# CFO in OFDM **URL:** \ **Category:** GNU Radio\ **Created:** [October 29, 2014, 1:35pm UTC](https://www.ruby-forum.com/t/cfo-in-ofdm/232812 "2014-10-29T13:35:26Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\ **Post date:** [October 29, 2014, 1:35pm UTC](https://www.ruby-forum.com/t/cfo-in-ofdm/232812/1 "2014-10-29T13:35:26Z")
Dear all, I am suffering from issues related to residual CFO error manifesting itself as a slight rotation over time. This can be partially corrected in the FD using tracking pilots which I have implemented but there is still a slight problem which may be caused by ICI. In the OFDM example code, how is the value for the Frequency Mod sensitivity (-2/FFT\_len) derived? Has anyone implemented other CFO estimation blocks for OFDM systems, other than the Schmidl and Cox technique? Regards, David * * * David Halls Ph.D. Research Engineer Toshiba Research Europe Limited 32 Queen Square, Bristol, BS1 4ND, UK Tel: +44 (0) 117 906 0790 * * * NOTE: The information in this email and any attachments may be confidential and/or legally privileged. This message may be read, copied and used only by the intended recipient. If you are not the intended recipient, please destroy this message, delete any copies held on your system and notify the sender immediately. Toshiba Research Europe Limited, registered in England and Wales (2519556). Registered Office 208 Cambridge Science Park, Milton Road, Cambridge CB4 0GZ, England. Web: www.toshiba.eu/research/trl ---
**Author:** ![divotdave](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/bbe5ce/32.png) [@divotdave](https://www.ruby-forum.com/u/divotdave)\ **Post date:** [October 29, 2014, 9:38pm UTC](https://www.ruby-forum.com/t/cfo-in-ofdm/232812/2 "2014-10-29T21:38:50Z")
On 10/29/2014 01:34 PM, David Halls wrote: > I am suffering from issues related to residual CFO error manifesting > itself as a slight rotation over time. This can be partially corrected > in the FD using tracking pilots which I have implemented but there is > still a slight problem which may be caused by ICI. > > In the OFDM example code, how is the value for the Frequency Mod > sensitivity (-2/FFT\_len) derived? Has anyone implemented other CFO > estimation blocks for OFDM systems, other than the Schmidl and Cox > technique? I recommend the S&C paper for the derivation, but the maths is not that hard. The very short version is that 1/fft\_len is the frequency offset of carriers in normalized frequency, and the fine frequency offset is unambiguous between \pm 1 carrier. The result of the estimator is thus in ‘fraction of carriers in normalized frequency’. I’ve played around with other techniques, but not in GNU Radio. The reason being, a tracking CFO estimator (i.e. one that changes the estimate over time during a packet) is much harder to implement (read, I was lazy). Now, if you don’t actually want to track, but only estimate once at the beginning, you won’t get much better than S&C. Note that you’ll always have ICI as soon as you have a Doppler spectrum. With decent equalization, it’s not _that_ big of a deal as long as it’s not too much ICI. M ---
**Author:** ![divotdave](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/bbe5ce/32.png) [@divotdave](https://www.ruby-forum.com/u/divotdave)\ **Post date:** [October 29, 2014, 10:20pm UTC](https://www.ruby-forum.com/t/cfo-in-ofdm/232812/3 "2014-10-29T22:20:35Z")
Thanks Martin, I dug out the S&C paper and think I’m clear on that, thanks. I have a currently uncoded system with superimposed QPSKs from multiple sources so it’s pretty sensitive! its fine using the Octoclock, though giving 0 BER end-to-end. I also have 32 symbol packets; so fairly long. I’ve implemented some basic CFO tracking using the pilots which helps. I think it’s residual CFO error causing the issue, manifesting as rotation of frequency over time, but as it’s high SNR this should be small. With tracking and correction the constellation _looks_ OK… I’m going to keep digging around… -------- Original message -------- From: Martin B. Date:2014/10/29 20:38 (GMT+00:00) To: [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) Subject: Re: [Discuss-gnuradio] CFO in OFDM On 10/29/2014 01:34 PM, David Halls wrote: > I am suffering from issues related to residual CFO error manifesting > itself as a slight rotation over time. This can be partially corrected > in the FD using tracking pilots which I have implemented but there is > still a slight problem which may be caused by ICI. > > In the OFDM example code, how is the value for the Frequency Mod > sensitivity (-2/FFT\_len) derived? Has anyone implemented other CFO > estimation blocks for OFDM systems, other than the Schmidl and Cox > technique? I recommend the S&C paper for the derivation, but the maths is not that hard. The very short version is that 1/fft\_len is the frequency offset of carriers in normalized frequency, and the fine frequency offset is unambiguous between \pm 1 carrier. The result of the estimator is thus in ‘fraction of carriers in normalized frequency’. I’ve played around with other techniques, but not in GNU Radio. The reason being, a tracking CFO estimator (i.e. one that changes the estimate over time during a packet) is much harder to implement (read, I was lazy). Now, if you don’t actually want to track, but only estimate once at the beginning, you won’t get much better than S&C. Note that you’ll always have ICI as soon as you have a Doppler spectrum. With decent equalization, it’s not _that_ big of a deal as long as it’s not too much ICI. M * * * Discuss-gnuradio mailing list [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) > **[Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio)** * * * NOTE: The information in this email and any attachments may be confidential and/or legally privileged. This message may be read, copied and used only by the intended recipient. If you are not the intended recipient, please destroy this message, delete any copies held on your system and notify the sender immediately. Toshiba Research Europe Limited, registered in England and Wales (2519556). Registered Office 208 Cambridge Science Park, Milton Road, Cambridge CB4 0GZ, England. Web: www.toshiba.eu/research/trl