Abstract: Many rate adaptation schemes have been proposed so far for IEEE 802.11 standard，but most of the schemes don’t consider the collision effect，resulting in the throughput degradation of the system.Though the Collision Aware Rate Adaptation（CARA） has considered the collision effect and has improved system performance effectively，it doesn’t differentiate the reason of collision and would produce RTS turn on/off alternately in the environment with hidden terminals.This paper presents an enhanced CARA——EN-CARA，combining detective mechanism of hidden terminals，which can avoid RTS turn on/off alternately，reduce the probabilities of the collision effectively and further improve system performance.Simulation results show that the proposed scheme outperforms the original CARA significantly under various randomly-generated network topologies.

Key words: IEEE 802.11, rate adaptation, hidden terminal

摘要： 针对IEEE 802.11标准的无线局域网已有大量速率自适应算法，但是大多数算法的速率选择未考虑冲突的影响，可能导致系统吞吐量反而下降。CARA算法虽然考虑了冲突因素，能有效提高系统性能，但是该算法没有对造成冲突的原因进行进一步区分，在存在隐藏终端的无线局域网中可能产生RTS振荡效应。为此提出一种基于CARA的改进算法——EN-CARA算法，该算法结合了隐藏终端检测机制，能避免RTS振荡效应，有效降低冲突概率，进一步改善和提高系统性能。仿真结果表明，该算法在随机生成的拓扑架构下能取得比CARA更好的性能。

关键词: IEEE 802.11, 速率自适应, 隐藏终端