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0707.0978
|
When Network Coding and Dirty Paper Coding meet in a Cooperative Ad Hoc
Network
|
cs.IT math.IT
|
We develop and analyze new cooperative strategies for ad hoc networks that
are more spectrally efficient than classical DF cooperative protocols. Using
analog network coding, our strategies preserve the practical half-duplex
assumption but relax the orthogonality constraint. The introduction of
interference due to non-orthogonality is mitigated thanks to precoding, in
particular Dirty Paper coding. Combined with smart power allocation, our
cooperation strategies allow to save time and lead to more efficient use of
bandwidth and to improved network throughput with respect to classical RDF/PDF.
|
0707.1008
|
On Cognitive Interference Networks
|
cs.IT math.IT
|
We study the high-power asymptotic behavior of the sum-rate capacity of
multi-user interference networks with an equal number of transmitters and
receivers. We assume that each transmitter is cognizant of the message it
wishes to convey to its corresponding receiver and also of the messages that a
subset of the other transmitters wish to send. The receivers are assumed not to
be able to cooperate in any way so that they must base their decision on the
signal they receive only. We focus on the network's pre-log, which is defined
as the limiting ratio of the sum-rate capacity to half the logarithm of the
transmitted power. We present both upper and lower bounds on the network's
pre-log. The lower bounds are based on a linear partial-cancellation scheme
which entails linearly transforming Gaussian codebooks so as to eliminate the
interference in a subset of the receivers. Inter alias, the bounds give a
complete characterization of the networks and side-information settings that
result in a full pre-log, i.e., in a pre-log that is equal to the number of
transmitters (and receivers) as well as a complete characterization of networks
whose pre-log is equal to the full pre-log minus one. They also fully
characterize networks where the full pre-log can only be achieved if each
transmitter knows the messages of all users, i.e., when the side-information is
"full".
|
0707.1025
|
The star trellis decoding of Reed-Solomon codes
|
cs.IT math.IT
|
The new method for Reed-Solomon codes decoding is introduced. The method is
based on the star trellis decoding of the binary image of Reed-Solomon codes.
|
0707.1063
|
High-resolution distributed sampling of bandlimited fields with
low-precision sensors
|
cs.IT math.IT
|
The problem of sampling a discrete-time sequence of spatially bandlimited
fields with a bounded dynamic range, in a distributed,
communication-constrained, processing environment is addressed. A central unit,
having access to the data gathered by a dense network of fixed-precision
sensors, operating under stringent inter-node communication constraints, is
required to reconstruct the field snapshots to maximum accuracy. Both
deterministic and stochastic field models are considered. For stochastic
fields, results are established in the almost-sure sense. The feasibility of
having a flexible tradeoff between the oversampling rate (sensor density) and
the analog-to-digital converter (ADC) precision, while achieving an exponential
accuracy in the number of bits per Nyquist-interval per snapshot is
demonstrated. This exposes an underlying ``conservation of bits'' principle:
the bit-budget per Nyquist-interval per snapshot (the rate) can be distributed
along the amplitude axis (sensor-precision) and space (sensor density) in an
almost arbitrary discrete-valued manner, while retaining the same (exponential)
distortion-rate characteristics. Achievable information scaling laws for field
reconstruction over a bounded region are also derived: With N one-bit sensors
per Nyquist-interval, $\Theta(\log N)$ Nyquist-intervals, and total network
bitrate $R_{net} = \Theta((\log N)^2)$ (per-sensor bitrate $\Theta((\log
N)/N)$), the maximum pointwise distortion goes to zero as $D = O((\log N)^2/N)$
or $D = O(R_{net} 2^{-\beta \sqrt{R_{net}}})$. This is shown to be possible
with only nearest-neighbor communication, distributed coding, and appropriate
interpolation algorithms. For a fixed, nonzero target distortion, the number of
fixed-precision sensors and the network rate needed is always finite.
|
0707.1064
|
The Effect of Noise Correlation in AF Relay Networks
|
cs.IT math.IT
|
In wireless relay networks, noise at the relays can be correlated possibly
due to common interference or noise propagation from preceding hops. In this
work we consider a parallel relay network with noise correlation. For the relay
strategy of amplify-and-forward (AF), we determine the optimal rate maximizing
relay gains when correlation knowledge is available at the relays. The effect
of correlation on the performance of the relay networks is analyzed for the
cases where full knowledge of correlation is available at the relays and when
there is no knowledge about the correlation structure. Interestingly we find
that, on the average, noise correlation is beneficial regardless of whether the
relays know the noise covariance matrix or not. However, the knowledge of
correlation can greatly improve the performance. Typically, the performance
improvement from correlation knowledge increases with the relay power and the
number of relays. With perfect correlation knowledge the system is capable of
canceling interference if the number of interferers is less than the number of
relays.
For a dual-hop multiple access parallel network, we obtain closed form
expressions for the maximum sum-rate and the optimal relay strategy. The relay
optimization for networks with three hops is also considered. For any relay
gains for the first stage relays, this represents a parallel relay network with
correlated noise. Based on the result of two hop networks with noise
correlation, we propose an algorithm for solving the relay optimization problem
for three-hop networks.
|
0707.1083
|
Delayed Correlations in Inter-Domain Network Traffic
|
cs.NI cs.IR
|
To observe the evolution of network traffic correlations we analyze the
eigenvalue spectra and eigenvectors statistics of delayed correlation matrices
of network traffic counts time series. Delayed correlation matrix D is composed
of the correlations between one variable in the multivariable time series and
another at a time delay \tau . Inverse participation ratio (IPR) of
eigenvectors of D deviates substantially from the IPR of eigenvectors of the
equal time correlation matrix C. We relate this finding to the localization and
discuss its importance for network congestion control. The time-lagged
correlation pattern between network time series is preserved over a long time,
up to 100\tau, where \tau=300 sec. The largest eigenvalue \lambda_{max} of D
and the corresponding IPR oscillate with two characteristic periods of 3\tau
and 6\tau . The existence of delayed correlations between network time series
fits well into the long range dependence (LRD) property of the network traffic.
The ability to monitor and control the long memory processes is crucial since
they impact the network performance. Injecting the random traffic counts
between non-randomly correlated time series, we were able to break the picture
of periodicity of \lambda_{max}. In addition, we investigated influence of the
periodic injections on both largest eigenvalue and the IPR, and addressed
relevance of these indicators for the LRD and self-similarity of the network
traffic.
|
0707.1099
|
Worst-Case Interactive Communication and Enhancing Sensor Network
Lifetime
|
cs.IT math.IT
|
We are concerned with the problem of maximizing the worst-case lifetime of a
data-gathering wireless sensor network consisting of a set of sensor nodes
directly communicating with a base-station.We propose to solve this problem by
modeling sensor node and base-station communication as the interactive
communication between multiple correlated informants (sensor nodes) and a
recipient (base-station). We provide practical and scalable interactive
communication protocols for data gathering in sensor networks and demonstrate
their efficiency compared to traditional approaches.
In this paper, we first develop a formalism to address the problem of
worst-case interactive communication between a set of multiple correlated
informants and a recipient. We realize that there can be different objectives
to achieve in such a communication scenario and compute the optimal number of
messages and bits exchanged to realize these objectives. Then, we propose to
adapt these results in the context of single-hop data-gathering sensor
networks. Finally, based on this proposed formalism, we propose a clustering
based communication protocol for large sensor networks and demonstrate its
superiority over a traditional clustering protocol.
|
0707.1193
|
Singular curves and cusp points in the joint space of 3-RPR parallel
manipulators
|
cs.RO
|
This paper investigates the singular curves in two-dimensional slices of the
joint space of a family of planar parallel manipulators. It focuses on special
points, referred to as cusp points, which may appear on these curves. Cusp
points play an important role in the kinematic behavior of parallel
manipulators since they make possible a nonsingular change of assembly mode.
