Local CSI Based Selection Beamforming for AF MIMO Relay System with Direct Link

We propose a novel transmission scheme for a relay system that consists of a source, an amplify-and-forward (AF) relay, and a destination, which are all multi-antenna nodes and employ beamforming. The proposed scheme is based merely on local channel state information. The source transmits to the relay only when the source finds that the source-destination link will be in outage. In so doing, the proposed scheme lends itself to providing better average spectral efficiency. We derive an exact closed-form expression of the outage probability for Rayleigh fading channels and determine the system diversity gain. Numerical results validate the analytical results and show that, for the scenarios considered and for given spectral efficiency, the proposed scheme provides improved outage probability compared to the recently developed technique.