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This paper presents cooperative spectrum sensing techniques for cognitive radio (CR) systems. The spectrum sensing detects the presence of a primary user (service) on an associated spectrum. The accuracy of spectrum sensing depends on both sensing time and number of sensing nodes (i.e. secondary users) participating in the sensing process. We derive optimal number of secondary users for cooperative spectrum sensing to minimize spectrum detection error probability and optimal spectrum sensing time to maximize data transmission throughput. A system model which consists of a primary TV spectrum and multiple Wi-Fi secondary users is considered. The proposed CR scheme can solve recent spectrum shortage problem in Wi-Fi networks due to traffic explosion by smart phones. We propose K-out-of-N-rule which is the most efficient cooperative sensing rule by analytical and numerical results.