Recently, I am learning the principle of OFDM, and the reference is http://defenseelectronicsmag.com/site-files/defenseelectronicsmag.com/files/archive/rfdesign.com/images/archive/0101Puegel30.pdf.
Everything is reasonable untill page 3 at the left and middle column.
The IFFT takes in N symbols at a time where N is the number of subcarriers in the system. Each of these N input symbols has a symbol period of T seconds.
Then, in expectation, the output of the IFFT, aka a OFDM symbol, should be combined with N symbols that are arranged to N orthogonal sinusoids. That is, a OFDM symbol with period of T seconds. However, at the middle column, the description is out of my expectation.
The block of N output samples from the IFFT make up a single OFDM symbol. The length of the OFDM symbol is NT where T is the IFFT input symbol period mentioned above.
Why the OFDM symbol period is N*T but not just T? A simple example in my understanding will be:
If it is, then the OFDM should not have the higher transmission rate or speed, because it costs the transmission time the same with the original modulation method that without the OFDM modulation.