delay of a signal

hi all

i try to find a part of signal in the main signal using FFT for exmaple

offset_time = 2048;

y = -2*randi([0,1],9000,1) + 1;

x = y(offset_time:offset_time + 100);

if NFFT is less than of offset_time how can i do this?

in my code NFFT is equal to 1024;