I want to transmit a number of symbols with QPSK-modulation over a channel by using OFDM. I have never used cyclic prefix before so I'm not so sure about this. In my lab instructions they seem to say that I add the cyclic prefix directly from the IFFT:ed version of the bit-stream and append it to the original IFFT sequence. b bits----QPSK-----> s(k) z = IFFT(s[k]) z_cyclic = z[N-Ncp]...z[N-1], z(0], z...z[N-1] y = z_cyclic + noise s_out = FFT(y) ---> bhat However, when I do this I get the wrong bits out. If i send 5 zeros: [0 0 0 0 0] and add a cyclic prefix after the IFFT then i should get a output bhat in the form of: cyclic + 0 0 0 0 0 where cyclic is additional ones an zeros due to the fact that i made the time-signal longer (appended the cyclic prefix). But in my case I get both ones and zeros in my output.