Research Of User Access Machenism In Multi-Rate Wlans

With the development and widespread of WLANs, the number of users is increasing in this networks day by day. However, the network resource is limited. Thus, in order to provide users with better service, optimizing the user access mechanism has becoming one of hot issues in recent years.The main goals of our thesis are to optimize the user access mechanism, improve the network throughput, and achieve fairness in the resource assignment among users. In this thesis, we first briefly introduce relative concepts and key techniques. Then, our research discusses three proposed methods for the user access mechanism in detail with their their processes, novelties, and performance analysis.First, we study the problem of achieving proportional fairness via AP association in multi-rate WLANs. This problem is formulated as a non-linear 0-1 program. Since this program is NP-hard, we propose a centralized algorithm NLAO-PF to obtain the approximate optimal solution via relaxation and rounding processes. This algorithm has a low complexity and a bounded performance ratio. To handle the case of dynamic user membership, we propose a distributed heuristic AP Association algorithm BPF based on a novel performance revenue function.Then, we investigate an access machenism combining with AP Association and Power Control. By introducing a new concept of AP utility, we establish the relationship between the network utility and the AP utility according to proportional fairness. This relationship is exploited to design an algorithm PCAP to optimize the network utility by increasing the average and decreasing the variance of the AP utility. What's more, this thesis also studies the problem of partially overlapping channel assignment to improve the performance of 802.11 wireless networks. We first conceive a novel interference model that takes into account both the channel separation and the physical distance separation of two nodes employing adjacent channels, and intoduce a new concept called "node orthogonality". In addition, we propose two approximate algorithms MICA and MUCA, to minimize the total weighted interference and to maximize the total bandwidth utilities, respectively. Rigorous mathematical analysis can prove that the performance ratios of both MICA and MUCA are equal to 2.In the end, the summary of the whole thesis, current shortcomings of the system and future work are given. Also, the summary of my work and research progress during master-degree study are described.