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Conventional 802.11 medium access control (MAC) protocols have been designed separately from the characteristics of the physical layer so as to simplify the analysis, but this hardly optimizes the overall performance from a system point of view. In this paper, we propose a channel state information (CSI) based random access protocol that takes advantage of multi-packet reception (MPR) in IEEE 802.11-like wireless local area networks (WLANs). Specifically, the proposed MAC protocol dynamically adjusts each nodes transmission probability according to the network population, estimated channel condition, as well as the maximum number of packets that can be simultaneously decoded. System throughput is analyzed by taking adaptive modulation and transmission errors into consideration. Based on such analytical result, an optimal transmission policy is derived to maximize the system throughput. In addition, the impact of imperfect CSI on the throughput performance is investigated and the tradeoff between the CSI quality and average SNR at the receiver is demonstrated. It is shown that compared with the schemes using conventional models, our proposed approach can significantly increase the throughput without sacrificing system resources such as bandwidth and transmission energy.