change in OFDM bandwidth

[MaXes]

Hi,

its been a year since i last touched a book and now i am doing a project involving OFDM. The hardware part will come later, for now i am refreshing my basics.

i just want to confirm something:

1) the OFDM bandwidth depends on OFDM symbol time. After i do the IFFT to form the symbol and then add the cyclic prefix, will the addition of a larger cyclic prefix decrease my bandwidth (because effectively it decreases the symbol rate).

2) considering the cyclic prefix changes the bandwidth, how does the new bandwidth change, because the spacing between sub-carriers remain same (isnt it :S)

best regards
MaXes

 

[Ahmed Alaa]

addition of CP only decreases the bandwidth of individual subcarriers but the overall allocated OFDM bandwidth depends on the subcarrier spcaing and this is not affected by CP. Decreasing subcarrier bandwidth while maintaining subcarrier spacing decreases BW efficiency because you allocated more BW than actually needed.