Cooperative communications among nodes is an efficient method to decrease signal fading and interference in MAC layer of wireless Ad hoc networks. However, the previous cooperative MAC protocols are designed for IEEE 802.11b, but not for later standard IEEE 802.11g or 802.11n. To increase performance, improve reliability and reduce energy consumption in communications, we propose a cooperative MAC protocol for IEEE 802.11g and being extended to 802.11n. By partitioning the relays with similar

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... ansmission rates into same groups, we can reduce efficiently the time for selecting better relays to help data transmission. Furthermore, we propose a novel retransmission scheme to reduce the retransmission time, while once data transmission is fail, only the relays which have received the data frame will help for retransmission instead of repeating all retransmission cycle. Simulation results show that our proposed protocol outperforms previous work by increasing the throughput and reducing the average delay time in transmission.