Select and Forward: Unifying Cooperative Diversity and Routing in a common framework

We study the diversity multiplexing tradeoff in cooperative diversity systems involving multiple relays. We show that distributed space time codes are not always fundamentally necessary in realizing the potential gains in these systems. In particular, under a wide variety of scenarios, we show that a smart relay selection algorithm in conjunction with conventional protocols for the three node system can achieve the same diversity-multiplexing tradeoff achieved by multiple relay systems using distributed space-time codes. We consider both, schemes that select relay before the the source transmission (proactive relay selection) and schemes that select the relay after the source transmission (reactive relay selection) and observe that their performance in terms of the diversity-multiplexing tradeoff are identical. We then consider systems where the destination is able to send a single-bit decision feedback and show that one can achieve the diversity multiplexing tradeoff of a multi-input-single-output antenna system. Our results present an alternative to distributed space-time codes for realizing the potential gains in multiple relay, user cooperation systems and open new avenues for fruitful interaction between routing and cooperative diversity.