Phase Measurements of OFDM Signal with USRP N210

[Al Amiri, Wesam (waalamiri42)]

Dears, 

I am doing phase measurements implementation of an OFDM signal transmitted from TX USRP N210 (with UHD_4.2.0)  and received by RX USRP N210 (with UHD_4.2.0) using GNURadio 3.10.3. In the implementation, I followed the GRC flow graph provided by GNU Radio Workshop (Slides 259 - 260) with changing the carrier frequency to 2 GHz, Sampling rate 1MHz at TX, and 10 MHz for Rx. 

The Tx USRP is connected to a Log Periodic antenna and similarly for the Rx USRP. There is no LOS between the TX and Rx antennas. They directed toward a reflective board so that the transmitted signal will be reflected by the board and received by the receiver.

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 the attached figure, the unwrapped phase values are not close to zero even if there is a time and frequency synchronization (using Schmidl and Cox). I am not sure why this happened? 

The phase values of the serializer are fixed (alternate between four values corresponding to four point), while when I do the phase unwrapping it gives as shown in the figure.

Regards,

Wesam Al Amiri

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