The purpose of this study is twofold. First, it reviews an important previous
work, which, to the authors' knowledge, has never been exploited yet. Second,
it determines the cusp points in any two-dimensional slice of the joint space.
First results show that the number of cusp points may vary from zero to eight.
This work finds applications in both design and trajectory planning.
|
0707.1241
|
Graph-Based Decoding in the Presence of ISI
|
cs.IT math.IT
|
We propose an approximation of maximum-likelihood detection in ISI channels
based on linear programming or message passing. We convert the detection
problem into a binary decoding problem, which can be easily combined with LDPC
decoding. We show that, for a certain class of channels and in the absence of
coding, the proposed technique provides the exact ML solution without an
exponential complexity in the size of channel memory, while for some other
channels, this method has a non-diminishing probability of failure as SNR
increases. Some analysis is provided for the error events of the proposed
technique under linear programming.
|
0707.1288
|
Espaces de repr\'esentation multidimensionnels d\'edi\'es \`a la
visualisation
|
cs.DB
|
In decision-support systems, the visual component is important for On Line
Analysis Processing (OLAP). In this paper, we propose a new approach that faces
the visualization problem due to data sparsity. We use the results of a
Multiple Correspondence Analysis (MCA) to reduce the negative effect of
sparsity by organizing differently data cube cells. Our approach does not
reduce sparsity, however it tries to build relevant representation spaces where
facts are efficiently gathered. In order to evaluate our approach, we propose
an homogeneity criterion based on geometric neighborhood of cells. The obtained
experimental results have shown the efficiency of our method.
|
0707.1295
|
Efficient supervised learning in networks with binary synapses
|
q-bio.NC cond-mat.stat-mech cs.NE q-bio.QM
|
Recent experimental studies indicate that synaptic changes induced by
neuronal activity are discrete jumps between a small number of stable states.
Learning in systems with discrete synapses is known to be a computationally
hard problem. Here, we study a neurobiologically plausible on-line learning
algorithm that derives from Belief Propagation algorithms. We show that it
performs remarkably well in a model neuron with binary synapses, and a finite
number of `hidden' states per synapse, that has to learn a random
classification task. Such system is able to learn a number of associations
close to the theoretical limit, in time which is sublinear in system size. This
is to our knowledge the first on-line algorithm that is able to achieve
efficiently a finite number of patterns learned per binary synapse.
Furthermore, we show that performance is optimal for a finite number of hidden
states which becomes very small for sparse coding. The algorithm is similar to
the standard `perceptron' learning algorithm, with an additional rule for
synaptic transitions which occur only if a currently presented pattern is
`barely correct'. In this case, the synaptic changes are meta-plastic only
(change in hidden states and not in actual synaptic state), stabilizing the
synapse in its current state. Finally, we show that a system with two visible
states and K hidden states is much more robust to noise than a system with K
visible states. We suggest this rule is sufficiently simple to be easily
implemented by neurobiological systems or in hardware.
|
0707.1304
|
Un index de jointure pour les entrep\^ots de donn\'ees XML
|
cs.DB
|
XML data warehouses form an interesting basis for decision-support
applications that exploit heterogeneous data from multiple sources. However,
XML-native database systems currently bear limited performances and it is
necessary to research ways to optimize them. In this paper, we propose a new
index that is specifically adapted to the multidimensional architecture of XML
warehouses and eliminates join operations, while preserving the information
contained in the original warehouse. A theoretical study and experimental
results demonstrate the efficiency of our index, even when queries are complex.
|
0707.1306
|
S\'election simultan\'ee d'index et de vues mat\'erialis\'ees
|
cs.DB
|
Indices and materialized views are physical structures that accelerate data
access in data warehouses. However, these data structures generate some
maintenance overhead. They also share the same storage space. The existing
studies about index and materialized view selection consider these structures
separately. In this paper, we adopt the opposite stance and couple index and
materialized view selection to take into account the interactions between them
and achieve an efficient storage space sharing. We develop cost models that
evaluate the respective benefit of indexing and view materialization. These
cost models are then exploited by a greedy algorithm to select a relevant
configuration of indices and materialized views. Experimental results show that
our strategy performs better than the independent selection of indices and
materialized views.
|
0707.1452
|
Clusters, Graphs, and Networks for Analysing Internet-Web-Supported
Communication within a Virtual Community
|
cs.AI cs.LG
|
The proposal is to use clusters, graphs and networks as models in order to
analyse the Web structure. Clusters, graphs and networks provide knowledge
representation and organization. Clusters were generated by co-site analysis.
The sample is a set of academic Web sites from the countries belonging to the
European Union. These clusters are here revisited from the point of view of
graph theory and social network analysis. This is a quantitative and structural
analysis. In fact, the Internet is a computer network that connects people and
organizations. Thus we may consider it to be a social network. The set of Web
academic sites represents an empirical social network, and is viewed as a
virtual community. The network structural properties are here analysed applying
together cluster analysis, graph theory and social network analysis.
|
0707.1470
|
Secrecy Capacity Region of Fading Broadcast Channels
|
cs.IT math.IT
|
The fading broadcast channel with confidential messages (BCC) is
investigated, where a source node has common information for two receivers
(receivers 1 and 2), and has confidential information intended only for
receiver 1. The confidential information needs to be kept as secret as possible
from receiver 2. The channel state information (CSI) is assumed to be known at
both the transmitter and the receivers. The secrecy capacity region is first
established for the parallel Gaussian BCC, and the optimal source power
allocations that achieve the boundary of the secrecy capacity region are
derived. In particular, the secrecy capacity region is established for the
Gaussian case of the Csiszar-Korner BCC model. The secrecy capacity results are
then applied to give the ergodic secrecy capacity region for the fading BCC.
|
0707.1534
|
An Architecture Framework for Complex Data Warehouses
|
cs.DB
|
Nowadays, many decision support applications need to exploit data that are
not only numerical or symbolic, but also multimedia, multistructure,
multisource, multimodal, and/or multiversion. We term such data complex data.
Managing and analyzing complex data involves a lot of different issues
regarding their structure, storage and processing, and metadata are a key
element in all these processes. Such problems have been addressed by classical
data warehousing (i.e., applied to "simple" data). However, data warehousing
approaches need to be adapted for complex data. In this paper, we first propose
a precise, though open, definition of complex data. Then we present a general
architecture framework for warehousing complex data. This architecture heavily
relies on metadata and domain-related knowledge, and rests on the XML language,
which helps storing data, metadata and domain-specific knowledge altogether,
and facilitates communication between the various warehousing processes.
|
0707.1548
|
Data Mining-based Materialized View and Index Selection in Data
Warehouses
|
cs.DB
|
Materialized views and indexes are physical structures for accelerating data
access that are casually used in data warehouses. However, these data
structures generate some maintenance overhead. They also share the same storage
space. Most existing studies about materialized view and index selection
consider these structures separately. In this paper, we adopt the opposite
stance and couple materialized view and index selection to take view-index
interactions into account and achieve efficient storage space sharing.
