I read LTE PHY profile and do not understand the relationship between number of FFT and bandwidth. For example LTE downlink channel bandwidth 1.25MHz and 5MHz have 128 FFTs and 512 FFTs accordingly. Another example is 802.11 a bandwidth 20MHz, FFT size 52.
I design an OFDM system system at carrier frequency 910MHz, with FFT size for example 64, how much coherent bandwidth that the system need? What is the formula? What other factors should be taken into account?
Add >> 18 Aug (I don't have enough reputaiton to add comment on Jason R reply)
When I test a simple software defined radio OFDM system with parameters as FFT = 256, data carrier per symbol = 125, number of pilot = 25, bandwidth = 5MHz, carrier frequency = 910MHz, the channel estimation wouks fine. If I change bandwidth to 1MHz, then the channel estimation goes wrong. In this case I reduce sampling frequency 5 times, so carrier spacing is reduced 5 times too.
I think the carrier spacing must be greater than a limit so that intersymbol interference (or inter-carrier interference?) is small enough to not cause channel estimation failture. How to find the carrier spacing limit?
Or delay spread $\tau \ll T$ where T is symbol interval may cause the problem. How to calculate the limit $\tau$?