61,378 research outputs found
Randomized Dimensionality Reduction for k-means Clustering
We study the topic of dimensionality reduction for -means clustering.
Dimensionality reduction encompasses the union of two approaches: \emph{feature
selection} and \emph{feature extraction}. A feature selection based algorithm
for -means clustering selects a small subset of the input features and then
applies -means clustering on the selected features. A feature extraction
based algorithm for -means clustering constructs a small set of new
artificial features and then applies -means clustering on the constructed
features. Despite the significance of -means clustering as well as the
wealth of heuristic methods addressing it, provably accurate feature selection
methods for -means clustering are not known. On the other hand, two provably
accurate feature extraction methods for -means clustering are known in the
literature; one is based on random projections and the other is based on the
singular value decomposition (SVD).
This paper makes further progress towards a better understanding of
dimensionality reduction for -means clustering. Namely, we present the first
provably accurate feature selection method for -means clustering and, in
addition, we present two feature extraction methods. The first feature
extraction method is based on random projections and it improves upon the
existing results in terms of time complexity and number of features needed to
be extracted. The second feature extraction method is based on fast approximate
SVD factorizations and it also improves upon the existing results in terms of
time complexity. The proposed algorithms are randomized and provide
constant-factor approximation guarantees with respect to the optimal -means
objective value.Comment: IEEE Transactions on Information Theory, to appea
Energy Efficiency in Cache Enabled Small Cell Networks With Adaptive User Clustering
Using a network of cache enabled small cells, traffic during peak hours can
be reduced considerably through proactively fetching the content that is most
probable to be requested. In this paper, we aim at exploring the impact of
proactive caching on an important metric for future generation networks,
namely, energy efficiency (EE). We argue that, exploiting the correlation in
user content popularity profiles in addition to the spatial repartitions of
users with comparable request patterns, can result in considerably improving
the achievable energy efficiency of the network. In this paper, the problem of
optimizing EE is decoupled into two related subproblems. The first one
addresses the issue of content popularity modeling. While most existing works
assume similar popularity profiles for all users in the network, we consider an
alternative caching framework in which, users are clustered according to their
content popularity profiles. In order to showcase the utility of the proposed
clustering scheme, we use a statistical model selection criterion, namely
Akaike information criterion (AIC). Using stochastic geometry, we derive a
closed-form expression of the achievable EE and we find the optimal active
small cell density vector that maximizes it. The second subproblem investigates
the impact of exploiting the spatial repartitions of users with comparable
request patterns. After considering a snapshot of the network, we formulate a
combinatorial optimization problem that enables to optimize content placement
such that the used transmission power is minimized. Numerical results show that
the clustering scheme enable to considerably improve the cache hit probability
and consequently the EE compared with an unclustered approach. Simulations also
show that the small base station allocation algorithm results in improving the
energy efficiency and hit probability.Comment: 30 pages, 5 figures, submitted to Transactions on Wireless
Communications (15-Dec-2016
An information-theoretic approach to the gravitational-wave burst detection problem
The observational era of gravitational-wave astronomy began in the Fall of
2015 with the detection of GW150914. One potential type of detectable
gravitational wave is short-duration gravitational-wave bursts, whose waveforms
can be difficult to predict. We present the framework for a new detection
algorithm for such burst events -- \textit{oLIB} -- that can be used in
low-latency to identify gravitational-wave transients independently of other
search algorithms. This algorithm consists of 1) an excess-power event
generator based on the Q-transform -- \textit{Omicron} --, 2) coincidence of
these events across a detector network, and 3) an analysis of the coincident
events using a Markov chain Monte Carlo Bayesian evidence calculator --
\textit{LALInferenceBurst}. These steps compress the full data streams into a
set of Bayes factors for each event; through this process, we use elements from
information theory to minimize the amount of information regarding the
signal-versus-noise hypothesis that is lost. We optimally extract this
information using a likelihood-ratio test to estimate a detection significance
for each event. Using representative archival LIGO data, we show that the
algorithm can detect gravitational-wave burst events of astrophysical strength
in realistic instrumental noise across different burst waveform morphologies.
We also demonstrate that the combination of Bayes factors by means of a
likelihood-ratio test can improve the detection efficiency of a
gravitational-wave burst search. Finally, we show that oLIB's performance is
robust against the choice of gravitational-wave populations used to model the
likelihood-ratio test likelihoods
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