enter image description hereenter image description hereI 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 ifft-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. [The complex symbols after fft-block at the Rx before the BPSK demodulator][1]

enter image description here

The complex symbols after fft-block at the Rx before the BPSK demodulator

The ideal complex modulated symbol after BPSK modulator at Tx

  • 1
    $\begingroup$ You said you have corrected the phase shift, but in the next sentence notice random phase shift. So, your correction, however you did that (you don't tell us!) Is not sufficient $\endgroup$ Oct 2, 2023 at 23:24
  • $\begingroup$ Have you tried to generate the OFDM symbols in a matlab Tx simulation, and create an OFDM Rx simulation to demod the Tx signal? That may be helpful before working with your device. Matlab can simulate Tx/Rx OFDM symbols, but working with your own simpler custom simulation may give you better insight. $\endgroup$
    – PaulH
    Oct 3, 2023 at 4:34
  • $\begingroup$ You said "At the transmiiter, I have noticed that many symbols after the fft-block". Did you mean "At the receiver", or did you mean, "after the ifft block"? I am pretty sure you know from your explanation that OFDM Tx uses ifft and the Rx uses fft. BTW, which of the figures is figure (18)? $\endgroup$
    – PaulH
    Oct 3, 2023 at 4:39
  • $\begingroup$ Thank you for your reply. Acyually, I have tested my code with matlab simulation it works fine both at tx and Rx. When I didplay the constellation diagram after the ifft block at the Tx, it shows that the symbols are spreading out towards the original but this is not a big isdue since the demodulator will retrieve the bits correctly at the Rx using only simulation. $\endgroup$
    – Walkürie
    Oct 3, 2023 at 10:36
  • $\begingroup$ The issue now is that when I transmit the ofdm signal with Adalm Pluto SDR, I corrected the time delay, random phase shift and after the fft block at the Rx I got the complex symbols rotating with dynamic phase shift as shown in the second figure while in fact I should only get real values of +1 &-1 and imaginary of zero after the fft block since these +1s & -1s will be feeding into the BPSK demodulator to retrieve my original bits as shown in the last figure of BPSK modulator at the Tx. What is the cause of the issue in the second figure? How can it be solved? $\endgroup$
    – Walkürie
    Oct 3, 2023 at 10:49


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