Significance of IFFT at the I TX of a n OFDM

Though the input is a time Domain Signal

The input is a disguising time domain input, I mean that every sample correspond to the weight of one of N frequencies if we have N sub-carriers. Actually, those are time samples that correspond to frequency weights so it is convenient to use IFFT.

Note that IFFT is a mathematical operation that doesn't care what goes in and out ! We found that multiplying by orthogonal carriers and adding resembles the mathematical formula of IFFT, thus it is a mathematical motivation not a conceptual one!

Also note that FFT and IFFT are the same! because in one we multiply by exp(x) and the other exp(-x), and both have the same real part.

Hope this helps,
Ahmed

I could not understand Disguising Time Domain,
Can u please write a dew more words
Thanks

I'll try to give you a good example:

Assume the following baseband stream:
1,1,-1,-1,1,1,1,-1,1,-1,-1,-1,-1,1,-1,-1,-1,1,…

We split this main stream to sub-signals. The serial to parallel conversion is given in the following table, every sub-channel will have lesser bandwidth (1/4 of main stream Bandwidth).

I will arrange the stream within sub-carriers as:

C1 C2 C3 C4
1 1 -1 -1
1 1 1 -1
1 -1 -1 -1
-1 1 -1 -1
-1 1 1 -1
-1 -1 1 1

Assume the carrier frequency C1 = 1 Hz, the harmonics are C2=2 Hz, C3 = 3 Hz and C4 = 4 Hz.

Now we multiply every row by the vector [cos(2*pi*1*t) cos(2*pi*2*t) cos(2*pi*3*t) cos(2*pi*4*t)]T to obtain an OFDM signal. Right ?

The row 1 of the table represents the amplitudes of a certain range of sinusoids because they'll be multiplied by sinusoids to obtain OFDM! Thus, the IFFT would retrieve a time domain signal (the OFDM signal). For example, at the first N instants, we capture the amplitude of a low frequency (C1) and a higher freq C2, C3 and so on to compose an OFDM signal.

Every row in the table can be considered as a spectrum !. Each row has only 4 frequencies because we multiply every element by a different sinusoid/carrier . Actually those rows are not spectra, they are extracted from a time domain baseband signal but the IFFT is a mathematical concept that doesn’t care what goes in and out, does it ?

Each of these rows can be converted to a time domain signal (OFDM) through IFFT, I know that this row is originally N adjacent baseband samples, but they are equal to the spectrum of the desired OFDM.Thus, the input to IFFT is a time domain signal disguising as spectrum.

Hope you got it
:|

Nicely put Ahmed Alaa

can somebody help me in inserting zeros at transmitter and removing them at receiver in xilinx system generator OFDM model.?
i ll be really greatful for ur help

Thanks Ahmed ****.but if multiplying by orthogonal sub carriers and adding them represents IFFT then where does modulation of signals take place?