Research On The System Optimization And The Access Schemes For The Wireless Access Networks

With the increasing requirement of the society and the development of the technology, wireless access network has been an indispensable portion of the access networks. Among all the wireless access networks, wireless local area networks (WLAN) and satellite mobile communication networks are two important systems to provide wireless access for users. The researches on the design and optimization of the networks, the access scheme, and the decision making of system solutions are useful in implement of future broad-band wireless access networks. In the WLAN research field, this dissertation mainly focuses on the system performance analysis, system throughput estimation, network design and optimization. On the other side, in the research of mobile satellite systems, access control schemes and decision making are also studied.Chapter 1 classifies the types of the wireless access networks and clarifies their features. WLAN and mobile satellite networks are introduced. The contents of this dissertation are also outlined.Chapter 2 summarizes all the factors that affect the system performance. In performance analysis, system throughput is the primary index to be considered. We make researches on the factors that affect the system throughput. In multi-cell WLAN, some particular factors, e.g. coverage overlapping, un-symmetrical restraint among cells, and channel assignment, are observed and analyzed by system simulation. These analyses are helpful for the discussion in the following chapters.Chapter 3 analyzes the system performance of the hybrid-data-rates WLAN and the multi-cell WLAN. If terminals with different data rates coexist in a WLAN system, the channel holding time and transmission efficiency of them are different. An analytical model is proposed to compute the total throughput in the case of hybrid data rates terminals coexists. The simulation results show the accuracy of this analytical model. Furthermore, the performance analysis is carried out in the multi-cell WLAN. In a multi-cell WLAN, co-channel interference between access points, and interference between co-channel users located in neighbored cells are existed, which leads the system performance complicated than the situation in a mono-cell WLAN. So a system throughput estimation method is proposed. Three important features of this method are that co-channel overlapping is allowed, hybrid data rates are allowed, and non-uniform distribution of users is allowed. Thesimulation results show that the estimation method is satisfied for system design.Chapter 4 proposes a joint algorithm for WLAN system design and optimization. To deploy a multi-cell 802.11 wireless LAN system, a generic design process consists of four phases; access point placement and channel assignment are two primary design issues. For a given set of traffic demands, maximizing not only the overall system throughput, but also the fairness in resource sharing among users are aimed at, which is measured by a throughput balance index. Unlike conventional approaches that decouple AP placement and channel assignment into two phases, the proposed objective function jointly solves the two problems for better performance. Due to the high computational complexity involved in the exhaustive searching, an efficient local searching algorithm, called patching algorithm, is designed. The simulation results show that for a typical indoor environment, patching algorithm can provide a close-to-optimal performance with much lower time complexity than the exhaustive searching.Chapter 5 proposes an access scheme in low earth orbit (LEO) satellite mobile communication systems. In LEO satellite networks, frequent handover requests exist because of the relative motions between the satellites and the mobile users. In order to decrease the handover requests, two access schemes besides the distance-prioritized scheme are proposed. One judges the priorities by estimating the overlaid time, the other scheme additionally considering the elevation angle performance. For the sake of simulation, a non-uniform distributed global traffic model is also constructed. By emulating the global traffic service through a LEO satellite mobile communication system under different access schemes, the performances of the networks are analyzed. The improvement of these two access schemes can be learned from the simulation results.Chapter 6 discusses the fuzzy algorithm for decision making in satellite mobile communication networks. A reliable decision making research is indispensable in the space engineering for engineering's complexity. So the general procedure of the decision making for the space engineering is discussed. And a transplantable hierarchical framework for the satellite mobile communication systems is proposed. The fuzzy analytic hierarchy process and the fuzzy comprehensive evaluation are combined to resolve the multi-attribute decision making problems. A decision support system is developed on three-tier browser/server mode. Finally, a practical example is analyzed based on the proposed methodology.