With the boom of wireless devices,the number of wireless users under wireless local area networks (WLANs) has increased dramatically.However,the standard backoff mechanism in IEEE 802.11 adopts fixed initial contention window (CW) size without considering changes of network load,which leads to a high collision probability and low channel utilization in bursty arrivals.In this paper,a novel CW dynamic adjustment scheme is proposed to achieve high throughput performance in dense user environment.In the proposed scheme,the initial CW size is dynamically adjusted to optimum according to the measured packet collision probability.Simulation results show that the proposed scheme can significantly improve the throughput performance.