Location based Network Optimizations for Mobile Wireless Networks: A study of the Impact of Mobility and Inaccurate Information

With an expected annual doubling in mobile data volumes, wireless operators are likely to face a major challenge in renewing and scaling their network infrastructure to support the future demands. Since cellular technologies might not be able to satisfy this demand single-handedly, local connectivity options such as Wi-Fi hotspots are seen as a means for relieving the load on the cellular networks. Smart phones and laptop/netbook computers are typically equipped with both cellular and Wi-Fi connectivity options, which allows the user or terminal to connect to Wi-Fi access points when available. In addition to cellular and Wi-Fi connectivity, smart phones are typically also equipped with Global Positioning System (GPS) receivers that enable outdoor localization. Further, cooperative network-based localization algorithms that enable indoor localization are gaining interest in the research community. This motivates the main topic of this thesis, which is to combine the connectivity and localization topics and investigate how useful location based network optimizations are for improving the user’s last hop connectivity. The main focus is on analyzing the impact of inaccurate location information, user mobility, measurement collection delays, and model parameter inaccuracies. The first contribution in this thesis concerns the impact of realistic communication constraints on network-based localization. In this joint work, the contribution is the modeling of the message exchanges necessary for centralized, cooperative network-based localization, as well as imposing realistic network constraints on these message flows. These constraints introduce additional delays in the message exchanges, which have a significant impact on tracking performance of the localization algorithm compared to the ideal situation where measurements are assumed to be instantaneously available. Secondly, two-hop relaying is considered for improving throughput and outage performance of downlink transmissions from a Wi-Fi hotspot to mobile destination nodes. The main focus is on relay selection, i.e. determining whether to use direct or relayed transmission and in the latter case, which mobile relay node to use. Here, two schemes are investigated: the first uses collected SNR measurements from all links, whereas the second uses collected position measurements. The performance of the schemes is investigated in terms of throughput performance in case of inaccurate input parameters, such as inaccurate position information, missing or crude Non-Line Of Sight (NLOS) information, and delayed measurements. Simulations have been used to investigate the performance of the schemes in different cases and the impact of different sources of inaccuracies. Thirdly, an extension to the two-hop relaying case is considered. This extension is a model-based cross-layer optimization, which uses position information to predict intra-system interference between different relay-destination pairs and to adapt relay transmit power levels, which thereby enables simultaneous relay-destination transmissions. Simulation results have shown that a significant throughput improvement is possible using this technique. In order to estimate the achievable throughput performance of different configuration choices, a distance dependent throughput model was developed. Fourthly, location based algorithms for prediction and planning of handovers between cellular and Wi-Fi networks is considered. The thesis focuses on the single user case, where the mobile user’s device can connect to a cellular network or one of many available Wi-Fi networks. A priori information regarding the average achievable throughput for the different networks is assumed to be available. An algorithm for determining the sequence of handovers that leads to the maximum throughput within a fixed time horizon is proposed. Also, a more efficient heuristic algorithm is proposed, and the performance of these algorithms is compared to a simple hysteresis-based algorithm, an upper bound reference algorithm, and a static algorithm that is always connected using the cellular network. In summary, this thesis has investigated the usefulness of location information for last hop route selection. The results have shown that while location-based two-hop relaying can help to decrease the signaling overhead and cope well with mobility, it requires a high-accuracy localization system as well as approximate environmental knowledge in order to cope with NLOS situations. It is also shown how location information enables simultaneous relaydestination transmissions through interference-awareness, which can further increase throughput performance. Finally, location-based handover prediction is shown to work well if the user’s movement trajectory is accurately estimated. The proposed heuristic algorithm is feasible in terms of computational complexity, and its performance is comparable to the optimal solution.