[[abstract]]In this project, we investigate the throughput performance of IEEE 802.11 Wireless Local Area Networks (WLAN) with multiple stations using different data rates. IEEE 802.11 WLAN PHYsical Layer (PHY) provides multiple data rates with different Modulation and Coding Schemes (MCS). If one station experiences bad channel conditions, it would degrade the data rate to achieve a more reliable transmission quality. However, 802.11 Distributed Coordination Function (DCF) protocol at Medium Access Control (MAC) layer essentially provides equal transmission opportunities to each transmitting station. Thus, the throughput of stations with high data rates will be restricted within the lowest rate used by some stations, resulting in the degradation of system throughput. Such the phenomenon is so called “performance anomaly” and can be an extremely severe problem in the network which provides large-scale PHY rates, e.g. 802.11g WLAN with rates ranging from 1 Mbps up to 54 Mbps. We will deeply investigate the impact of “performance anomaly” on WLAN system throughput by theoretical analysis, and assess the performance of the existing Link Adaptation (LA) schemes. Furthermore, we will propose our cross-layer LA scheme which performs the optimization between PHY and MAC layer, and evaluate the performance of the proposed method by comparison with the existing LA schemes. The proposed scheme is expected to improve system throughput and also provide access fairness among stations by taking into account the PHY/MAC dependence and therefore performing the cross-layer optimization. By both the theoretical analysis and the cross-layer solution, we hope to thoroughly provide an in depth view about the design and the implementation of the LA scheme in IEEE 802.11 WLAN environments.