Why fft and ifft used in ofdm?

we use IFFT in OFDM to convert the signal from frequency domain to time domain

the idea in OFDM generation, the transmitter accepts a stream of data and converts them to symbols using modulation technique, for example QPSK.

Then the S/P converter takes the output 4 symbols and mixes each one with one of the subcarrier, we now have 4 sine waves then add the 4 sine.

Now we notice that S/P conversion stage the data represent as a function of frequency. After addition stage stage the data represent as a function of time.

This conversion is actually a well-known computational technique called the inverse Fast Fourier transform.