In this paper, the problem of optimal power allocation among the relays in two-hop decode-and-forward cooperative networks is considered, and a new power allocation scheme which minimizes the average symbol error probability under limited total power of all relays and limited maximum powers of individual relays is developed. It is shown that the corresponding power allocation optimization problem is convex and, therefore, can be efficiently solved using interior point methods. Moreover, an

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... ximate closed-form solution in the form of waterfilling is also developed. Simulation results confirm the improved performance of the proposed power allocation scheme over other schemes. Index Terms-Power allocation, decode-and-forward relay networks, cooperative communications, convex optimization.