Cooperative diversity using per-user power control in the multiuser MAC channel

We consider a multiuser MAC fading channel with two users communicating with a common destination, where each user mutually acts as a relay for the other one as well as transmits his own information. We propose a power control-enhanced cooperative transmission scheme allowing each user to allocate a certain amount of power for his own transmitted data while the rest is devoted to relaying. The underlying protocol is based on a modification of the so-called non-orthogonal amplify and forward (NAF) protocol in Azarian, K. et al, (2005). We develop capacity expressions for our scheme and derive the rate-optimum power allocation, in closed form. Our results indicate that even in a mutual cooperation setting like ours, on any given realization of the channel, one of the users will always allocate zero power to relaying the data of the other one, and thus act selfishly.