1Improving Amplify-and-Forward Relay Networks: Optimal Power Allocation versus Selection

Abstract

We consider an Amplify-and-Forward cooperative diversity system where a source node communicates with a destination node with the help of one or more relay nodes. The conventional system model assumes all relay nodes participate, with the available channel and power resources equally distributed over all nodes. This approach being clearly sub-optimal, we first present two power allocation schemes to minimize the system outage probability, based on complete channel state information and channel statistics, respectively. We further show that the proposed optimal power allocation methods minimize system symbol error rate as well. Next, we propose a selection scheme where only one “best ” relay node is chosen to assist in the transmission. We show that the selection-AF scheme maintains full diversity order, and at reasonable power levels has significantly better outage behavior and average throughput than the conventional all-participate scheme or that with optimal power allocation. Finally we combine power allocation and selection to further improve performance.