Orthogonal or Non Orthogonal Amplify and Forward Protocol: How to Cooperate?

In this paper we consider a source, N relays, a destination, a channel realization and a per-slot global power constraint and we try to resolve the following problem: what is the best way to communicate: with direct transmission or using the orthogonal Amplify and Forward protocol (OAF) or the non orthogonal (NAF) one? This is equivalent to resolve a power allocation problem on the cooperation frame which aims to maximize the channel capacity. The obtained solution shows that the selected cooperative protocol depends only on the available power at the relays. However the decision to cooperate depends on the channel realization. We propose a power allocation scheme with relay selection that improves the outage probability and the channel capacity compared to the selective OAF and the NAF protocols.