This book introduces cooperative spectrum sensing using OSTBC in order to achieve cooperative diversity in cognitive radio networks. Transmit diversity or gain is achieved by using cognitive relays between primary and cognitive controller. The signals received from the primary users are amplied by the cognitive relays and further forwarded to the cognitive controller where decisions are made on the basis of the information collected from each cognitive relay. The cooperative relaying protocol used here in cognitive relays is based on an amplifying-forward scheme. The energy detector performance is analyzed over an independent Rayleigh fading channel. Detection and false alarm probabilities are derived and manipulated using OSTBC on PU and SU through AF protocol in cooperative communication. The performance of the system is analyzed with single and multiple relays and with and without direct path between the PUs and SUs. Maximum ratio and selection combining schemes are used in energy detector and the results are compared with and without direct link between primary and secondary users. The results are processed/validated with the aid of Matlab and Mathematica.