Candidate materialized views and indexes are selected through a data mining
process. We also exploit cost models that evaluate the respective benefit of
indexing and view materialization, and help select a relevant configuration of
indexes and materialized views among the candidates. Experimental results show
that our strategy performs better than an independent selection of materialized
views and indexes.
|
0707.1558
|
Autonomy with regard to an Attribute
|
cs.MA
|
This paper presents a model of autonomy called autonomy with regard to an
attribute applicable to cognitive and not cognitive artificial agents. Three
criteria (global / partial, social / nonsocial, absolute / relative) are
defined and used to describe the main characteristics of this type of autonomy.
A software agent autonomous with regard to the mobility illustrates a possible
implementation of this model.
|
0707.1588
|
A Tight Lower Bound to the Outage Probability of Discrete-Input
Block-Fading Channels
|
cs.IT math.IT
|
In this correspondence, we propose a tight lower bound to the outage
probability of discrete-input Nakagami-m block-fading channels. The approach
permits an efficient method for numerical evaluation of the bound, providing an
additional tool for system design. The optimal rate-diversity trade-off for the
Nakagami-m block-fading channel is also derived and a tight upper bound is
obtained for the optimal coding gain constant.
|
0707.1644
|
Fast and Simple Relational Processing of Uncertain Data
|
cs.DB cs.PF
|
This paper introduces U-relations, a succinct and purely relational
representation system for uncertain databases. U-relations support
attribute-level uncertainty using vertical partitioning. If we consider
positive relational algebra extended by an operation for computing possible
answers, a query on the logical level can be translated into, and evaluated as,
a single relational algebra query on the U-relation representation. The
translation scheme essentially preserves the size of the query in terms of
number of operations and, in particular, number of joins. Standard techniques
employed in off-the-shelf relational database management systems are effective
for optimizing and processing queries on U-relations. In our experiments we
show that query evaluation on U-relations scales to large amounts of data with
high degrees of uncertainty.
|
0707.1739
|
On slow-fading non-separable correlation MIMO systems
|
cs.IT math.IT
|
In a frequency selective slow-fading channel in a MIMO system, the channel
matrix is of the form of a block matrix. We propose a method to calculate the
limit of the eigenvalue distribution of block matrices if the size of the
blocks tends to infinity. We will also calculate the asymptotic eigenvalue
distribution of $HH^*$, where the entries of $H$ are jointly Gaussian, with a
correlation of the form $E[h_{pj}\bar h_{qk}]= \sum_{s=1}^t
\Psi^{(s)}_{jk}\hat\Psi^{(s)}_{pq}$ (where $t$ is fixed and does not increase
with the size of the matrix). We will use an operator-valued free probability
approach to achieve this goal. Using this method, we derive a system of
equations, which can be solved numerically to compute the desired eigenvalue
distribution.
|
0707.1824
|
The Kinematics of Manipulators Built From Closed Planar Mechanisms
|
cs.RO
|
The paper discusses the kinematics of manipulators builts of planar closed
kinematic chains. A special kinematic scheme is extracted from the array of
these mechanisms that looks the most promising for the creation of different
types of robotic manipulators. The structural features of this manipulator
determine a number of its original properties that essentially simplify its
control. These features allow the main control problems to be effectively
overcome by application of the simple kinematic problems. The workspace and
singular configurations of a basic planar manipulator are studied. By using a
graphic simulation method, motions of the designed mechanism are examined. A
prototype of this mechanism was implemented to verify the proposed approach.
|
0707.1859
|
On the Degrees of Freedom in Cognitive Radio Channels
|
cs.IT math.IT
|
After receiving useful peer comments, we would like to withdraw this paper.
|
0707.1912
|
Throughput Scaling Laws for Wireless Networks with Fading Channels
|
cs.IT math.IT
|
A network of n communication links, operating over a shared wireless channel,
is considered. Fading is assumed to be the dominant factor affecting the
strength of the channels between transmitter and receiver terminals. It is
assumed that each link can be active and transmit with a constant power P or
remain silent. The objective is to maximize the throughput over the selection
of active links. By deriving an upper bound and a lower bound, it is shown that
in the case of Rayleigh fading (i) the maximum throughput scales like $\log n$
(ii) the maximum throughput is achievable in a distributed fashion. The upper
bound is obtained using probabilistic methods, where the key point is to upper
bound the throughput of any random set of active links by a chi-squared random
variable. To obtain the lower bound, a decentralized link activation strategy
is proposed and analyzed.
|
0707.1913
|
Removing Manually-Generated Boilerplate from Electronic Texts:
Experiments with Project Gutenberg e-Books
|
cs.DL cs.CL
|
Collaborative work on unstructured or semi-structured documents, such as in
literature corpora or source code, often involves agreed upon templates
containing metadata. These templates are not consistent across users and over
time. Rule-based parsing of these templates is expensive to maintain and tends
to fail as new documents are added. Statistical techniques based on frequent
occurrences have the potential to identify automatically a large fraction of
the templates, thus reducing the burden on the programmers. We investigate the
case of the Project Gutenberg corpus, where most documents are in ASCII format
with preambles and epilogues that are often copied and pasted or manually
typed. We show that a statistical approach can solve most cases though some
documents require knowledge of English. We also survey various technical
solutions that make our approach applicable to large data sets.
|
0707.1957
|
Moveability and Collision Analysis for Fully-Parallel Manipulators
|
cs.RO
|
The aim of this paper is to characterize the moveability of fully-parallel
manipulators in the presence of obstacles. Fully parallel manipulators are used
in applications where accuracy, stiffness or high speeds and accelerations are
required \cite{Merlet:97}. However, one of its main drawbacks is a relatively
small workspace compared to the one of serial manipulators. This is due mainly
to the existence of potential internal collisions, and the existence of
singularities. In this paper, the notion of free aspect is defined which
permits to exhibit domains of the workspace and the joint space free of
singularity and collision. The main application of this study is the
moveability analysis in the workspace of the manipulator as well as
path-planning, control and design.
|
0707.2006
|
Working Modes and Aspects in Fully-Parallel Manipulator
|
cs.RO
|
The aim of this paper is to characterize the notion of aspect in the
workspace and in the joint space for parallel manipulators. In opposite to the
serial manipulators, the parallel manipulators can admit not only multiple
inverse kinematic solutions, but also multiple direct kinematic solutions. The
notion of aspect introduced for serial manipulators in [Borrel 86], and
redefined for parallel manipulators with only one inverse kinematic solution in
[Wenger 1997], is redefined for general fully parallel manipulators. Two
Jacobian matrices appear in the kinematic relations between the joint-rate and
the Cartesian-velocity vectors, which are called the "inverse kinematics" and
the "direct kinematics" matrices. The study of these matrices allow to
respectively define the parallel and the serial singularities. The notion of
working modes is introduced to separate inverse kinematic solutions. Thus, we
can find out domains of the workspace and the joint space exempt of
singularity. Application of this study is the moveability analysis in the
workspace of the manipulator as well as path-planing and control. This study is
illustrated in this paper with a RR-RRR planar parallel manipulator.
|
0707.2014
|
On the error exponent of variable-length block-coding schemes over
finite-state Markov channels with feedback
|
cs.IT math.IT
|
The error exponent of Markov channels with feedback is studied in the
variable-length block-coding setting. Burnashev's classic result is extended
and a single letter characterization for the reliability function of
finite-state Markov channels is presented, under the assumption that the
channel state is causally observed both at the transmitter and at the receiver
side. Tools from stochastic control theory are used in order to treat channels
with intersymbol interference. In particular the convex analytical approach to
Markov decision processes is adopted to handle problems with stopping time
horizons arising from variable-length coding schemes.
|
0707.2017
|
The Isoconditioning Loci of A Class of Closed-Chain Manipulators
|
cs.RO
|
The subject of this paper is a special class of closed-chain manipulators.
