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OFDM complex symbols have dynamic phase shift after the fft-block & before the BPSK demodulator at the receiver ,

Category: Software
Product Number: ADALM Pluto

I have transmitted and received an OFDM-signal using the same ADALM-Pluto-SDR's Tx/Rx Antenna.

I have used BPSK baseband modulation with symbol freq=312.5 KHz, L (length of ifft)=64, fsampling=fsym*L=20 MHz. I basically sent 4 OFDM-symbols, each symbol has 64 symbol-length(52 subchannels and 12 null-subcarriers). The cyclic prefix is 0.25% of the ifft length. I used a barker code with my signal for synchronization.

At the transmiiter, I have noticed that many symbols after the fft-block are approaching to zero and spreading around! After correcting the time delay and phase shift at the receiver, I have observed that my baseband complex symbols, after the fft-block at the receiver and before the BPSK-demodulator, have an obvious dynamic phase shift as you see below in the figure (18) and also in the constellation diagram. At this stage, I was supposed to get real values of +1s and -1s with imaginary parts equal to zero as I used BPSK baseband modulation. Instead I got rotating amplitudes of +1s and -1s.

My question is what is the cause of this problem and how can I solve it in matlab. I have tried to implement several algorithms after calculating the dynamic phase shift but all did not work.

I would highly appreciate your support. 

Thread Notes

lallison - Moved from StudentZone (Analog Dialogue) to Virtual Classroom for ADI University Program. Post date updated from Monday, October 2, 2023 to Thursday, December 14, 2023 to reflect the move.


lallison - Moved from StudentZone (Analog Dialogue) to Virtual Classroom for ADI University Program. Post date updated from Thursday, December 14, 2023 to Thursday, December 14, 2023 to reflect the move.