We study power allocation for the decode-andforward cooperative diversity protocol in a wireless network under the assumption that only mean channel gains are available at the transmitters. In a Rayleigh fading channel with uniformly distributed node locations, we aim to find the power allocation that minimizes the outage probability under a short-term power constraint, wherein the total power for all nodes is less than a prescribed value during each two-stage transmission. Due to the

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... nal and implementation complexity of the optimal solution, we derived a simple near-optimal solution. In this nearoptimal scheme, a fixed fraction of the total power is allocated to the source node in Stage I. In Stage II, the remaining power is split equally among a set of selected nodes if the selected set is not empty, and otherwise is allocated to the source node. A node is selected if it can decode the message from the source and its mean channel gain to the destination is above a threshold. In this scheme, each node only needs to know its own mean channel gain to the destination and the number of selected nodes. Simulation results show that the proposed scheme achieves an outage probability close to that for the optimal scheme obtained by numerical search, and achieves significant performance gain over other schemes in the literature. Index Terms-Cooperative diversity, decode-and-forward, power allocation, outage probability, relay networks I. INTRODUCTION Cooperative diversity is a set of techniques that exploit the potential of spatially dispersed user antennas to improve communications reliability [1]- [3] . The use of cooperation to achieve diversity in wireless systems was studied in [1] from an information theoretic point of view, where superposition block Markov encoding and backward decoding were employed to achieve the rate region of cooperation. Later, several low-complexity amplify-and-forward and decode-andforward relay schemes were proposed and studied in [2], [3] . These algorithms were extended to large networks, and both repetition-based and space-time-coded cooperation were considered. In these papers, a common transmission power was assumed at each node. Recently, various resource allocation problems were studied in [4]-[10] for cooperative diversity systems. For parallelrelay AWGN channels, the optimum power allocations were derived for both amplify-and-forward and decode-and-forward