The quality of channel state information (CSI) plays a significant role in the performance of relay selection (RS) schemes in cooperative communications. Particularly in a highly mobile environment when there exists delay between the selection and data transmission instants, the quality of CSI often degrades, given to what is known as outdated CSI phenomena. In this paper, we first outline several single RS schemes that achieve full diversity when using perfect CSI and prove that the diversity order of these schemes reduces to unity if outdated CSI exists. To deal with the performance deterioration due to the outdated CSI phenomena, we propose a new multiple RS scheme for both amplify-and-forward (AaF) and decode-and-forward (DaF) relay systems, namely, the plus normalized threshold opportunistic RS . In particular, we first opportunistically select best relays from all candidate relays with respect to end-to-end signal-to-noise ratios (SNRs) for AaF and relay-to-destination SNRs for DaF. Then, the ratios of the SNRs on the rest of the candidate relays to that of the th highest SNR are tested against a normalized threshold , and only those relays passing this test are selected in addition to the best relays. The performance of the scheme is theoretically analyzed in terms of outage probability, and its asymptotic diversity order is also examined. In addition, we outline a distributed selection protocol that makes no assumption of the global CSI at each relay. Numerical results confirm the analysis and show the advantage of the proposed scheme against existing counterparts in highly dynamic networks.
['Moyuan Chen', 'Ted C.-K. Liu', 'Xiaodai Dong']
Opportunistic Multiple Relay Selection With Outdated Channel State Information