Abstract: Today’s IEEE 802.11 devices provide multiple transmission rates in PHY layer，so the optimal rate can be chosen in an adaptive manner depending on the underlying channel condition in order to maximize the system performance.In the model of Distributed Coordination Function（DCF），the collision can’t be avoided absolutely when there are multiple users.The Collision-Aware Rate Adaptation（CARA）can extremely increase the throughput by adaptively enabling the Request-To-Send/Clear-To-Send（RTS/CTS） exchange.But the throughput of CARA drops greatly with the number of users increasing.A novel rate adaptation scheme called Adaptive Avoid Collision Rate Adaptation（Adaptive-ACRA） is proposed that effectively boosts the throughput with collision by varying threshold value.The effectiveness of Adaptive-ACRA is demonstrated by comparing with exiting approach through extensive simulations.

Key words: rate adaptation, IEEE802.11 Distributed Coordination Function（DCF）, Media Access Control（MAC） protocol, Wireless Local Area Networks（WLANs）

摘要： 目前的IEEE802.11在物理层提供了不同的传输速率，因此根据不同的信道条件可以选择一个合适的传输速率，以使系统吞吐量达到最大。在WLANs中，当有多个用户向同一个接入点传输数据时，碰撞就会产生。Collision-Aware Rate Adaptation（CARA）算法能够有选择地启用Request-To-Send/Clear-To-Send（RTS/CTS）识别出这种碰撞，很大程度上避免了由于碰撞引起的吞吐量下降。但随着碰撞的增加，CARA吞吐量还是有明显下降。为了提高吞吐量，提出了一种新的速率自适应算法Adaptive Avoid Collision Rate Adaptation（Adaptive-ACRA），该算法根据信道条件不同改变门限值，有效地提高了大量碰撞存在情况下的吞吐量。最后，通过大量NS2仿真证明了新算法提高系统吞吐量的有效性。

关键词: 速率自适应, IEEE802.11分布式协调功能（DCF）, 媒体访问控制（MAC）协议, 无线局域网络（WLAN）