Conditionally-Distributed Link Selection in Multiuser Multirelay Networks with Transmit Beamforming

This work analyzes the outage probability and the mean spectral efficiency of a link-selection scheme for a multiuser multirelay cooperative network with transmit beamforming. The proposed scheme is conditionally distributed, in the sense that it can be implemented in a distributed manner depending on the channel conditions. In the considered network, a base station equipped with M antennas communicates with one out of L single-antenna mobile stations by selecting either the direct link or a relaying link. The latter is assisted by one out of N singleantenna amplify-and-forward relay stations using a fixed-gain relaying protocol. Expressions in integral form for the upper and lower bounds of the outage probability and the mean spectral efficiency are provided and validated byMonte Carlo simulations. Moreover, an asymptotic analysis of the outage probability is performed and reveals that the system achieves full diversity order, equal to ML+N. Importantly, it is shown that the mean spectral efficiency of the proposed scheme is always above half of that one attained by direct transmission.