56 research outputs found
Wireless ad-hoc networks: Strategies and Scaling laws for the fixed SNR regime
This paper deals with throughput scaling laws for random ad-hoc wireless
networks in a rich scattering environment. We develop schemes to optimize the
ratio, of achievable network sum capacity to the sum of the
point-to-point capacities of source-destinations pairs operating in isolation.
For fixed SNR networks, i.e., where the worst case SNR over the
source-destination pairs is fixed independent of , we show that
collaborative strategies yield a scaling law of in contrast to multi-hop strategies which yield a
scaling law of . While, networks where
worst case SNR goes to zero, do not preclude the possibility of collaboration,
multi-hop strategies achieve optimal throughput. The plausible reason is that
the gains due to collaboration cannot offset the effect of vanishing receive
SNR. This suggests that for fixed SNR networks, a network designer should look
for network protocols that exploit collaboration. The fact that most current
networks operate in a fixed SNR interference limited environment provides
further motivation for considering this regime.Comment: 26 pages single column, submitted to Transactions on Information
Theor
Robust Hydraulic Fracture Monitoring (HFM) of Multiple Time Overlapping Events Using a Generalized Discrete Radon Transform
In this work we propose a novel algorithm for multiple-event localization for
Hydraulic Fracture Monitoring (HFM) through the exploitation of the sparsity of
the observed seismic signal when represented in a basis consisting of space
time propagators. We provide explicit construction of these propagators using a
forward model for wave propagation which depends non-linearly on the problem
parameters - the unknown source location and mechanism of fracture, time and
extent of event, and the locations of the receivers. Under fairly general
assumptions and an appropriate discretization of these parameters we first
build an over-complete dictionary of generalized Radon propagators and assume
that the data is well represented as a linear superposition of these
propagators. Exploiting this structure we propose sparsity penalized algorithms
and workflow for super-resolution extraction of time overlapping multiple
seismic events from single well data
Methods for Large Scale Hydraulic Fracture Monitoring
In this paper we propose computationally efficient and robust methods for
estimating the moment tensor and location of micro-seismic event(s) for large
search volumes. Our contribution is two-fold. First, we propose a novel
joint-complexity measure, namely the sum of nuclear norms which while imposing
sparsity on the number of fractures (locations) over a large spatial volume,
also captures the rank-1 nature of the induced wavefield pattern. This
wavefield pattern is modeled as the outer-product of the source signature with
the amplitude pattern across the receivers from a seismic source. A rank-1
factorization of the estimated wavefield pattern at each location can therefore
be used to estimate the seismic moment tensor using the knowledge of the array
geometry. In contrast to existing work this approach allows us to drop any
other assumption on the source signature. Second, we exploit the recently
proposed first-order incremental projection algorithms for a fast and efficient
implementation of the resulting optimization problem and develop a hybrid
stochastic & deterministic algorithm which results in significant computational
savings.Comment: arXiv admin note: text overlap with arXiv:1305.006
Algorithms for item categorization based on ordinal ranking data
We present a new method for identifying the latent categorization of items
based on their rankings. Complimenting a recent work that uses a Dirichlet
prior on preference vectors and variational inference, we show that this
problem can be effectively dealt with using existing community detection
algorithms, with the communities corresponding to item categories. In
particular we convert the bipartite ranking data to a unipartite graph of item
affinities, and apply community detection algorithms. In this context we modify
an existing algorithm - namely the label propagation algorithm to a variant
that uses the distance between the nodes for weighting the label propagation -
to identify the categories. We propose and analyze a synthetic ordinal ranking
model and show its relation to the recently much studied stochastic block
model. We test our algorithms on synthetic data and compare performance with
several popular community detection algorithms. We also test the method on real
data sets of movie categorization from the Movie Lens database. In all of the
cases our algorithm is able to identify the categories for a suitable choice of
tuning parameter.Comment: To appear in IEEE Allerton conference on computing, communications
and control, 201
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