The rapid growth of wireless communication system has put pressure on radio spectrum usage. Due to the widely used fixed spectrum access, spectrum utilization is very low. In order to overcome this problem, cognitive radio, which leads the way for dynamic spectrum access capability is necessary. Cognitive radio listens to the channel and if that channel is idle, it will transmit through it which enables to access the unused spectrum and vacate it if needed by PU (primary user). The detection decision would be done by either a single SU (secondary user) or by multiple SUs cooperating with each other. Adaptive threshold based Energy detection is the detection technique used in this thesis. In this technique, the received signals SNR (Signal to noise ratio) will be estimated by the SUs and if that SU is not efficient enough to detect the idleness of the channel due to the reception of weak signal, the SUs will cooperate to detect the PU signal by applying SC (selection combining) scheme at the central fusion center otherwise they will decide by themselves. This provides a reliable detection with less overhead by using single and cooperative energy detection adaptively. From the simulation results, with same parameter adjustments the AWGN (Additive white Gaussian noise) channel had given a better performance than Rayleigh fading channel. This was upgraded in using cooperative energy detection which provides a better detection probability with low SNR and sensing time. Also noise uncertainty factor degrades the detection performance which was improved by cooperative detection. The proposed system gives a much better detection performance at low SNR region cooperating three SUs and using non cooperative energy detection for high SNR regions.