Opportunistic Role Selection for Two-User Amplify-and-Forward Relaying Systems

In this paper, we establish a two-user opportunistic cooperative framework, where two users compete to transmit their own information to a common destination. For such a framework, by relying on the instantaneous channel conditions, either of the two users can opportunely play the role of an information source, and the other user will serve as an amplify-and-forward (AF) relay. To enhance the transmission reliability in terms of system outage probability, an optimal and centralized role selection (C-ROSE) scheme is first proposed, which maximizes the received signal-to-noise ratio (SNR) at the destination. To alleviate the signaling overhead of C-ROSE, a distributed ROSE (D-ROSE) scheme is presented by invoking a local channel state information (CSI) exploitation/decision mechanism, which can lower the signaling overhead while maintaining almost the same outage performance as that of C-ROSE. With the user fairness in mind and owing to the distributed decision rule of D-ROSE, a proportional fair scheduling (PFS) strategy is incorporated into D-ROSE, such that the two users have an equal opportunity to act as a source. We refer to this new ROSE scheme as P-ROSE. To reveal the inherent impacts of different link statistics on the system outage behavior, closed-form lower and upper bound expressions are derived for the outage probability of the three ROSE schemes. Moreover, asymptotic outage analysis points out that the high-SNR outage behaviors of the three schemes are exactly the same and are dominated by the user-destination links, irrespective of the interuser link. Furthermore, both theoretical analysis and numerical results manifest that 1) under the scenario of balanced user-destination links, the outage performance of the three ROSE schemes are almost the same over the entire SNR regions and that 2) under the scenario of unbalanced user-destination links, the outage performance of P-ROSE is inferior to that of C-ROSE and D-ROSE from low-to-medium SNR regions, particularly in the case of a weak interuser link. In addition, representative numerical examples are shown to demonstrate the effects of node placement on the system outage performance, signaling overhead, and user fairness of the three ROSE schemes, from which the tradeoff between outage performance and user fairness of the three schemes are numerically examined.