Adaptive Optimization of Rate Adaptation Algorithms in Multi-Rate WLANs

Rate adaptation is one of the basic functionalities in today´s 802.11 wireless LANs (WLANs). Although it is primarily designed to cope with the variability of wireless channels and achieve higher system spectral efficiency, its design needs careful consideration of cross-layer dependencies, in particular, link-layer collisions. Most practical rate adaptations focus on the time-varying characteristics of wireless channels, ignoring the impact of link-layer collisions. As a result, they may lose their effectiveness due to unnecessary rate downshift wrongly triggered by the collisions. Some recently proposed rate adaptations use RTS/CTS to suppress the collision effect by differentiating collisions from channel errors. The RTS/CTS handshake, however, incurs significant overhead and is rarely activated in infrastructure WLANs. In this paper, we introduce a new approach for optimizing the operation of rate adaptations by adjusting the rate-increasing and decreasing parameters based on link-layer measurement. To construct the algorithm, we study the impact of rate-increasing and decreasing thresholds on performance and show that dynamic adjustment of thresholds is an effective way to mitigate the collision effect in multi-user environments. Our method does not require additional probing overhead incurred by RTS/CTS exchanges and may be practically deployed without change in firmware. We demonstrate the effectiveness of our solution, comparing with existing approaches through extensive simulations.