Re: [Discuss-gnuradio] Channel estimation in OFDM examples

[Martin Braun]

On 01/18/2016 10:58 AM, Jurgis Aleksandravičius wrote:
> I am using the OFDM tx/rx example flowgraphs on USRP N210 radios to get
> complex channel coefficients from tags produced by OFDM Channel
> Estimation block.

I assume you're using the ofdm_tx and ofdm_rx examples.

> When I record and plot the channel coefficient in time, I can see that
> its magnitude is constant, but phase is 'rolling' at a frequency of ~100
> Hz when center frequency is 2.4 GHz. I guess that is because of
> differences in clock frequency at transmitter and receiver. I tried
> using the MIMO cable for clock synchronization - that just made the
> phase of channel coefficient less noisy, but phase roll was still there.
> Also, my understanding was that OFDM should correct the frequency offset.
> Does anyone have thoughts on what might going on here?

There's not all to much to go off of here, but keep in mind that the
receiver will always estimate a frequency offset (it doesn't you you
have perfectly synced tx/rx) and that estimate is noisy. When you have a
frequency offset, it will update that between symbols according to the
rotation during the cyclic prefix. Maybe that's what you're seeing.

M