Implementation of OFDM TX and RX using GNURadio and USRP

[Al Amiri, Wesam (waalamiri42)]

Dears, 

I am working on the implementation of OFDM TX and RX using GNURadio 3.10.3 and USRP N210 (with UHD_4.2.0). 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. The implementation works fine, and I was able to receive the OFDM signal by the RX USRP.  

What I am trying to do is to extract some carriers from the FFT bins at the demodulator to do phase measurements on each one of the extracted carriers. 
I tried to create my own block to do that, but that didn't work and caused the GRC graph to freeze. Then, I tried to do that offline using Matlab, where I attached the FFT block to a file sink. 
In Matlab, I plotted the absolute of I/Q samples of the file sink as shown in the attached figure. But still, I am not able to distinguish between the subcarriers. 

Could anyone please give me advice how to extract certain carriers from the FFT bins of the demodulator?

Regards,
Wesam Al Amiri

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Description: Screenshot from 2022-11-24 09-55-38.png

Screenshot from 2022-11-24 10-55-38.png
Description: Screenshot from 2022-11-24 10-55-38.png