Performance Analysis And Dynamic Optimization Of IEEE 802.11 MAC

Wireless Local Area Networks (WLANs) are now becoming a popular access technology for "last mile" and IEEE 802.11 is the most important international standard for WLAN. The IEEE 802.11 MAC layer protocol adopts the mechanism of Distributed Coordination Function (DCF), which is based on CSMA/CA(Carrier Sense Multiple Access with Collision Avoidance) to perform its fundamental function, wireless medium access control. However, the performance of the above protocol would deteriorate in many cases, because it has not taken many factors of real wireless environments into accounts. The research target of this thesis is to optimize DCF and make it adaptive to the characteristics of wireless channel, different scales of users and different application scenarios and thus to improve the whole system performance of IEEE 802.11 MAC. By thoroughly analyzing the related protocols, former research and the results of real measurements, the thesis points out some questions, which requires further study and improvement.In order to overcome the deficiency of former research of dynamic optimization, this thesis proposes an adaptive optimization mechanism, DOOR (Dynamic Optimization on Range), for IEEE 802.11 DCF. For the first time, the author replaces the accurate measurement in the former optimal algorithms by the measurement and estimation of the subrange of competing station number and DOOR would adjust the protocol parameters, based on the subrange in which the number of competing stations is, to improve the whole system performance of DCF. The thesis presents the reasons of range partitioning, operation mechanisms and range estimation method of DOOR. Detailed system model and performance evaluation for DOOR are also given. The new self-adaptive mechanism DOOR not only has low algorithm complexity and low system cost but also is applicable to the complex wireless networks. At the same time, the results of simulations show that DOOR could achieve much higher throughput and shorter system delay than the standard IEEE 802.11 DCF in almost all the different numbers of competing stations. In addition, this thesis proposes a priority mechanism to improve the whole system performance of IEEE 802.11, which based on multirate and FER together. Different from other proposals, this mechanism first takes random frame loss into account. In the thesis, the detailed mechanisms, analytical model and elaborate numerical results are introduced for the mechanism.