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In this paper, we propose a new cooperative spectrum sensing method based on adaptive activation of energy detector (ED) and preamble detector (PD) for cognitive radio networks. The ED performance is highly degraded under low signal to noise ratio and noise uncertainty condition. To alleviate the problem of ED and increase the sensing performance, we have used adaptive activation of energy efficient ED and reliable PD. As the first step of our proposed method, we have used ED to take a decision in the clear region where the detector can easily make its own local decision. There are two thresholds for the measured energy in the first step. If the sensed energy in the first step is between these two thresholds, the second step which involves the activation of cooperative PD is triggered to make an appropriate decision on the presence or absence of primary users's signal. Otherwise, the second step detector PD is not activated. In this way, we can enhance the detection performance and energy efficiency by taking the collaborative advantages of ED and PD at the same time. Simulation results validate the effectiveness of our proposed method as compared with conventional schemes.