Why Padding Zeros before OFDM IFFT?

Hi all,

In many OFDM system(like 802.11a and DVB-T), they always padding zeros
to data before go to the IFFT module. Why use padding zeros?

Best regards,
Davy

Davy said the following on 16/04/2006 02:46:
> 
> In many OFDM system(like 802.11a and DVB-T), they always padding zeros
> to data before go to the IFFT module. Why use padding zeros?

They're known as virtual sub-carriers (or null sub-carriers). 
Essentially, they cause the baseband time-domain signal (post-IFFT) to 
be oversampled, which makes the job of the DAC easier.

-- 
Oli