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The time-division-multiple-access-based (TDMA) protocol has been widely utilized as a reliable media access control (MAC) mechanism. By allocating each user a dedicated time slot, a given TDMA user transmits its packets in its exclusively assigned time slots, whereas other users are in idle mode (not transmitting). However, in this paper, a cooperative TDMA (C-TDMA) method is investigated, which enables cooperative transmissions among multiple TDMA users to improve the probability of success of packet transmissions. Specifically, a TDMA user not only transmits packets in its assigned slots but monitors/overhears other users' packets in other time slots as well. It will then be able to assist the other users, if needed, to retransmit their failed packets through cooperative diversity. For performance evaluations, expressions of three metrics, i.e., network throughput, packet-dropping rate, and average packet delay, are derived, considering a Rayleigh fading channel. Both theoretical and simulation results are presented to demonstrate that the throughput, dropping rate, and delay performance are all improved significantly with the use of C-TDMA.