have two User one of them sending over freq (0-B] and the other transmit over (B-2B]. Both of them are using OFDM and each one has Nc subcarriers. I wanted to simulate the received signal using Matlab.

  • One way to implement, I beleive is that I say user one have 2*Nc subcarriers and only use the first Nc subcarriers and the second user also has 2Nc subcarrier and only ise subcarriers [Nc+1-2Nc). in this case I should use FFT/IFFT size of 2Nc:

    Received signal would be Y = ifft([d_1 0*d2],2Nc)+ifft([0*d_1 d_2],2Nc) where d_1 and d_2 are the data of the first and the second users. and at the receiver I can compute d=fft(Y, 2Nc) and the first half would be for user1 and second half for user2.

  • But I want to use FFT size of Nc, i.e., compute ifft(d_1,Nc) and ifft(d_2,Nc). If I add these two iffts directly, I will get incorrect results as the IFFT results does not show that the two data occupying different subcarriers. My question is, in this case, how I should change (probably up-convert) the ifft result of the second UE?

    Furthermore, after constructing the received signal, what operation do I need to get back the transmitted data? Should I again use fft(y,2Nc) ?

Your help is highly appreciated.


  • $\begingroup$ What you'd like to do is feasable. But could you comment on your motivation for doing so? Using two Nc-IFFTs requires oversampling and up-conversion. I'm not sure whether the complexity of this scheme is really lower compared to the first one. $\endgroup$ – Deve Dec 9 '14 at 15:46
  • $\begingroup$ Thanks a lot. I was thinking that with the second method, it might be easier when we have more than 2 UEs or when I do not know how many UEs are there so it can be more distributed, i.e., each UE only use IFFT of size Nc and then upconvert to its freq range and then transmit it, does not need to wory about the size of the IFFT based on the number of users. But I am sure about the steps and the parameters that I need to use for oversampling and upconversion. that would be great if you can elaborate more on the steps and parameters of each step. $\endgroup$ – Vahid Dec 9 '14 at 19:59

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