How does spectrum sensing work in cognitive radio systems?

Cognitive radio dynamically accesses unused spectrum (white spaces) without interfering with primary users. Spectrum sensing methods: Energy detection (compares received power to threshold, simple but susceptible to noise uncertainty), Feature detection (exploits signal characteristics like cyclostationarity, pilots), Matched filter (optimal for known signals), Cooperative sensing (multiple CRs share decisions for improved reliability against shadowing). Challenges: Detecting weak primary signals, hidden node problem, sensing-throughput trade-off, and fast detection under mobility. Database approaches complement sensing by querying spectrum availability from regulatory database.