First, we analyze a family of two-degree-of-freedom (dof) five-bar planar
linkages. Two Jacobian matrices appear in the kinematic relations between the
joint-rate and the Cartesian-velocity vectors, which are called the ``inverse
kinematics" and the "direct kinematics" matrices. It is shown that the loci of
points of the workspace where the condition number of the direct-kinematics
matrix remains constant, i.e., the isoconditioning loci, are the coupler points
of the four-bar linkage obtained upon locking the middle joint of the linkage.
Furthermore, if the line of centers of the two actuated revolutes is used as
the axis of a third actuated revolute, then a three-dof hybrid manipulator is
obtained. The isoconditioning loci of this manipulator are surfaces of
revolution generated by the isoconditioning curves of the two-dof manipulator,
whose axis of symmetry is that of the third actuated revolute.
|
0707.2027
|
Workspace and Assembly modes in Fully-Parallel Manipulators : A
Descriptive Study
|
cs.RO
|
The goal of this paper is to explain, using a typical example, the
distribution of the different assembly modes in the workspace and their
effective role in the execution of trajectories. The singular and non-singular
changes of assembly mode are described and compared to each other. The
non-singular change of assembly mode is more deeply analysed and discussed in
the context of trajectory planning. In particular, it is shown that, according
to the location of the initial and final configurations with respect to the
uniqueness domains in the workspace, there are three different cases to
consider before planning a linking trajectory.
|
0707.2034
|
Conception Isotropique D'Une Morphologie Parall\`Ele : Application \`a
L'Usinage
|
cs.RO
|
The aim of this paper is the isotropic design of a hybrid morphology
dedicated to 3-axis machining applications. It is necessary to ensure the
feasibility of continuous, singularity-free trajectories, as well as a good
manipulability in position and velocity. We want to propose an alternative
design to conventional serial machine-tools. We compare a serial PPP
machine-tool (three prismatic orthogonal axes) with a hybrid architecture which
we optimize only the first two axes. The critrerion used for the optimization
is the conditioning of the Jacobian matrices. The optimum, namely isotropy, can
be obtained which provides our architecture with excellent manipulability
properties.
|
0707.2042
|
A distributed Approach for Access and Visibility Task under Ergonomic
Constraints with a Manikin in a Virtual Reality Environment
|
cs.RO
|
This paper presents a new method, based on a multi-agent system and on
digital mock-up technology, to assess an efficient path planner for a manikin
for access and visibility task under ergonomic constraints. In order to solve
this problem, the human operator is integrated in the process optimization to
contribute to a global perception of the environment. This operator cooperates,
in real-time, with several automatic local elementary agents. The result of
this work validates solutions brought by digital mock-up and that can be
applied to simulate maintenance task.
|
0707.2090
|
A Training based Distributed Non-Coherent Space-Time Coding Strategy
|
cs.IT math.IT
|
Unitary space-time modulation is known to be an efficient means to
communicate over non-coherent Multiple Input Multiple Output (MIMO) channels.
In this letter, differential unitary space-time coding and non-coherent
space-time coding for the training based approach of Kim and Tarokh are
addressed. For this approach, necessary and sufficient conditions for
multi-group decodability are derived in a simple way assuming a Generalized
Likelihood Ratio Test receiver and a unitary codebook. Extending Kim and
Tarokh's approach for colocated MIMO systems, a novel training based approach
to distributed non-coherent space-time coding for wireless relay networks is
proposed. An explicit construction of two-group decodable distributed
non-coherent space-time codes achieving full cooperative diversity for all even
number of relays is provided.
|
0707.2176
|
Benefit of Delay on the Diversity-Multiplexing Tradeoffs of MIMO
Channels with Partial CSI
|
cs.IT math.IT
|
This paper re-examines the well-known fundamental tradeoffs between rate and
reliability for the multi-antenna, block Rayleigh fading channel in the high
signal to noise ratio (SNR) regime when (i) the transmitter has access to
(noiseless) one bit per coherence-interval of causal channel state information
(CSI) and (ii) soft decoding delays together with worst-case delay guarantees
are acceptable. A key finding of this work is that substantial improvements in
reliability can be realized with a very short expected delay and a slightly
longer (but bounded) worst-case decoding delay guarantee in communication
systems where the transmitter has access to even one bit per coherence interval
of causal CSI. While similar in spirit to the recent work on communication
systems based on automatic repeat requests (ARQ) where decoding failure is
known at the transmitter and leads to re-transmission, here transmit
side-information is purely based on CSI. The findings reported here also lend
further support to an emerging understanding that decoding delay (related to
throughput) and codeword blocklength (related to coding complexity and delays)
are distinctly different design parameters which can be tuned to control
reliability.
|
0707.2185
|
Mod\'elisation Dynamique d'un Robot Parall\`ele \`a 3-DDL : l'Orthoglide
|
cs.RO
|
In this article, we propose a method for calculation of the inverse and
direct dynamic models of the Orthoglide, a parallel robot with threedegrees of
freedom in translation. These models are calculated starting from the elements
of the dynamic model of the kinematic chain structure and equations of
Newton-Euler applied to the platform. These models are obtained in explicit
form having an interesting physical interpretation.
|
0707.2191
|
Word statistics in Blogs and RSS feeds: Towards empirical universal
evidence
|
cs.IT math.IT
|
We focus on the statistics of word occurrences and of the waiting times
between such occurrences in Blogs. Due to the heterogeneity of words'
frequencies, the empirical analysis is performed by studying classes of
"frequently-equivalent" words, i.e. by grouping words depending on their
frequencies. Two limiting cases are considered: the dilute limit, i.e. for
those words that are used less than once a day, and the dense limit for
frequent words. In both cases, extreme events occur more frequently than
expected from the Poisson hypothesis. These deviations from Poisson statistics
reveal non-trivial time correlations between events that are associated with
bursts of activities. The distribution of waiting times is shown to behave like
a stretched exponential and to have the same shape for different sets of words
sharing a common frequency, thereby revealing universal features.
|
0707.2227
|
Degeneracy study of the forward kinematics of planar 3-RPR parallel
manipulators
|
cs.RO
|
This paper investigates two situations in which the forward kinematics of
planar 3-RPR parallel manipulators degenerates. These situations have not been
addressed before. The first degeneracy arises when the three input joint
variables r1, r2 and r3 satisfy a certain relationship. This degeneracy yields
a double root of the characteristic polynomial in t, which could be erroneously
interpreted as two coalesce assembly modes. But, unlike what arises in
non-degenerate cases, this double root yields two sets of solutions for the
position coordinates (x, y) of the platform. In the second situation, we show
that the forward kinematics degenerates over the whole joint space if the base
and platform triangles are congruent and the platform triangle is rotated by
180 deg about one of its sides. For these "degenerate" manipulators, which are
defined here for the first time, the forward kinematics is reduced to the
solution of a 3rd-degree polynomial and a quadratics in sequence. Such
manipulators constitute, in turn, a new family of analytic planar manipulators
that would be more suitable for industrial applications.
