I have an OFDM system with Number of sub-carriers $N= 1024$, modulated data using $QAM$ modulation is transmitted via those subcarriers as follows:
modulation --> ifft --> adding CP --> upsampling --> conv channel --> adding noise
Then at the receiving side, the received signal is processed as follows, discarding the channel
delay --> downsampling --> CP removal --> fft --> MMSE equalizer --> demodulation
The issue which I am facing in that system is in the equzlizer step, When using the original channel, I can not get the performance back. Howenver, when I estimate that channel after the step of fft and then use the estimated channel, The performance is OK !!
Why does that issue happens? I think because of the upsampling step, because when I don't upsample the signal, that becomes fine but when I use the upsampline and use then equalize using the original channel, I can't get the performance back.