Research On Rate Adaptive Algorithm Based On IEEE 802.11n Block ACK Mechanism

With rapid development of wireless local area network(WLAN) and popularity of portable terminals, people require better performance of WLAN. As the widely used standard, IEEE 802.11 supports multiple rates in the physical(PHY) layer. However, it doesn’t specify how to switch between these PHY rates in communication. To increase the system throughput, different rates should be exploited in an adaptive manner depending on the underlying channel condition, this is what rate adaptation algorithms research on. However, with the increasing of PHY rates, system throughput is severely limited by high MAC overhead. The IEEE 802.11n standard dramatically improve the efficiency in the MAC layer by using Block ACK scheme and frame aggregate scheme. Most rate adaptation algorithms based on IEEE 802.11n focus on technologies of OFDM and MIMO, but they don’t account for the Block ACK. Hence, we will extend the work by considering these algorithms under Block-ACK scheme.Firstly, the paper surveys three kinds of rate adaptation algorithms, and analyzes their respective advantages and disadvantages. Then, it studies the performance of ARF based on ACK scheme under Block ACK scheme. Since Block ACK scheme compared to ACK scheme has longer time to response channel and can’t change rate arbitrarily, there are some problems: 1)it increases or decreases rate by multiple times within a BA frame; 2)it can’t reasonably deal with the timeout of BA frame.To solve these problems, the paper presents a WFD algorithm based on Block ACK scheme which uses a weighted moving average, where acknowledge weights are controlled by MPDUs arrival time and more recent samples carry more weight, so it is more accurate to estimate current channel condition. The saturation throughput produced by WFD is then compared to the saturation throughput produced by other algorithms in many scenarios by NS-3 network simulator. The simulation results have shown that WFD yields higher throughput performance than other well-known rate adaptation algorithms(ARF and AARF) in all tested scenarios, and it realizes throughput optimization of multi-rate under Block ACK scheme.