I'm not sure what's going on here, but I think it could be likely that your estimate of the preamble-autocorrelation simply doesn't work all too well.
Generally: ditch benchmark_rx and the blocks it uses. They are really superseeded by the OFDM blocks that were introduced in 2013 – they are far better, and also, will simply give you the estimates you want, I think.
Make sure you have a recent version of GNU Radio, and look for the rx_ofdm.grc example (likely somewhere in /usr/share/gnuradio/examples/digital/ofdm). The "Schmidl & Cox OFDM synch." block actually has an output "freq_offset", and the "OFDM Channel Estimation" block adds a stream tag containing the full subcarrier offset (if any).
Other than that:
I've been trying to figure out the frequency offset of USRP recently.Could you specify this a bit more?
Best regards,
Marcus
On 19.05.2016 05:53, DingJing wrote:
Hi all,I've been trying to figure out the frequency offset of USRP recently. So I run the OFDM benchmark program and output the compensation value of fine offset.For detail,in benchmark.rx.py, we import ofdm_receiver,and ofdm_reciver import ofdm_sync_pn to calculate the fine offset and do time synchronization.The main procedure of ofdm_sync_pn(using Schmidl and Cox algorithm) is as following:
And I output the S/H(sample and hold) block to file sink block to record the fine offset.However,the result figure are as following:X label can be regarded as time,Y label is the output fine offset by angle between -Pi to Pi.It is fiercely different from my expectation in which the figure should be a straight line approximately.I wonder why this happens.Thank you so much!
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