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The IEEE 802.15.4 standard is able to achieve low-power transmissions in low-rate and short-distance wireless personal area network (WPAN). The slotted carrier sense multiple access with collision avoidance (CSMA/CA) is used for contention mechanism. Sensor nodes perform backoff process immediately when the clear channel assessment (CCA) detecting busy channel. It may neglect the implicit information of the failed CCA detection and further cause the redundant senses. The blind backoff process in the slotted CSMA/CA will cause lower channel utilization. This paper proposes an additional carrier sensing (ACS) algorithm based on IEEE 802.15.4 to enhance the carrier sensing mechanism for the original slotted CSMA/CA. An analytical Markov chain model is developed to evaluate the performance of ACS algorithm. Both analytical and simulation results show that the proposed algorithm performs better than IEEE 802.15.4, which significantly improves throughput, average medium access control (MAC) delay and power consumption of CCA detecting.