schmidl & Cox OFDM sync paper [1];
their algorithm is pretty much what is used GNU Radio. It also explains
the difference between coarse and fine frequency offset (short: coarse =
integer multiples of sub-carrier offset, fine=smaller than that).

In the original paper, they suggest 2 symbols for synchronisation (of
time & frequency) as well as initial CSI estimation. This can also be
done with 1 symbol, which is done in the current OFDM stack.

The OFDM work that was recently added can handle both 1 or 2
sync-symbols; the estimation of the coarse freq. offset is slightly
different for these cases.

However, this type of sync needs modification if you want to adapt it to
802.11.

MB

[1] Schmidl, T.M.; Cox, D.C., "Robust frequency and timing
synchronization for OFDM," Communications, IEEE Transactions on ,
vol.45, no.12, pp.1613,1621, Dec 1997




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