Browsing by Author "Dash, Debashis"

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    Bounds and protocols for a two-way multiple node channel
    (2007) Dash, Debashis; Sabharwal, Ashutosh
    Often in multiuser wireless systems like cellular networks, data is present in both directions (uplink and downlink). One way to formulate protocols for such networks, is to design the uplink and downlink independently. But such a design ignores the two-way nature of data. We show that by jointly designing the uplink and downlink, we get rate gains as long as there is data in both the directions. We make a distinction between data being two-way and the channel being two-way and show that for the case where only the channel is two-way, a disjoint design is optimal. The rate gains can be attributed to the inherent feedback in two-way schemes which enables cooperation in all topologies (including hidden node topologies). Furthermore, in near-far situations, the weak user gains appreciably from such cooperation. Finally we demonstrate that several well known inner bounds can be derived as special cases of our rate region.
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    Distributed Resource Allocation with Local Information
    (2013-12-09) Dash, Debashis; Sabharwal, Ashutosh; Aazhang, Behnaam; Knightly, Edward W.; Heinkenschloss, Matthias
    Making distributed decisions based on incomplete information is inevitable in dynamic wireless networks due to a multitude of constraints. We study the effects of incomplete information on system performance in two parts. We first analyze the effect of incomplete topology information on network capacity and then the effect of partial traffic information on the capacity of a two-flow interference network. In the first part of the thesis, we study the effect of local topology information based resource allocation on the number of conflicts (called defects) produced in the network. First we show its equivalence to sum rate maximization of the network. Then we prove the non-existence of an universal local coloring protocol that can produce defect-free coloring. Next we find the optimal protocol with no information and a local coloring protocol for path graphs that can achieve Nash equilibrium. We develop a general framework to analyze any local coloring protocol based on a randomized starting point that can be applied to any graph. Finally we develop a graph decomposition method to apply it to any graph with non-overlapping cliques and cycles. In the second part of the thesis, we study a two-user cognitive channel, where the primary flow is sporadic, cannot be re-designed and operating below its link capacity. To study the impact of primary traffic uncertainty, we propose a block activity model that captures the random on-off periods of primary's transmissions. Each block in the model can be split into parallel Gaussian-mixture channels, such that each channel resembles a multiple user channel from the point of view of the secondary user. The secondary senses the current state of the primary at the start of each block. We show that the optimal power transmitted depends on the sensed state and the optimal power profile is either growing or decaying in power as a function of time. We show that such a scheme achieves capacity when there is no noise in the sensing. The optimal transmission for the secondary performs rate splitting and follows a layered water-filling power allocation for each parallel channel to achieve capacity.