Energy efficiency contours for amplify-and-forward and decode-and-forward cooperative protocols

In this paper we compare the energy efficiency of different cooperative schemes by means of energy efficiency contours. We consider a three-nodes cooperative scenario, employing either the amplify-and-forward (AF) or the decode-and-forward (DF) protocol. In the case of the DF protocol we analyse the performance of both repetition coding (RC) and parallel coding (PC). Our analysis shows that the maximum energy efficiency operating point is usually related to the allocation of different transmission rates to each user, while enforcing rate fairness leads the system to a sub-optimum operating point in terms of energy efficiency. Depending on the particular topology, the most energy efficient scheme may be AF or DF, with a clear advantage of DF when the two users are close. Moreover, we demonstrate that the exploitation of a return channel may have a large impact in the energy efficiency, being able to almost double it.