Conventional multi-carrier operation as it is used for FDM works simply by selecting a number of center frequencies – one for each carrier to be used.
The center frequencies must be spaced. In fact there is a trade-off between minimizing interference between different carriers and using the total bandwidth efficiently. In other words each carrier uses an upper and lower guard band to protect itself from its adjacent carriers.
Nevertheless, there will always be some interference between the adjacent carriers – known as Adjacent Carrier Interference (ACI) Especially for rectangular pulses the guard bands must be quite big, as the rectangular pulse has a huge spectrum. Otherwise we would have to apply a pulse shaping filter, but this would destroy the rectangular form of our pulse and thus complicate inter-symbol interference handling. For the rectangular pulse there is a better option possible and it is even easier to implement.
The spectrum of a rectangular pulses shows null points exactly at integer multiples of the frequency given by the symbol duration. Orthogonally avoids ACI to some extent. Thus OFDM simply places the next carrier exactly in the first null point of the previous one. With this we don’t need any pulse-shaping and between OFDM carriers using the same symbol duration TS and the same grid of center frequencies no guard bands are required.
This allows a tight packing of small carrier -called the sub-carriers or tones- into a bigger frequency band. Of course at the edges of this bigger band there might be some guard bands required to protect systems on adjacent bands from out-of spectrum emissions by the OFDM system.