|
0707.2228
|
Kinematic Analysis of a Family of 3R Manipulators
|
cs.RO
|
The workspace topologies of a family of 3-revolute (3R) positioning
manipulators are enumerated. The workspace is characterized in a half-cross
section by the singular curves. The workspace topology is defined by the number
of cusps that appear on these singular curves. The design parameters space is
shown to be divided into five domains where all manipulators have the same
number of cusps. Each separating surface is given as an explicit expression in
the DH-parameters. As an application of this work, we provide a necessary and
sufficient condition for a 3R orthogonal manipulator to be cuspidal, i.e. to
change posture without meeting a singularity. This condition is set as an
explicit expression in the DH parameters.
|
0707.2229
|
The Computation of All 4R Serial Spherical Wrists With an Isotropic
Architecture
|
cs.RO
|
A spherical wrist of the serial type with n revolute (R) joints is said to be
isotropic if it can attain a posture whereby the singular values of its
Jacobian matrix are all equal to sqrt(n/3). What isotropy brings about is
robustness to manufacturing, assembly, and measurement errors, thereby
guaranteeing a maximum orientation accuracy. In this paper we investigate the
existence of redundant isotropic architectures, which should add to the
dexterity of the wrist under design by virtue of its extra degree of freedom.
The problem formulation, for, leads to a system of eight quadratic equations
with eight unknowns. The Bezout number of this system is thus 2^8=256, its BKK
bound being 192. However, the actual number of solutions is shown to be 32. We
list all solutions of the foregoing algebraic problem. All these solutions are
real, but distinct solutions do not necessarily lead to distinct manipulators.
Upon discarding those algebraic solutions that yield no new wrists, we end up
with exactly eight distinct architectures, the eight corresponding manipulators
being displayed at their isotropic postures.
|
0707.2238
|
A design oriented study for 3R Orthogonal Manipulators With Geometric
Simplifications
|
cs.RO
|
This paper proposes a method to calculate the largest Regular Dextrous
Workspace (RDW) of some types of three-revolute orthogonal manipulators that
have at least one of their DH parameters equal to zero. Then a new performance
index based on the RDW is introduced, the isocontours of this index are plotted
in the parameter space of the interesting types of manipulators and finally an
inspection of the domains of the parameter spaces is conducted in order to
identify the better manipulator architectures. The RDW is a part of the
workspace whose shape is regular (cube, cylinder) and the performances
(conditioning index) are bounded inside. The groups of 3R orthogonal
manipulators studied have interesting kinematic properties such as, a
well-connected workspace that is fully reachable with four inverse kinematic
solutions and that does not contain any void. This study is of high interest
for the design of alternative manipulator geometries.
|
0707.2265
|
Separable convex optimization problems with linear ascending constraints
|
cs.IT math.IT math.OC
|
Separable convex optimization problems with linear ascending inequality and
equality constraints are addressed in this paper. Under an ordering condition
on the slopes of the functions at the origin, an algorithm that determines the
optimum point in a finite number of steps is described. The optimum value is
shown to be monotone with respect to a partial order on the constraint
parameters. Moreover, the optimum value is convex with respect to these
parameters. Examples motivated by optimizations for communication systems are
used to illustrate the algorithm.
|
0707.2270
|
Design of a Spherical Wrist with Parallel Architecture: Application to
Vertebrae of an Eel Robot
|
cs.RO
|
The design of a spherical wrist with parallel architecture is the object of
this article. This study is part of a larger project, which aims to design and
to build an eel robot for inspection of immersed piping. The kinematic analysis
of the mechanism is presented first to characterize the singular configurations
as well as the isotropic configurations. We add the design constraints related
to the application, such as (i) the compactness of the mechanism, (ii) the
symmetry of the elements in order to ensure static and dynamic balance and
(iii) the possibility of the mechanism to fill the elliptic form of the ell
sections.
|
0707.2275
|
Passive Control Architecture for Virtual Humans
|
cs.RO
|
In the present paper, we introduce a new control architecture aimed at
driving virtual humans in interaction with virtual environments, by motion
capture. It brings decoupling of functionalities, and also of stability thanks
to passivity. We show projections can break passivity, and thus must be used
carefully. Our control scheme enables task space and internal control, contact,
and joint limits management. Thanks to passivity, it can be easily extended.
Besides, we introduce a new tool as for manikin's control, which makes it able
to build passive projections, so as to guide the virtual manikin when sharp
movements are needed.
|
0707.2376
|
Tripartitions do not always discriminate phylogenetic networks
|
q-bio.PE cs.CE cs.DM
|
Phylogenetic networks are a generalization of phylogenetic trees that allow
for the representation of non-treelike evolutionary events, like recombination,
hybridization, or lateral gene transfer. In a recent series of papers devoted
to the study of reconstructibility of phylogenetic networks, Moret, Nakhleh,
Warnow and collaborators introduced the so-called {tripartition metric for
phylogenetic networks. In this paper we show that, in fact, this tripartition
metric does not satisfy the separation axiom of distances (zero distance means
isomorphism, or, in a more relaxed version, zero distance means
indistinguishability in some specific sense) in any of the subclasses of
phylogenetic networks where it is claimed to do so. We also present a subclass
of phylogenetic networks whose members can be singled out by means of their
sets of tripartitions (or even clusters), and hence where the latter can be
used to define a meaningful metric.
|
0707.2432
|
Pricing Asian Options for Jump Diffusions
|
cs.CE
|
We construct a sequence of functions that uniformly converge (on compact
sets) to the price of Asian option, which is written on a stock whose dynamics
follows a jump diffusion, exponentially fast. Each of the element in this
sequence solves a parabolic partial differen- tial equation (not an
integro-differential equation). As a result we obtain a fast numerical
approximation scheme whose accuracy versus speed characteristics can be
controlled. We analyze the performance of our numerical algorithm on several
examples.
|
0707.2482
|
Multiple-Description Lattice Vector Quantization
|
cs.IT math.IT
|
In this thesis, we construct and analyze multiple-description codes based on
lattice vector quantization.
|
0707.2506
|
Mixed Integer Linear Programming For Exact Finite-Horizon Planning In
Decentralized Pomdps
|
cs.AI
|
We consider the problem of finding an n-agent joint-policy for the optimal
finite-horizon control of a decentralized Pomdp (Dec-Pomdp). This is a problem
of very high complexity (NEXP-hard in n >= 2). In this paper, we propose a new
mathematical programming approach for the problem. Our approach is based on two
ideas: First, we represent each agent's policy in the sequence-form and not in
the tree-form, thereby obtaining a very compact representation of the set of
joint-policies. Second, using this compact representation, we solve this
problem as an instance of combinatorial optimization for which we formulate a
mixed integer linear program (MILP). The optimal solution of the MILP directly
yields an optimal joint-policy for the Dec-Pomdp. Computational experience
shows that formulating and solving the MILP requires significantly less time to
solve benchmark Dec-Pomdp problems than existing algorithms. For example, the
multi-agent tiger problem for horizon 4 is solved in 72 secs with the MILP
whereas existing algorithms require several hours to solve it.
|
0707.2527
|
Rate and Power Allocation for Discrete-Rate Link Adaptation
|
cs.IT math.IT
|
Link adaptation, in particular adaptive coded modulation (ACM), is a
promising tool for bandwidth-efficient transmission in a fading environment.
