Energy Efficient Relaying and Coalition-Forming in Relay Networks

Relaying is often advocated for improving system performance by enhancing spatial diversity in wireless networks. In this paper, we address the issue of energy tradeoff made by relay nodes between transmitting their own data and forwarding other nodes´ information in fading channels. We first propose a power control policy in a two-node relay network under which total energy consumption across both nodes is minimized while meeting both outage probability requirements. Based on this power control algorithm, we consider the problem of forming optimal partial coalitions of relays in an N node system subject to selfish constraints: A node participates in a relay pair (or chain) if and only if the energy cost of relaying is lower than the cost of direct transmission by the node to the destination itself. We develop a simple (1,2)-polynomial time bi-criteria approximation for this NP-hard problem. The energy cost provided by the approximation is at most that of the optimal relay pairing, while the constraints are violated by at most a factor of two. The running time of the approximation algorithm is polynomial, as it requires the solution of a relaxed linear programming instance of the original integer programming problem.