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Due to the limited energy supplies of nodes, in many applications like wireless sensor networks, energy-efficiency is crucial for extending the networks lifetime. We study the routing problem for multihop wireless ad hoc networks based on cooperative transmission. The source node wants to transmit messages to a single destination. Other nodes in the network may operate as relay nodes. In this paper, we propose a cooperative multihop routing for the purpose of power savings, constrained on a required bit error rate (BER) at the destination. Two efficient power allocation schemes are proposed, which depend only on the statistics of the channels. In the first scheme, each node needs to know only the local channel statistics, and can be implemented in a distributed manner. The second scheme is a centralized power control strategy, which has a higher energy efficiency, at the expense of more complexity and signaling overhead. It is shown that energy savings of 75% are achievable in line networks with 3 relays for BER = 10-4 constraint at the destination.