For determining FFT and GI (guard interval) modes within a receiver, a correlation signal is generated from an in-phase and quadrature (I/Q) stream. An plurality of GI mode division signals are generated by processing the correlation signal delayed by different delays. For each of the GI mode division signals, a respective peak-value position and a respective peak-value is determined to be used for determining the GI and FFT modes of the I/Q stream. The correlation calculator generates the correlation signal dependent on the FFT mode to minimize memory capacity and cost of the receiver.