# How to simulate two OFDM Users with low complexity FFT/IFFT?

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) ?