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I am doing some analysis on a received OFDM signal using USRP N210 with GNURadio. Figure OFDM RX shows the OFDM receiver (please refer to GNURdaio workshop slides) . The transmitted OFDM signal is generated from another N210 USRP.

To do phase measurements, I used Complex to Mag Phase block after the OFDM serializer and recorded the phase measurement of using File sink. Then, I read the file using the matlab and did phase unwrapping for the phase values, as shown in
As shown the unwrapped phase values are not close to zero even there are a time and frequency synchronization (using Schmidl and Cox). I am not sure why this happened? The phase values of the serializer are fixed (four values corresponding to four point), while when I do the phase unwrapping it gives as shown in the figure Unwrapped Phase.

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The unwrapping matlab function produces fake wraps (phase jumps) due to noise. This creates a cumulative error in all following phase samples. This usually happens when the SNR is low. To solve this problem, recurrent phase unwrapping algorithm can be used instead of the matlab function.

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