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IEEE 802.11 is currently the most popular standard for Wireless LANs. The Distributed Coordination Function (DCF) defines the primary medium access control of 802.11, which uses the CSMA/CA mechanism and Binary Exponential Backoff (BEB) for Contention Window (CW) adjustment when collisions occur. In this paper, we propose a new CW adjustment scheme, CW Idle-Slots-based Control (WISC), using a control-theoretic approach. Specifically, we design and implement a PD (Proportional and Derivative) controller at each contending station, that dynamically adjusts CW based on a locally available channel state, i.e., the average number of consecutive idle slots between two transmissions, such that the channel state converges to the optimal value. Simulation results demonstrate that the new scheme outperforms the standard BEB in terms of both throughput and fairness, especially at high contention levels.