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In response to the growing market demand for high-performance wireless local area networks (WLANs) with quality-of-service (QoS) support, the IEEE 802.11 standard adopts the 802.11e as the medium access protocol and 802.11n-based products, which utilize multiple-input-multiple-output (MIMO) transmission systems. The medium access collision avoidance of IEEE 802.11e, even with IEEE 802.11n at the physical layer, still utilizes the enhanced distributed coordination function (EDCF) mechanism. It is known that collisions cause significant performance degradation in IEEE 802.11e EDCF-based systems. In this paper, we utilize the multiple spatial channels of MIMO technology to propose collision-mitigation enhancements to the IEEE 802.11e EDCF. The key idea of the scheme, which is called MIMO-based EDCF (M-EDCF), is the sharing of the spatial channels during the medium contention period, i.e., instead of accessing the medium using all the spatial channels, the accessing nodes use a subset of the available spatial channels. As the number of concurrent spatial channels used is less than or equal to the spatial degree of freedom, receivers can decode the transmitted signals of different medium access contenders and then coordinate their responses to avoid potential collisions. The spatial channel sharing also enables transmitters to sense the medium and terminate if they detect other ongoing medium access attempts. Simulation results show that the M-EDCF scheme substantially reduces medium access collisions. Based on the preliminary results, an adaptive optimized length of the collision-avoidance window is derived. The performance of the M-EDCF scheme based on optimized online spatial channel sharing demonstrates that the scheme further reduces medium access collisions and boosts medium utilization.