3,085 research outputs found

    On the set of optimal policies in discrete dynamic programming

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    A Simple Cooperative Diversity Method Based on Network Path Selection

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    Cooperative diversity has been recently proposed as a way to form virtual antenna arrays that provide dramatic gains in slow fading wireless environments. However most of the proposed solutions require distributed space-time coding algorithms, the careful design of which is left for future investigation if there is more than one cooperative relay. We propose a novel scheme, that alleviates these problems and provides diversity gains on the order of the number of relays in the network. Our scheme first selects the best relay from a set of M available relays and then uses this best relay for cooperation between the source and the destination. We develop and analyze a distributed method to select the best relay that requires no topology information and is based on local measurements of the instantaneous channel conditions. This method also requires no explicit communication among the relays. The success (or failure) to select the best available path depends on the statistics of the wireless channel, and a methodology to evaluate performance for any kind of wireless channel statistics, is provided. Information theoretic analysis of outage probability shows that our scheme achieves the same diversity-multiplexing tradeoff as achieved by more complex protocols, where coordination and distributed space-time coding for M nodes is required, such as those proposed in [7]. The simplicity of the technique, allows for immediate implementation in existing radio hardware and its adoption could provide for improved flexibility, reliability and efficiency in future 4G wireless systems.Comment: To appear, IEEE JSAC, special issue on 4

    Project design with limited commitment and teams

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    We study the interaction between a group of agents who exert effort to complete a project and a manager who chooses its objectives. The manager has limited commitment power so that she can commit to the objectives only when the project is sufficiently close to completion. We show that the manager has incentives to extend the project as it progresses. This result has two implications. First, the manager will choose a larger project if she has less commitment power. Second, the manager should delegate the decision rights over the project size to the agents unless she has sufficient commitment power

    Donor conception and (dis)closure in the UK::siblingship, friendship and kinship

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    As many aspects of assisted conception become familiar, others emerge not only as unfamiliar, but also unpredicted. This article focuses on a newly emergent kin figure – the donor sibling. It suggests that the anthropology of friendship might have as much to tell us about the significance of the donor sibling in Euro-American kinship thinking as the anthropology of kinship. The article locates the donor-sibling in changes in UK legislation and policy that have increasingly promoted more transparency and ‘openness’ in donor conception

    Calculation of the effect of random superfluid density on the temperature dependence of the penetration depth

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    Microscopic variations in composition or structure can lead to nanoscale inhomogeneity in superconducting properties such as the magnetic penetration depth, but measurements of these properties are usually made on longer length scales. We solve a generalized London equation with a non-uniform penetration depth, lambda(r), obtaining an approximate solution for the disorder-averaged Meissner effect. We find that the effective penetration depth is different from the average penetration depth and is sensitive to the details of the disorder. These results indicate the need for caution when interpreting measurements of the penetration depth and its temperature dependence in systems which may be inhomogeneous
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