Opportunistic space-time coding to exploit cooperative diversity in fast-fading channels

In this paper, we propose a novel scheme coined opportunistic space-time coding (OSTC) to deal with the outdated channel state information (CSI) problem for opportunistic relay selection (ORS) systems. A pair of relays, instead of a single relay, is opportunistically selected in terms of the instantaneous CSIs. Alamouti scheme, a unique space-time code achieving both full-rate and full-diversity, is applied to encode the regenerated signals at this pair of relays. To evaluate the performance, a closed-form expression of outage probability is derived for OSTC. The analytical and simulated results reveal that a full diversity on the number of cooperating relays is achieved by OSTC when using perfect CSI. In the fast-fading channels, where the outdated CSI drastically degrades the performance of ORS and reduces its diversity order to 1 (i.e., no diversity), a cooperative diversity with an order of 2 can still be achieved by the proposed scheme. Most importantly, this performance gain comes without any sacrifice in the spectral efficiency, which is the drawback of Generalized Selection Combining schemes.