Improving utilization of the radio spectrum is the main goal of Cognitive Radio Networks (CRN). Recent studies made use of cooperative relay technology in cognitive networks, to increase transmission diversity gain. In this paper, the authors consider an OFDM based cooperative cognitive network with a pair of source-destination nodes as the Primary User (PU), and a pair of source-destination nodes - which is assisted with a relay - as the Secondary (cognitive) User (SU). Both primary and secondary users share a same spectrum. In a two hop transmission, the source transmits in the first hop, and the half-duplex relay decodes the message, re-encodes and forwards it to the destination in the second hop on a different subcarrier.