Cognitive radio is emerging wireless technology which offers reallocation and reutilization of the unused spectrum. Not all the network runs with highest load at all the time. Therefore part of spectrum always remains unused. In a multi radio environment cognitive radio allows the user to request for spectrum while roaming from another service provider. In case the service provider has free spectrum, it can reallocate the spectrum to the demanding user. In this paper, this provides a framework for analyzing spectrum sensing, void user detection, reallocation using two methods namely frequency domain analysis method and QR decomposition method.