This document discusses cooperative spectrum sensing based on an adaptive threshold for cognitive radio. It begins with an introduction to cognitive radio and spectrum sensing. It then discusses energy detection, the most common spectrum sensing method due to its low complexity. However, energy detection may not perform well in low signal-to-noise ratio environments. To address this, an adaptive threshold is proposed that varies based on the received primary user signal's SNR. The document also discusses cooperative spectrum sensing, where multiple cognitive radios sense the spectrum individually and share their decisions with a fusion center. The fusion center combines the decisions to make an overall determination with better performance than non-cooperative sensing. Simulation results showed that an adaptive threshold combined with cooperative spectrum sensing provides better detection