Spectrum sensing is the basic and important operation in Cognitive Radio (CR) to find the unused spectrum. Energy detector is a popular sensing method because it doesn't require transmitted signal properties, channel information, of even the type of modulation. This study summarizes the performance result of energy detector over Additive White Gaussian Noise (AWGN), Rayleigh fading and Nakagami fading channels. Energy detection with soft decision and hard decision are also studied for different

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... number of cognitive nodes as well as each cognitive node having multiple antennas. The performance of hard decision and soft decision are evaluated by means of complementary Receiver Operating Characteristic (ROC) curves. It clearly shows that the probability of missing detection decreases for increasing the number of antennas in cognitive node and also increasing of cooperated cognitive users.