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 [email protected] https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
