To Relay or Not to Relay? Optimizing Multiple Relay Transmissions by Listening in Slow Fading Cooperative Diversity Communication

The formation of virtual antenna arrays among cooperating nodes, distributed in space, has been shown to provide improved resistance to slow fading and has attracted considerable interest. Scaling cooperation in practice to a large number of participating relay nodes is an open area of research. It was recently shown that appropriate selection of a single, “opportunistic” available relay that maximizes a function of the end-to-end, instantaneous channel conditions, achieves the same diversity-multiplexing gain tradeoff with schemes that require multiple and simultaneous relay transmissions (possibly at the same frequency band) and employ distributed space-time coding. In this work, we present low SNR analysis in slow fading environments that shows equivalence of opportunistic relaying to optimal decode-and-forward, under an aggregate power constraint, outperforming schemes based on distributed space-time coding. Amplify-and-forward schemes under an aggregate power constraint are also examined, demonstrating improved performance when a single, opportunistic relay is used. The findings suggest that cooperative diversity benefits, under the assumptions followed in this work, are maximized when cooperative relays choose not to transmit, but rather choose to This work was supported by NSF under grant number CNS-0434816, the MIT Media Laboratory Digital Life Program and a Nortel Networks graduate fellowship award. Part of this paper has been submitted to IEEE WCNC 2006. November 22, 2005 DRAFT cooperatively listen, giving priority to the transmission of a single, opportunistic relay. In other words, cooperation benefits are maximized when relays act as sensors of the wireless channel and not necessarily as active retransmitters. Since no simultaneous transmissions are utilized, opportunistic relaying can be implemented in existing radio front ends and a radio implementation is briefly discussed.