Abstract: Outage probabilities of all relays selection scheme and opportunistic relaying selection scheme based on decode-and-forward (DF) protocol were analyzed. Especially, exact closed-form results of the outage probability for the two schemes over Rayleigh fading channels were derived. Further investigation of the outage performance was made and asymptotic expressions for outage probability were obtained in the high signal-to-noise ratio (SNR) regime. Theoretical analysis and numerical simulation results show that the outage results of the two methods arc equal in high SNR or in the case that the perfor-mance of relay-destination channels is better than that of sourccrclay channels. The outage performance of all relays selection scheme is better than that of opportunistic relaying selection scheme when the performance of channels from the source to relays is better than that of the channels form relays to the destination.

Key words: Cooperative communications,Decode-and-forward DF,Rayleigh fading,0utage probability,Relay selection