OFDM demodulation, how to set the sampling rate? I have a signal for which I know the OFDM burst length (from the autocorrelation function of the signal) T_burst and the FFT size 1024 (by measuring the channel spacing) and the guard interval (128). 


How should sample such a signal for OFDM demodulation ? 
What is the ideal sample rate for that ? 
Is it (1024+128)/T_burst?


I plan to use GNU radio for demodulation, like this (this image is from IRC channel - credit to drmpeg):
Your receiver will consist of, apart from synchronization blocks, an FFT. You want that FFT to generate one output bin for each of the subcarriers. Therefore, you should make use of the relationship between the input sample rate of a DFT to its output bin spacing:

$$
\Delta f = \frac{f_s}{N_{FFT}}
$$

where $\Delta f$ is the subcarrier spacing and $f_s$ is the sample rate (both in Hz). Assuming you know the subcarrier spacing and the FFT size, then it's straightforward to choose the sample rate:

$$
f_s = N_{FFT} \Delta f
$$