What are coase freq. estimation and fine freq. estimation in OFDM? Did the implementation in gnuradi

GNU Radio
1

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?

2

On Sun, Apr 14, 2013 at 03:57:31PM +0800, Yingjie C. 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

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

Dipl.-Ing. Martin B.
Research Associate

Kaiserstraße 12
Building 05.01
76131 Karlsruhe

Phone: +49 721 608-43790
Fax: +49 721 608-46071
www.cel.kit.edu

KIT – University of the State of Baden-Württemberg and
National Laboratory of the Helmholtz Association

3

Thanks for you reply, Tom.
So what you mean is that gnuradio only use one symbol to do the time &
frequency (including coarse and fine)synchronization, as well as CSI
estimation, is that right?

You also said the coarse freq. offset estimation is slight different.
Can
you show me some references only related to coarse freq. offset
estimation
in gnuradio. Thanks in advance.

Best,
Kay

2013/4/15 Martin B. (CEL) [email protected]

4

Hi Martin,
I am not able to find the file
digital_ofdm_frame_acquisition.cc or digital_ofdm_chanest_vcvc.cc in my
gnuradio. I have gnuradio 3.6.2 installed on my ubuntu 10.04. I tried
searching in /usr/local/share/gnuradio/examples/digital/ofdm. But there
were no c++ files. The find command also doesn’t show it. where should i
look for that c++ file.

On Tue, Apr 16, 2013 at 12:49 PM, Martin B. (CEL)

5

On Tue, Apr 16, 2013 at 12:22:28AM +0800, Yingjie C. wrote:

Thanks for you reply, Tom.

Hm, I guess we both have a beard and wear glasses.

So what you mean is that gnuradio only use one symbol to do the time &
frequency (including coarse and fine)synchronization, as well as CSI
estimation, is that right?

The old stuff yes, the new stuff, if you like.

You also said the coarse freq. offset estimation is slight different. Can you
show me some references only related to coarse freq. offset estimation in
gnuradio. Thanks in advance.

digital_ofdm_frame_acquisition.cc or digital_ofdm_chanest_vcvc.cc.

Grepping for ‘coarse’ would have helped you!

MB

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

Dipl.-Ing. Martin B.
Research Associate

Kaiserstraße 12
Building 05.01
76131 Karlsruhe

Phone: +49 721 608-43790
Fax: +49 721 608-46071
www.cel.kit.edu

KIT – University of the State of Baden-Württemberg and
National Laboratory of the Helmholtz Association

6

Martin,
Thanks I got it. I am planning to update my ubuntu to 12.04
and
then install latest version of gnuradio. should i manually delete all
the
existing folders and run the installation or does build-gnuradio take
care
of that.

7

On Tue, Apr 16, 2013 at 06:17:56PM +0530, Karan T. wrote:

           I am not able to find the file digital_ofdm_frame_acquisition.cc

or digital_ofdm_chanest_vcvc.cc in my gnuradio. I have gnuradio 3.6.2 installed
on my ubuntu 10.04. I tried searching in /usr/local/share/gnuradio/examples/
digital/ofdm. But there were no c++ files. The find command also doesn’t show
it. where should i look for that c++ file.

Hi Karan,

there’s no signal processing files in ‘examples’. I recommend the
introductory pages to GNU Radio on the wiki.

On Linux, the ‘find’ command will help.

Such as:

src/gnuradio % find gr-digital -name digital_ofdm_frame_acquisition.cc
gr-digital/lib/digital_ofdm_frame_acquisition.cc

There it is.

MB

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

Dipl.-Ing. Martin B.
Research Associate

Kaiserstraße 12
Building 05.01
76131 Karlsruhe

Phone: +49 721 608-43790
Fax: +49 721 608-46071
www.cel.kit.edu

KIT – University of the State of Baden-Württemberg and
National Laboratory of the Helmholtz Association