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  1. 1. Abstract In most existing localized topology control protocols for mobile ad hoc networks (MANETs), each node selects a few logical neighbors based on location information, and uses a small transmission range to cover those logical neighbors. Transmission range reduction conserves energy and bandwidth consumption, while still maintaining the network connectivity. However, the majority of these approaches assume a static network without mobility. In a mobile environment, however, network connectivity can be compromised by two types of “bad” location information: inconsistent information, which makes a node select too few logical neighbors, and outdated information, which makes a node use a too small transmission range. In this paper, we first demonstrate the existing problems, and then propose a mobility-sensitive topology control method that extends many existing mobility-insensitive protocols. Two mechanisms are introduced: consistent local views that avoid inconsistent information and delay and mobility management that tolerate outdated information. The effectiveness of the proposed approach is confirmed through an extensive simulation study.
  2. 2. INTRODUCTION In mobile ad hoc networks (MANETs), all nodes cooperate to achieve a global task, such as area monitoring and data gathering/communication. To reduce energy consumption and signal interference, it is important to select an appropriate transmission power for each node, also called topology control, while still satisfying certain global constraints. Most existing topology control protocols in MANETs use the localized approach to find a small transmission range subject to some global constraints, including connectivity and other reliability and throughput related measures. However, the majority of these approaches assume a static network without mobility. In a typical localized approach, each node collects neighborhood information through periodic, asynchronous “Hello” messages. We refer to neighborhood information collected at each node as local view at a particular time. A wireless ad hoc network consists of a group of autonomous wireless devices that communicate with each other over the shared wireless channels. These wireless nodes are either mobile (e.g., in wireless mobile ad hoc networks) or static (e.g., in wireless sensor networks). Wireless ad hoc networks can be established very quickly since they do not require the support of a fixed infrastructure. Typical applications of wireless ad hoc networks include disaster recovery , surveillance, environment monitoring, just to name a few. Wireless ad hoc networks are different from wired networks, wireless cellular networks [83], or wireless local area networks (WLANs) in that the topology may change constantly . Communication links are formed on the fly according to the distribution and mobility of nodes, and are dependent on the status of other links due to wireless interference in the physical and the MAC layers. This characteristic gives rise to several interesting phenomena. For one thing, network connectivity becomes a function of both the spatial distribution of nodes and the transmission power of each individual node. For another, the network capacity is determined not only by the connectivity , but also by MAC and routing protocols. As a consequence, several new challenges have emerged in the system design and analysis of wireless ad hoc networks, including:
  3. 3. Energy efficiency. Since many wireless devices, especially mobile devices and sensors, are battery-powered, how to carry out the necessary functions with the minimal energy is critical to prolong the network lifetime. Network capacity. The capacity of wireless ad hoc networks is limited in the sense that the wireless channels has to be shared among all devices that are within the transmission or carrier sensing range of each other . How to determine the transmission power and coordinate transmissions among devices so as to increase spatial reuse and maximize the network capacity has thus become an important issue. Channel access. How to arbitrate channel access among devices is crucial to enable the efficient sharing of the wireless channel and to provide certain level of performance guarantee (e.g., for real-time applications). Routing. This issue is concerned with the efficient and correct delivery of messages, given the dynamic network topology and the unreliable communication channel. Security and privacy . Since wireless communication is broadcast in nature, wireless traffic is subject to malicious attacks and privacy violation. How to ensure security and preserve privacy is a key issue to the wide deployment of wireless ad hoc networks. Among the above challenges, energy efficiency and network capacity are the most fundamental to the network performance, as energy and bandwidth are the two major wireless resources. In this thesis, we aim to achieve energy efficiency and improve network capacity with topology control.