What I'll get if I take FFT of a frequency domain data.

I have two questions related to this.

  1. For ex, x is a time domain data where X = fft(x), now fft(X)= what i'll get ?

  2. While reading SCFDMA, data is first DFT'd and a subcarrier mapping happens then we are taking an IFFT which will transmitted using a radio. According to my knowledge the input to radio is a time domain data.But in SCFDMA taking DDT itself converts my time domain data to Frequency domain then again if i take IFFT the frequency domain data converted back to time domain which is fed to radio. How this works ?

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    $\begingroup$ One question per question please - this seems like it's two very different questions. $\endgroup$ – Paul R Oct 3 '13 at 9:40
  • $\begingroup$ Also, as pointed out in the answer, please be sure to define acronyms that you're using. Not everyone is familiar with the exact problem domain that you might be working in. $\endgroup$ – Jason R Oct 3 '13 at 15:13
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    $\begingroup$ SCFDMA most probably refers to single-carrier frequency division multiple access as beeing used in Long Term Evolution (LTE) uplink. DDT is a typo I guess and should be DFT. $\endgroup$ – Deve Oct 3 '13 at 16:54

The FFT of the FFT bascially gets you the original signal again, it's just scaled and time flipped, i.e. $$FFT\left \{ FFT \left \{ x(n) \right \} \right \} = N \cdot x(-n)$$

That's a simple consequence of the fact that the forward and inverse Fourier Transforms are almost the same.

I have no idea what all the acronyms in the second question mean.

  • $\begingroup$ I think SC-FDMA is single carrier-frequency domain multiple access. Or he actually means SC-OFDMA. $\endgroup$ – user304584 Mar 20 '17 at 14:05

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