In this paper, we propose a distributed relay selection algorithm based on statistical channel state information (CSI) in amplify-and-forward (AF) cooperative networks. A tradeoff is made between energy efficiency and system outage performance. A forwarding threshold is obtained by minimizing the average total power consumption. Each relay individually decides whether to participate in forwarding the source signals according to a forwarding threshold. A distributed multiple relay selection (DMRS) algorithm is proposed, in which all candidate relays have the possibility of forwarding the signals and the threshold is obtained by numerical search. The DMRS algorithm obtains good outage performance with low power consumption.
A distributed single relay selection (DSRS) algorithm with low complexity is then investigated under the assumption that only one relay forwards the signals. Simulation results indicate that the proposed algorithms provide significant performance gain in terms of energy efficiency over the existing AF relay selection algorithms.