The main motivation behind employing ACM schemes is to improve the spectral
efficiency of wireless communication systems. In this paper, using a finite
number of capacity achieving component codes, we propose new transmission
schemes employing constant power transmission, as well as discrete and
continuous power adaptation, for slowly varying flat-fading channels.
We show that the proposed transmission schemes can achieve throughputs close
to the Shannon limits of flat-fading channels using only a small number of
codes. Specifically, using a fully discrete scheme with just four codes, each
associated with four power levels, we achieve a spectral efficiency within 1 dB
of the continuous-rate continuous-power Shannon capacity. Furthermore, when
restricted to a fixed number of codes, the introduction of power adaptation has
significant gains with respect to ASE and probability of no transmission
compared to a constant power scheme.
|
0707.2718
|
Animation of virtual mannequins, robot-like simulation or motion
captures
|
cs.RO
|
In order to optimize the costs and time of design of the new products while
improving their quality, concurrent engineering is based on the digital model
of these products, the numerical model. However, in order to be able to avoid
definitively physical model, old support of the design, without loss of
information, new tools must be available. Especially, a tool making it possible
to check simply and quickly the maintainability of complex mechanical sets
using the numerical model is necessary. Since one decade, our team works on the
creation of tool for the generation and the analysis of trajectories of virtual
mannequins. The simulation of human tasks can be carried out either by
robot-like simulation or by simulation by motion capture. This paper presents
some results on the both two methods. The first method is based on a
multi-agent system and on a digital mock-up technology, to assess an efficient
path planner for a manikin or a robot for access and visibility task taking
into account ergonomic constraints or joint and mechanical limits. In order to
solve this problem, the human operator is integrated in the process
optimization to contribute to a global perception of the environment. This
operator cooperates, in real-time, with several automatic local elementary
agents. In the case of the second approach, we worked with the CEA and EADS/CCR
to solve the constraints related to the evolution of human virtual in its
environment on the basis of data resulting from motion capture system. An
approach using of the virtual guides was developed to allow to the user the
realization of precise trajectory in absence of force feedback. The result of
this work validates solutions through the digital mock-up; it can be applied to
simulate maintenability and mountability tasks.
|
0707.2721
|
A Framework to Illustrate Kinematic Behavior of Mechanisms by Haptic
Feedback
|
cs.RO
|
The kinematic properties of mechanisms are well known by the researchers and
teachers. The theory based on the study of Jacobian matrices allows us to
explain, for example, the singular configuration. However, in many cases, the
physical sense of such properties is difficult to explain to students. The aim
of this article is to use haptic feedback to render to the user the
signification of different kinematic indices. The framework uses a Phantom Omni
and a serial and parallel mechanism with two degrees of freedom. The
end-effector of both mechanisms can be moved either by classical mouse, or
Phantom Omni with or without feedback.
|
0707.2780
|
On the ergodic sum-rate performance of CDD in multi-user systems
|
cs.IT math.IT
|
The main focus of space-time coding design and analysis for MIMO systems has
been so far focused on single-user systems. For single-user systems, transmit
diversity schemes suffer a loss in spectral efficiency if the receiver is
equipped with more than one antenna, making them unsuitable for high rate
transmission. One such transmit diversity scheme is the cyclic delay diversity
code (CDD). The advantage of CDD over other diversity schemes such as
orthogonal space-time block codes (OSTBC) is that a code rate of one and delay
optimality are achieved independent of the number of transmit antennas. In this
work we analyze the ergodic rate of a multi-user multiple access channel (MAC)
with each user applying such a cyclic delay diversity (CDD) code. We derive
closed form expressions for the ergodic sum-rate of multi-user CDD and compare
it with the sum-capacity. We study the ergodic rate region and show that in
contrast to what is conventionally known regarding the single-user case,
transmit diversity schemes are viable candidates for high rate transmission in
multi-user systems. Finally, our theoretical findings are illustrated by
numerical simulation results.
|
0707.2792
|
Distributed Compression and Multiparty Squashed Entanglement
|
quant-ph cs.IT math.IT
|
We study a protocol in which many parties use quantum communication to
transfer a shared state to a receiver without communicating with each other.
This protocol is a multiparty version of the fully quantum Slepian-Wolf
protocol for two senders and arises through the repeated application of the
two-sender protocol. We describe bounds on the achievable rate region for the
distributed compression problem. The inner bound arises by expressing the
achievable rate region for our protocol in terms of its vertices and extreme
rays and, equivalently, in terms of facet inequalities. We also prove an outer
bound on all possible rates for distributed compression based on the multiparty
squashed entanglement, a measure of multiparty entanglement.
|
0707.2833
|
A Comparative Study between Two Three-DOF Parallel Kinematic Machines
using Kinetostatic Criteria and Interval Analysis
|
cs.RO
|
This paper addresses the workspace analysis of two 3-DOF translational
parallel mechanisms designed for machining applications. The two machines
features three fixed linear joints. The joint axes of the first machine are
orthogonal whereas these of the second are parallel. In both cases, the mobile
platform moves in the Cartesian $x-y-z$ space with fixed orientation. The
workspace analysis is conducted on the basis of prescribed kinetostatic
performances. Interval analysis based methods are used to compute the dextrous
workspace and the largest cube enclosed in this workspace.
|
0707.2836
|
Multimedia Capacity Analysis of the IEEE 802.11e Contention-based
Infrastructure Basic Service Set
|
cs.IT cs.MM math.IT
|
We first propose a simple mathematical analysis framework for the Enhanced
Distributed Channel Access (EDCA) function of the recently ratified IEEE
802.11e standard. Our analysis considers the fact that the distributed random
access systems exhibit cyclic behavior. The proposed model is valid for
arbitrary assignments of AC-specific Arbitration Interframe Space (AIFS) values
and Contention Window (CW) sizes and is the first that considers an arbitrary
distribution of active Access Categories (ACs) at the stations. Validating the
theoretical results via extensive simulations, we show that the proposed
analysis accurately captures the EDCA saturation performance. Next, we propose
a framework for multimedia capacity analysis of the EDCA function. We calculate
an accurate station- and AC-specific queue utilization ratio by appropriately
weighing the service time predictions of the cycle time model for different
number of active stations. Based on the calculated queue utilization ratio, we
design a simple model-based admission control scheme. We show that the proposed
call admission control algorithm maintains satisfactory user-perceived quality
for coexisting voice and video connections in an infrastructure BSS and does
not present over- or under-admission problems of previously proposed models in
the literature.
|
0707.2841
|
The Virtual Manufacturing concept: Scope, Socio-Economic Aspects and
Future Trends
|
cs.RO
|
The research area "Virtual Manufacturing (VM)'' is the use of information
technology and computer simulation to model real world manufacturing processes
for the purpose of analysing and understanding them. As automation technologies
such as CAD/CAM have substantially shortened the time required to design
products, Virtual Manufacturing will have a similar effect on the manufacturing
phase thanks to the modelling, simulation and optimisation of the product and
the processes involved in its fabrication. After a description of Virtual
Manufacturing (definitions and scope), we present some socio-economic factors
of VM and finaly some "hot topics'' for the future are proposed.
|
0707.2842
|
A Classification of 3R Orthogonal Manipulators by the Topology of their
Workspace
|
cs.RO
|
A classification of a family of 3-revolute (3R) positining manipulators is
established. This classification is based on the topology of their workspace.
The workspace is characterized in a half-cross section by the singular curves.
