120,389 research outputs found
Jet Substructure Without Trees
We present an alternative approach to identifying and characterizing jet
substructure. An angular correlation function is introduced that can be used to
extract angular and mass scales within a jet without reference to a clustering
algorithm. This procedure gives rise to a number of useful jet observables. As
an application, we construct a top quark tagging algorithm that is competitive
with existing methods.Comment: 22 pages, 16 figures, version accepted by JHE
Ordered Tomlinson-Harashima Precoding in G.fast Downstream
G.fast is an upcoming next generation DSL standard envisioned to use
bandwidth up to 212 MHz. Far-end crosstalk (FEXT) at these frequencies greatly
overcomes direct links. Its cancellation based on non-linear
Tomlinson-Harashima Precoding (THP) proved to show significant advantage over
standard linear precoding. This paper proposes a novel THP structure in which
ordering of successive interference pre-cancellation can be optimized for
downstream with non-cooperating receivers. The optimized scheme is compared to
existing THP structure denoted as equal-rate THP which is widely adopted in
wireless downlink. Structure and performance of both methods differ
significantly favoring the proposed scheme. The ordering that maximizes the
minimum rate (max-min fairness) for each tone of the discrete multi-tone
modulation is the familiar V-BLAST ordering. However, V-BLAST does not lead to
the global maximum when applied independently on each tone. The proposed novel
Dynamic Ordering (DO) strategy takes into account asymmetric channel statistics
to yield the highest minimum aggregated rate.Comment: 7 pages, 11 figures, Accepted at the 2015 IEEE Globecom 2015,
Selected Areas in Communications: Access Networks and Systems, 6-10 December,
201
Efficient Generation of Geographically Accurate Transit Maps
We present LOOM (Line-Ordering Optimized Maps), a fully automatic generator
of geographically accurate transit maps. The input to LOOM is data about the
lines of a given transit network, namely for each line, the sequence of
stations it serves and the geographical course the vehicles of this line take.
We parse this data from GTFS, the prevailing standard for public transit data.
LOOM proceeds in three stages: (1) construct a so-called line graph, where
edges correspond to segments of the network with the same set of lines
following the same course; (2) construct an ILP that yields a line ordering for
each edge which minimizes the total number of line crossings and line
separations; (3) based on the line graph and the ILP solution, draw the map. As
a naive ILP formulation is too demanding, we derive a new custom-tailored
formulation which requires significantly fewer constraints. Furthermore, we
present engineering techniques which use structural properties of the line
graph to further reduce the ILP size. For the subway network of New York, we
can reduce the number of constraints from 229,000 in the naive ILP formulation
to about 4,500 with our techniques, enabling solution times of less than a
second. Since our maps respect the geography of the transit network, they can
be used for tiles and overlays in typical map services. Previous research work
either did not take the geographical course of the lines into account, or was
concerned with schematic maps without optimizing line crossings or line
separations.Comment: 7 page
On the diffeomorphism commutators of lattice quantum gravity
We show that the algebra of discretized spatial diffeomorphism constraints in
Hamiltonian lattice quantum gravity closes without anomalies in the limit of
small lattice spacing. The result holds for arbitrary factor-ordering and for a
variety of different discretizations of the continuum constraints, and thus
generalizes an earlier calculation by Renteln.Comment: 16 pages, Te
goSLP: Globally Optimized Superword Level Parallelism Framework
Modern microprocessors are equipped with single instruction multiple data
(SIMD) or vector instruction sets which allow compilers to exploit superword
level parallelism (SLP), a type of fine-grained parallelism. Current SLP
auto-vectorization techniques use heuristics to discover vectorization
opportunities in high-level language code. These heuristics are fragile, local
and typically only present one vectorization strategy that is either accepted
or rejected by a cost model. We present goSLP, a novel SLP auto-vectorization
framework which solves the statement packing problem in a pairwise optimal
manner. Using an integer linear programming (ILP) solver, goSLP searches the
entire space of statement packing opportunities for a whole function at a time,
while limiting total compilation time to a few minutes. Furthermore, goSLP
optimally solves the vector permutation selection problem using dynamic
programming. We implemented goSLP in the LLVM compiler infrastructure,
achieving a geometric mean speedup of 7.58% on SPEC2017fp, 2.42% on SPEC2006fp
and 4.07% on NAS benchmarks compared to LLVM's existing SLP auto-vectorizer.Comment: Published at OOPSLA 201
STR2: Optimized Simple Tabular Reduction for Table Constraints
International audienceTable constraints play an important role within constraint programming. Recently, many schemes or algorithms have been proposed to propagate table constraints and/or to compress their representation. In this paper, we describe an optimization of simple tabular reduction (STR), a technique proposed by J. Ullmann to dynamically maintain the tables of supports when generalized arc consistency (GAC) is enforced/maintained. STR2, the new refined GAC algorithm we propose, allows us to limit the number of operations related to validity checking and search of supports. Interestingly enough, this optimization makes simple tabular reduction potentially r times faster where r is the arity of the constraint(s). The results of an extensive experimentation that we have conducted with respect to random and structured instances indicate that STR2 is usually around twice as fast as the original STR, two or three times faster than the approach based on the hidden variable encoding, and can be up to one order of magnitude faster than previously state-of-the art (generic) GAC algorithms on some series of instances. When comparing STR2 with the more recently developed algorithm based on multi-valued decision diagrams (MDDs), we show that both approaches are rather complementary
Decompositions of Grammar Constraints
A wide range of constraints can be compactly specified using automata or
formal languages. In a sequence of recent papers, we have shown that an
effective means to reason with such specifications is to decompose them into
primitive constraints. We can then, for instance, use state of the art SAT
solvers and profit from their advanced features like fast unit propagation,
clause learning, and conflict-based search heuristics. This approach holds
promise for solving combinatorial problems in scheduling, rostering, and
configuration, as well as problems in more diverse areas like bioinformatics,
software testing and natural language processing. In addition, decomposition
may be an effective method to propagate other global constraints.Comment: Proceedings of the Twenty-Third AAAI Conference on Artificial
Intelligenc
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