Investigation On The Mac Access Mechanisms Performance Of Very High Throughput Wlan

As the product of the combination of computer networks and wireless communications technologies in their rapid developments, Wireless Local Area Networks (WLANs), featured in fast and efficient to install, agile to adjust, economical for network rollout and flexible to extend, have been adequately improved and broadly deployed in various circumstances of modern society since1990s. The rapid enhancement of WLANs has greatly put forward the research and formulation of the theoretical standards in this hot field. There are several standards for WLANs---IEEE802.11, which is most widely applied and developed, HyperLAN protocols and Bluetooth regulations.With the surging deployment of wireless mobile terminals as well as in-depth academic researches about the related area, based on the national scientific and technological major projects-Key Technologies Research and Demonstration of VHT-WLAN (Very High Throughput-Wireless Local Area Network) Systems (Project No.2010ZX03005-001-03), this thesis has made in-depth research of performance in Medium Access Control (MAC) layer from both theoretical and practical facets. First of all, historical background, related standard specifications, and development prospects are thoroughly presented. By analyzing the outstanding merits of WLANs, compared to wired computer networks, we can easily realize the irresistible trend of social applications. And then the accessing mechanisms in MAC layer of WLANs and the two-dimensional model, the analytical model from the theoretical angle, will be presented, followed by pointing out the defect of low and unsatisfying throughput of this system. Then the detail of one-dimensional Markov model for the legacy accessing mechanism of MAC layer in WLANs will be presented, followed by the proposed one-dimensional model of single-channel MAC scheme and the fundamental factors influencing the systematic parameters including saturation throughput, collision probability as well as fairness. Subsequently, the two-dimensional hybrid OFDMA/CSMA (Orthogonal Frequency Division Multiple Access/Carrier Sense Multiple Access) scheme will be introduced on the basis of the legacy accessing schedule in MAC layer of WLANs. In addition, we will demonstrate that the hybrid OFDMA/CSMA scheme outperforms the legacy CSMA/CA mechanism as far as the comprehensive systematic performance from the perspective of both theory and simulation are concerned. Nevertheless, there are still certain defects inherent in OFDMA/CSMA scheme because of the shortcomings of the CSMA/CA technology. Therefore, an adaptive contention window algorithm to hybrid OFDMA/CSMA mechanism based on contention weights will be presented and then demonstrated to achieve better systematic performance than the original OFDMA/CSMA scheme.