The present invention is directed to a method for use in a cognitive radio. A Fourier transform of a time domain signal is performed to obtain a series of frequency domain samples covering substantially the entire predetermined RF spectrum, the time domain signal corresponding to the RF waveform, each frequency domain sample being characterized by a predetermined waveform characteristic. The predetermined waveform characteristic of each frequency domain sample is evaluated in each subcarrier block to obtain a subcarrier block activity level for each subcarrier block. A subcarrier block is selected when its subcarrier block activity level is greater than a predetermined threshold.