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: [email protected]
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
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