Wireless networks technologies are on a rapid increase in demand nowadays. Wireless networks are characterized by fixed spectrum assignment policy that is imposed by FCC allowing more and more number of unlicensed users to transmit their signals in licensed bands so as to efficiently utilize the available spectrum due to the increasing demand for frequency spectrum and limited resource availability. Cognitive radios(CR) increase the efficiency of spectrum utilization(Quality of Service-QoS) through interference avoidance and local re-use of frequency bands originally designated for exclusive operation. Cognitive Radio can smartly senses and adapts with the changing environment by altering its transmitting parameters，such as modulation，frequency，frame format etc. The main challenges with cognitive radios are that it should not interfere with the licensed users and should vacate the band when required. These radios(CR) should be able to sense the signals at a very fast rate so as to make necessary decision whether to vacate or use the available spectrum. In this thesis，performance evaluation is done between cooperative and non cooperative spectrum sensing schemes which uses Amplify-and-Forward algorithm，agility gain，energy detection，hard and Soft decision rules in CR in order to evaluate the performance advantages of cooperation over non-cooperative Cognitive Radio networks .I tried to go a bit further to include another relay node at different relay time slot but the analysis of this scenario is not emphasized. Furthermore，based on the energy detection scheme OFDM system model was analyzed by placing a variable channel gain between the unlicensed users and the fusion center in vector notation.
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