The workspace topology is defined by the number of cusps and nodes that appear
on these singular curves. The design parameters space is shown to be divided
into nine domains of distinct workspace topologies, in which all manipulators
have similar global kinematic properties. Each separating surface is given as
an explicit expression in the DH-parameters.
|
0707.2926
|
Robust Hypothesis Testing with a Relative Entropy Tolerance
|
cs.IT math.IT
|
This paper considers the design of a minimax test for two hypotheses where
the actual probability densities of the observations are located in
neighborhoods obtained by placing a bound on the relative entropy between
actual and nominal densities. The minimax problem admits a saddle point which
is characterized. The robust test applies a nonlinear transformation which
flattens the nominal likelihood ratio in the vicinity of one. Results are
illustrated by considering the transmission of binary data in the presence of
additive noise.
|
0707.2998
|
Building a Cooperative Communications System
|
cs.IT math.IT
|
In this paper, we present the results from over-the-air experiments of a
complete implementation of an amplify and forward cooperative communications
system. Our custom OFDM-based physical layer uses a distributed version of the
Alamouti block code, where the relay sends one branch of Alamouti encoded
symbols. First we show analytically and experimentally that amplify and forward
protocols are unaffected by carrier frequency offsets at the relay. This result
allows us to use a conventional Alamouti receiver without change for the
distributed relay system. Our full system implementation shows gains up to
5.5dB in peak power constrained networks. Thus, we can conclusively state that
even the simplest form of relaying can lead to significant gains in practical
implementations.
|
0707.3030
|
Optimal Design of Ad Hoc Injection Networks by Using Genetic Algorithms
|
cs.NE cs.AI cs.NI
|
This work aims at optimizing injection networks, which consist in adding a
set of long-range links (called bypass links) in mobile multi-hop ad hoc
networks so as to improve connectivity and overcome network partitioning. To
this end, we rely on small-world network properties, that comprise a high
clustering coefficient and a low characteristic path length. We investigate the
use of two genetic algorithms (generational and steady-state) to optimize three
instances of this topology control problem and present results that show
initial evidence of their capacity to solve it.
|
0707.3043
|
p-Adic Modelling of the Genome and the Genetic Code
|
q-bio.OT cs.IT math.IT physics.bio-ph
|
The present paper is devoted to foundations of p-adic modelling in genomics.
Considering nucleotides, codons, DNA and RNA sequences, amino acids, and
proteins as information systems, we have formulated the corresponding p-adic
formalisms for their investigations. Each of these systems has its
characteristic prime number used for construction of the related information
space. Relevance of this approach is illustrated by some examples. In
particular, it is shown that degeneration of the genetic code is a p-adic
phenomenon. We have also put forward a hypothesis on evolution of the genetic
code assuming that primitive code was based on single nucleotides and
chronologically first four amino acids. This formalism of p-adic genomic
information systems can be implemented in computer programs and applied to
various concrete cases.
|
0707.3087
|
Universal Reinforcement Learning
|
cs.IT cs.LG math.IT
|
We consider an agent interacting with an unmodeled environment. At each time,
the agent makes an observation, takes an action, and incurs a cost. Its actions
can influence future observations and costs. The goal is to minimize the
long-term average cost. We propose a novel algorithm, known as the active LZ
algorithm, for optimal control based on ideas from the Lempel-Ziv scheme for
universal data compression and prediction. We establish that, under the active
LZ algorithm, if there exists an integer $K$ such that the future is
conditionally independent of the past given a window of $K$ consecutive actions
and observations, then the average cost converges to the optimum. Experimental
results involving the game of Rock-Paper-Scissors illustrate merits of the
algorithm.
|
0707.3095
|
Channel Capacity Estimation using Free Probability Theory
|
cs.IT math.IT
|
In many channel measurement applications, one needs to estimate some
characteristics of the channels based on a limited set of measurements. This is
mainly due to the highly time varying characteristics of the channel. In this
contribution, it will be shown how free probability can be used for channel
capacity estimation in MIMO systems. Free probability has already been applied
in various application fields such as digital communications, nuclear physics
and mathematical finance, and has been shown to be an invaluable tool for
describing the asymptotic behaviour of many large-dimensional systems. In
particular, using the concept of free deconvolution, we provide an
asymptotically (w.r.t. the number of observations) unbiased capacity estimator
for MIMO channels impaired with noise called the free probability based
estimator. Another estimator, called the Gaussian matrix mean based estimator,
is also introduced by slightly modifying the free probability based estimator.
This estimator is shown to give unbiased estimation of the moments of the
channel matrix for any number of observations. Also, the estimator has this
property when we extend to MIMO channels with phase off-set and frequency
drift, for which no estimator has been provided so far in the literature. It is
also shown that both the free probability based and the Gaussian matrix mean
based estimator are asymptotically unbiased capacity estimators as the number
of transmit antennas go to infinity, regardless of whether phase off-set and
frequency drift are present. The limitations in the two estimators are also
explained. Simulations are run to assess the performance of the estimators for
a low number of antennas and samples to confirm the usefulness of the
asymptotic results.
|
0707.3175
|
Stacked OSTBC: Error Performance and Rate Analysis
|
cs.IT math.IT
|
It is well known, that the Alamouti scheme is the only space-time code from
orthogonal design achieving the capacity of a multiple-input multiple-output
(MIMO) wireless communication system with n_T=2 transmit antennas and n_R=1
receive antenna. In this work, we propose the n-times stacked Alamouti scheme
for n_T=2n transmit antennas and show that this scheme achieves the capacity in
the case of n_R=1 receive antenna. This result may regarded as an extension of
the Alamouti case. For the more general case of more than one receive antenna,
we show that if the number of transmit antennas is higher than the number of
receive antennas we achieve a high portion of the capacity with this scheme.
Further, we show that the MIMO capacity is at most twice the rate achieved with
the proposed scheme for all SNR. We derive lower and upper bounds for the rate
achieved with this scheme and compare it with upper and lower bounds for the
capacity. In addition to the capacity analysis based on the assumption of a
coherent channel, we analyze the error rate performance of the stacked OSTBC
with the optimal ML detector and with the suboptimal lattice-reduction (LR)
aided zero-forcing detector. We compare the error rate performance of the
stacked OSTBC with spatial multiplexing (SM) and full-diversity achieving
schemes. Finally, we illustrate the theoretical results by numerical
simulations.
|
0707.3186
|
Kinematic and stiffness analysis of the Orthoglide, a PKM with simple,
regular workspace and homogeneous performances
|
cs.RO
|
The Orthoglide is a Delta-type PKM dedicated to 3-axis rapid machining
applications that was originally developed at IRCCyN in 2000-2001 to meet the
advantages of both serial 3-axis machines (regular workspace and homogeneous
performances) and parallel kinematic architectures (good dynamic performances
and stiffness). This machine has three fixed parallel linear joints that are
mounted orthogonally. The geometric parameters of the Orthoglide were defined
as function of the size of a prescribed cubic Cartesian workspace that is free
of singularities and internal collision. The interesting features of the
Orthoglide are a regular Cartesian workspace shape, uniform performances in all
directions and good compactness. In this paper, a new method is proposed to
analyze the stiffness of overconstrained Delta-type manipulators, such as the
Orthoglide. The Orthoglide is then benchmarked according to geometric,
kinematic and stiffness criteria: workspace to footprint ratio, velocity and
force transmission factors, sensitivity to geometric errors, torsional
stiffness and translational stiffness.
