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Lifetime extension is a key design issue for wireless sensor networks (WSN) with battery-operated nodes. Compared with direct transmission, transmission power can be significantly reduced by cooperative communication, because it can effectively mitigate multi-path fading by introducing space diversity. In this paper, BER analysis for M-PSK and M-QAM modulation in noncooperative and cooperative situations is firstly presented. Then power is optimally allocated to source and relay nodes with the objective of minimizing the total transmission power under the average BER constraint. Based on these works, a new distributed cooperative MAC protocol is proposed to improve the lifetime of WSN. In this protocol, both channel state information (CSI) and residual energy information (REI) of sensor nodes are considered to choose the cooperative nodes. Simulation results show that when the access point (AP) is above a certain height, the proposed cooperative protocol can significantly prolong network lifetime compared with non-cooperative MAC protocols.