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Like most wireless network protocols, IEEE 802.15.4 adopts a CA (Congestion Avoidance) algorithm to avoid early collisions of new frames by randomizing its first transmission delay instead of delivering them immediately. The traditional CA scheme of IEEE 802.15.4 selects its random access time from the predetermined range without considering the degree of underlying network congestion. This static behavior often incurs a long delay to settle in the suitable range for the current network load or frequent clashes especially when heavy traffic lasts long. This paper proposes an ACA (Adaptive Congestion Avoidance) algorithm which dynamically adjusts the initial back off range according to the measurement of the average collision rate. It also introduces an analytical model to predict the performance spectrum in which ACA algorithm's throughput falls. With only small deviations from corresponding simulations, our model shows that ACA scheme can improve the throughput of IEEE 802.15.4 by up-to 41% while shortening the frame delays.