  Spectrum sensing is essential in cognitive radio to enable dynamic spectrum
access. In many scenarios, primary user signal must be detected reliably in low
signal-to-noise ratio (SNR) regime under required sensing time. We propose to
use stochastic resonance, a nonlinear filter having certain resonance
frequency, to detect primary users when the SNR is very low. Both block and
sequential detection schemes are studied. Simulation results show that, under
the required false alarm rate, both detection probability and average detection
delay can be substantially improved. A few implementation issues are also
discussed.
