IEEE 802.11 WLAN physical layers provide multi-rate capabilities. The station selects an appropriate transmission rate to maximize the throughput performance according to the channel condition. So far, many rate adaptation schemes have been proposed. The mechanism to choose a transmission rate from multiple transmission rates is referred to as rate adaptation. Most rate adaptation schemes do not consider the transmission failure may be caused by collisions. The throughput performance could decrease severely because that most of transmission failures are caused by collisions. In Collision-Aware Rate Adaptation (CARA) [8], the station differentiates collisions from transmission failures caused by channel error so as to operate correctly with rate adaptation. However, there are two flaws in CARA. Firstly, CARA could not detect collisions precisely enough. Secondly, RTS/CTS exchange may turn on and off frequently. In this paper, we propose a rate adaptation scheme called Enhanced CARA (E-CARA) to overcome the two problems of CARA. The main idea of E-CARA is that the station utilizes Clear Channel Assessment (CCA) with Busy Tone and Freeze Counter (FC) [11] to differentiate frame collisions from channel errors. Our simulations have shown that E-CARA outperforms CARA and Auto Rate Fallback (ARF).