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The key enabling functionality of a cognitive radio is to ensure that it would not interfere with primary users, by reliably detecting primary user signals. Due to the low computational complexity, Energy detector is a widely used spectrum sensing method. The performance of energy detector in fading channels has been previously analyzed in some papers. However, these analyses are based on the exact model which makes the results computationally complex. In this paper, we propose some simple and reliable closed forms solutions for probability of detection in Nakagami fading channels. Furthermore, since cognitive users should be able to adjust the spectrum sensing time according to the desired probability of detection and tolerable probability of false alarm, a direct relationship between sensing time and SNR in terms of the aforementioned probabilities is needed. In this article, the N-SNR relationship is also calculated for Nakagami fading channels under some tolerable simplifications.