Dynamic fragmentation scheme for rate-adaptive wireless LANs

Many rate-adaptive MAC protocols have been proposed in the past for wireless LANs over time-varying media to provide efficient channel usage and throughput enhancement. Most of these protocols are receiver-based and employ RTS/CTS collision avoidance handshakes specified in the distributed coordination function (DCF) of the IEEE802.11 MAC. However, these protocols do not consider the possible bursty transmission of fragments in a rate-adaptive fashion. In this paper, a rate-adaptive protocol with a dynamic fragmentation scheme is proposed in order to enhance the throughput by employing bursty fragment transmissions based on the channel condition. Instead of using one fragmentation threshold, multiple thresholds for different data rates are proposed so that more data can be transmitted at higher data rates when the channel is good. In the fragmentation scheme of IEEE802.11, all fragments are generated with equal size at the same time, and all fragments will not change until they are transmitted. In our proposed scheme, whenever the rate for the next transmission is chosen based on the channel condition estimated from the previous fragment transmission, a new fragment is generated using the fragment threshold for the new rate. In this way, the channel can be more effectively utilized to squeeze in more bits. This scheme is evaluated under a time-correlated fading channel model in terms of the achieved throughput. Simulation results show that this scheme achieves a much higher throughput than other rate-adaptive protocols do.