Active user cooperation in fading multiple-access channels

Consider a two-user fading cooperative multiple-access channel (CMAC), where each user, along with transmitting its own information to the destination, helps the other user by forwarding the latterpsilas information. It is assumed that the transceivers (nodes) are capable of half-duplex communication and that full channel state information (CSI) is available at all the nodes. Cooperative transmission strategies based on a flow-theoretic approach are proposed. The proposed strategies include a sub-optimal but simple orthogonal relaying strategy for the CMAC (OR-CMAC), and a flow-optimized strategy for the CMAC (FO-CMAC). The proposed protocols are evaluated in terms of achievable rate regions and outage performances, for different scenarios. The boundaries of the achievable rate regions for the proposed strategies are characterized by convex optimization formulations, and FO-CMAC is shown to provide improved achievable rate regions as compared to the conventional multiple-access channel (MAC), and a previously proposed strategy based on the decode-and-forward (DF) approach. Although sub-optimal in terms of the achievable rate region, OR-CMAC is shown to provide outage performance close to FO-CMAC for the scenarios considered. Moreover, by comparing to a lower bound on the outage probability, both the proposed strategies are shown to achieve the optimal diversity order of two for the required rate region of interest.