Browsing by Author "Hunter, Christopher R."

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    Distributed Protocols for Signal-Scale Cooperation
    (2012) Hunter, Christopher R.; Sabharwal, Ashutosh
    Signal-scale cooperation is a class of techniques designed to harness the same gains offered by multi-antenna communication in scenarios where devices are too small to contain an array of antennas. While the potential improvements in reliability at the physical layer are well known, three key challenges must be addressed to harness these gains at the medium access layer: (a) the distributed synchronization and coordination of devices to enable cooperative behavior, (b) the conservation of energy for devices cooperating to help others, and (c) the management of increased inter-device interference caused by multiple spatially separate transmissions in a cooperative network. In this thesis, we offer three contributions that respectively answer the above three challenges. First, we present two novel cooperative medium access control protocols: Distributed On-demand Cooperation (DOC) and Power-controlled Distributed On-demand Cooperation (PDOC). These protocols utilize negative acknowledgments to synchronize and trigger cooperative relay transmissions in a completely distributed manner. Furthermore, they avoid cooperative transmissions that would likely be unhelpful to the source of the traffic. Second, we present an energy conservation algorithm known as Distributed Energy-Conserving Cooperation (DECC). DECC allows devices to alter their cooperative behavior based on measured changes to their own energy efficiency. With DECC, devices become self-aware of the impact of signal-scale cooperation -- they explicitly monitor their own performance and scale the degree to which they cooperate with others accordingly. Third and finally, we present a series of protocols to combat the challenge of inter-device interference. Whereas energy efficiency can be addressed by a self-aware device monitoring its own performance, inter-device interference requires devices with network awareness that understand the impact of their behavior on the devices around them. We investigate and quantify the impact of incomplete network awareness by proposing a modeling approximation to derive relaying policy behaviors. We then map these policies to protocols for wireless channels.
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    Random access cooperative systems
    (2008) Hunter, Christopher R.; Sabharwal, Ashutosh
    User-cooperation has been studied extensively in the literature. This mechanism achieves many of the same gains that can be had by using multi-antenna (MIMO) communications in applications where the size of a node is limited. Existing analysis assumes that the system is scheduled; both the relay knows to listen when a packet is being sent. We present analysis of a class of random-access cooperative systems and show that the same performance as the scheduled systems can be achieved as long as certain requirements are met in the packet detection scheme of the relay. In particular, we find that a static decision threshold on energy detection results in a cooperative network that performs asymptotically no better than a simple point-to-point network with no relay at all. However, a decision threshold that dynamically shifts with average SNR allows the system to achieve full spatial diversity.