Green cooperative communication using threshold-based relay selection protocols

This paper proposes new approach to relay selection using a threshold-based transmission protocol for a system with multiple amplify-and-forward relays and users. The novelty of this protocol includes (1) economical feedback that reduces signal processing complexity, as well as (2) adaptively interrupted transmission that can save significant RF power when the selection requirement is not met. Applied to the downlink, this protocol uses a threshold on the first-hop (base-station-to-relay) SNR for relay selection. One-bit feedback is sent from each relay to the base station, reporting the comparison result between the base station-relay link SNR and threshold. Transmission takes place for a relay satisfying the selection condition of having its received SNR greater than a threshold or is otherwise switched off. Numerical results show that for low average SNR scenarios (where relaying is required), the selection threshold can be appropriately chosen relative to the number of relays to dramatically reduce transmit power without sacrificing outage performance.