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According to the IEEE 802.11n, the next generation wireless local area networks (WLAN) aim to support data rates at least 100 Mbps, as measured at the medium access control (MAC) layer. Thus, it is important to develop a cross-layer analytical model to evaluate the MAC layer goodput performance with consideration of the physical layer effects. This paper suggests such an analytical model to calculate the MAC layer goodput for the IEEE 802.11a WLAN in the Nakagami fading channel, while incorporating the effects of channel estimation, delay spread and signal detection scheme in the physical layer. Furthermore, we develop a simple and efficient channel-driven rate adaptation (CDKA) scheme to dynamically adjust the transmission parameters to maximize the MAC layer goodput. From out numerical results, we find that some physical (PHY) modes in the IEEE 802.11a are unnecessary. We also find that as rms delay spread increases, the goodput in the MAC layer can be improved due to frequency diversity. For the case of rms delay spread changing from 50 to 200 nsec with 15 dB Eh/No, the goodput is improved from 12 Mbps to 15 Mbps.