Simulation of symbol synchronization in a OFDM receiver

Dear all
 I am doing a block for a OFDM receiver that receives as input the ofdm
received signal and gives as output a rectangular signal (for
synchronization) in which each pulse represents a peak in the
autocorrelation function realised over the ofdm received signal. How
can i define a threshold to detect this peak of the correlation
function? Is there another way to do this? I will use different kind of
channels AWGN, TU6,... to test it.
Thanks