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The contention-based DCF suffers from collision seriously under heavy load conditions. In this paper, a novel medium access control protocol called contention-polling duality coordination function (CPDCF) is proposed to enhance the DCF-based MAC protocol. A polling ACK mechanism is introduced to admit the designated station to transmit without performing any contention process. By eliminating collisions and preventing waste from idle backoff slots, the system capacity is increased effectively. A mathematical model is developed to analyze the saturation throughput of the CPDCF with various CP combinations. The performance in terms of fairness is studied as well. The model accuracy is verified via extensive simulations. Numerical results show that the well-managed CPDCF can achieve a significantly higher system capacity than the legacy DCF does. The CPDCF is also employed to enhance the performance of IEEE 802.11e EDCA. We conclude that the proposed CPDCF is quite feasible, effective, and can be easily integrated with the widespread DCF-based devices.