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Hermann Simon Lichte, "Quantifying and Reducing the Cost of Cooperative Relaying in Wireless Multi-Hop Networks," PhD Thesis, Department of Computer Science, Paderborn University (UPB), June 2011. (Advisor: Holger Karl; Referee: Falko Dressler)


Fast and reliable data transmission in wireless networks is hard to achieve due to interference and fading. Interference limits spatial reuse in the network and fading leads to high error rates for transmissions. These error rates can be exponentially reduced by cooperative relaying. With this approach, neighboring nodes can help the transmission by repeating the signal via independent channels. In large wireless multi-hop networks, this poses two major problems. First, the additional transmission of relays causes additional interference. To achieve a desired outage capacity, receivers span guard zones to mitigate interference and thereby affect spatial reuse by consuming area – the so-called spatial consumption. Second, retransmission requires additional channel resources which typically reduces data rate – the so-called multiplexing loss. Without careful studies it is not obvious when these costs pay off. Therefore, I study how the additional interference due to relaying affects the network's performance in two steps: First, I analyze the spatial consumption of the cooperative transmission using geometry. Second, I combine this analysis with outage capacity expressions. With the resulting analytical framework, I show that although relays require additional space by blocking neighbors, the diversity gain compensates for interference. I identify the operating regions of cooperative relaying and show that, especially at high robustness requirements, cooperative relaying even leads to better spatial reuse than conventional transmission approaches. To reduce multiplexing loss for unicasts, I develop a routing-informed Medium Access Control (MAC) protocol that schedules a single relay retransmission such that it assists in two successive point-to-point transmissions, so-called two-for-one cooperation. I show how to integrate the proposed protocol into the IEEE 802.11 Wireless Local Area Network (WLAN) standard by modifying an open-source Linux WLAN driver. Measurement results show the efficiency of two-for-one cooperation in multi-hop networks. Contrary to unicasts, multiplexing loss is not a cost anymore for broadcasts as in this case retransmissions happen anyway. Thus, to make broadcasts robust against fading, cooperative relaying can be used without sacrificing data rate. To broadcast as quickly as possible (i.e., with low latency), only a subset of nodes should retransmit messages. I find that existing heuristics for determining this so-called broadcast set have poor delivery ratios with fading channels. I identify the static unit disk model of transmission ranges to be the source of the problem and propose a probabilistic model as a replacement. I additionally exploit cooperation diversity during construction of the broadcast set to gain improved reliability while still keeping the size of the set and hence the latency low. Simulation results show that the proposed Probabilistic Cooperation Diversity Broadcast (PCDB) decreases the time for broadcasts while still distributing packets to all nodes with high probability. The simulation and measurement results in this thesis require implementations of the same MAC protocols on different platforms. To untie this complex development process, I propose to automate its most error-prone parts: Implementation of MAC automata, analysis, and code generation. To do so, I formalize cooperative MAC protocols by an easy-to-use specification language and propose strategies to construct compilers to automatically analyze validity and performance of the specification and to translate the specified protocols into program code for implementation.

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Hermann Simon Lichte

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    author = {Lichte, Hermann Simon},
    title = {{Quantifying and Reducing the Cost of Cooperative Relaying in Wireless Multi-Hop Networks}},
    advisor = {Karl, Holger},
    institution = {Department of Computer Science},
    location = {Paderborn, Germany},
    month = {6},
    referee = {Dressler, Falko},
    school = {Paderborn University (UPB)},
    type = {PhD Thesis},
    year = {2011},

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