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Add cyclic prefix to OFDM system

  1. Feb 17, 2014 #1
    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[1]...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.
  2. jcsd
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