I. Introduction
Cooperative diversity is a new diversity technique that was proposed in the literature as a form of spatial diversity system [1]–[5]. It uses the broadcast nature of the wireless medium and its basic idea is that the terminals, which are in the coverage area of a transmitter, can forward an “overheard” version of the transmitted signal. This virtual antenna array produces the desired diversity gain at the destination. Among the proposed re-forward strategies, the Amplify-and-Forward (AF), which involves a simple scale and retransmission of the received signal by the relay nodes, seems to give a good performance and complexity trade-off [6].