|
0707.3205
|
Neutrality and Many-Valued Logics
|
cs.LO cs.AI
|
In this book, we consider various many-valued logics: standard, linear,
hyperbolic, parabolic, non-Archimedean, p-adic, interval, neutrosophic, etc. We
survey also results which show the tree different proof-theoretic frameworks
for many-valued logics, e.g. frameworks of the following deductive calculi:
Hilbert's style, sequent, and hypersequent. We present a general way that
allows to construct systematically analytic calculi for a large family of
non-Archimedean many-valued logics: hyperrational-valued, hyperreal-valued, and
p-adic valued logics characterized by a special format of semantics with an
appropriate rejection of Archimedes' axiom. These logics are built as different
extensions of standard many-valued logics (namely, Lukasiewicz's, Goedel's,
Product, and Post's logics). The informal sense of Archimedes' axiom is that
anything can be measured by a ruler. Also logical multiple-validity without
Archimedes' axiom consists in that the set of truth values is infinite and it
is not well-founded and well-ordered. On the base of non-Archimedean valued
logics, we construct non-Archimedean valued interval neutrosophic logic INL by
which we can describe neutrality phenomena.
|
0707.3248
|
Decentralized sequential change detection using physical layer fusion
|
cs.IT math.IT
|
The problem of decentralized sequential detection with conditionally
independent observations is studied. The sensors form a star topology with a
central node called fusion center as the hub. The sensors make noisy
observations of a parameter that changes from an initial state to a final state
at a random time where the random change time has a geometric distribution. The
sensors amplify and forward the observations over a wireless Gaussian multiple
access channel and operate under either a power constraint or an energy
constraint. The optimal transmission strategy at each stage is shown to be the
one that maximizes a certain Ali-Silvey distance between the distributions for
the hypotheses before and after the change. Simulations demonstrate that the
proposed analog technique has lower detection delays when compared with
existing schemes. Simulations further demonstrate that the energy-constrained
formulation enables better use of the total available energy than the
power-constrained formulation in the change detection problem.
|
0707.3263
|
Autonomous tools for Grid management, monitoring and optimization
|
cs.DC cs.CE hep-ex
|
We outline design and lines of development of autonomous tools for the
computing Grid management, monitoring and optimization. The management is
proposed to be based on the notion of utility. Grid optimization is considered
to be application-oriented. A generic Grid simulator is proposed as an
optimization tool for Grid structure and functionality.
|
0707.3269
|
International Standard for a Linguistic Annotation Framework
|
cs.CL
|
This paper describes the Linguistic Annotation Framework under development
within ISO TC37 SC4 WG1. The Linguistic Annotation Framework is intended to
serve as a basis for harmonizing existing language resources as well as
developing new ones.
|
0707.3270
|
A Formal Model of Dictionary Structure and Content
|
cs.CL
|
We show that a general model of lexical information conforms to an abstract
model that reflects the hierarchy of information found in a typical dictionary
entry. We show that this model can be mapped into a well-formed XML document,
and how the XSL transformation language can be used to implement a semantics
defined over the abstract model to enable extraction and manipulation of the
information in any format.
|
0707.3336
|
Statistical mechanical analysis of the linear vector channel in digital
communication
|
cs.IT cond-mat.dis-nn cond-mat.stat-mech math.IT
|
A statistical mechanical framework to analyze linear vector channel models in
digital wireless communication is proposed for a large system. The framework is
a generalization of that proposed for code-division multiple-access systems in
Europhys. Lett. 76 (2006) 1193 and enables the analysis of the system in which
the elements of the channel transfer matrix are statistically correlated with
each other. The significance of the proposed scheme is demonstrated by
assessing the performance of an existing model of multi-input multi-output
communication systems.
|
0707.3390
|
Consistency of the group Lasso and multiple kernel learning
|
cs.LG
|
We consider the least-square regression problem with regularization by a
block 1-norm, i.e., a sum of Euclidean norms over spaces of dimensions larger
than one. This problem, referred to as the group Lasso, extends the usual
regularization by the 1-norm where all spaces have dimension one, where it is
commonly referred to as the Lasso. In this paper, we study the asymptotic model
consistency of the group Lasso. We derive necessary and sufficient conditions
for the consistency of group Lasso under practical assumptions, such as model
misspecification. When the linear predictors and Euclidean norms are replaced
by functions and reproducing kernel Hilbert norms, the problem is usually
referred to as multiple kernel learning and is commonly used for learning from
heterogeneous data sources and for non linear variable selection. Using tools
from functional analysis, and in particular covariance operators, we extend the
consistency results to this infinite dimensional case and also propose an
adaptive scheme to obtain a consistent model estimate, even when the necessary
condition required for the non adaptive scheme is not satisfied.
|
0707.3409
|
Faster exon assembly by sparse spliced alignment
|
cs.DS cs.CC cs.CE q-bio.QM
|
Assembling a gene from candidate exons is an important problem in
computational biology. Among the most successful approaches to this problem is
\emph{spliced alignment}, proposed by Gelfand et al., which scores different
candidate exon chains within a DNA sequence of length $m$ by comparing them to
a known related gene sequence of length n, $m = \Theta(n)$. Gelfand et al.\
gave an algorithm for spliced alignment running in time O(n^3). Kent et al.\
considered sparse spliced alignment, where the number of candidate exons is
O(n), and proposed an algorithm for this problem running in time O(n^{2.5}). We
improve on this result, by proposing an algorithm for sparse spliced alignment
running in time O(n^{2.25}). Our approach is based on a new framework of
\emph{quasi-local string comparison}.
|
0707.3457
|
A Generalized Information Formula as the Bridge between Shannon and
Popper
|
cs.IT cs.AI math.IT
|
A generalized information formula related to logical probability and fuzzy
set is deduced from the classical information formula. The new information
measure accords with to Popper's criterion for knowledge evolution very much.
In comparison with square error criterion, the information criterion does not
only reflect error of a proposition, but also reflects the particularity of the
event described by the proposition. It gives a proposition with less logical
probability higher evaluation. The paper introduces how to select a prediction
or sentence from many for forecasts and language translations according to the
generalized information criterion. It also introduces the rate fidelity theory,
which comes from the improvement of the rate distortion theory in the classical
information theory by replacing distortion (i.e. average error) criterion with
the generalized mutual information criterion, for data compression and
communication efficiency. Some interesting conclusions are obtained from the
rate-fidelity function in relation to image communication. It also discusses
how to improve Popper's theory.
|
0707.3461
|
Lattices for Distributed Source Coding: Jointly Gaussian Sources and
Reconstruction of a Linear Function
|
cs.IT math.IT
|
Consider a pair of correlated Gaussian sources (X1,X2). Two separate encoders
observe the two components and communicate compressed versions of their
observations to a common decoder. The decoder is interested in reconstructing a
linear combination of X1 and X2 to within a mean-square distortion of D. We
obtain an inner bound to the optimal rate-distortion region for this problem. A
portion of this inner bound is achieved by a scheme that reconstructs the
linear function directly rather than reconstructing the individual components
X1 and X2 first. This results in a better rate region for certain parameter
values. Our coding scheme relies on lattice coding techniques in contrast to
more prevalent random coding arguments used to demonstrate achievable rate
regions in information theory. We then consider the case of linear
reconstruction of K sources and provide an inner bound to the optimal
rate-distortion region. Some parts of the inner bound are achieved using the
following coding structure: lattice vector quantization followed by
"correlated" lattice-structured binning.
|
0707.3479
|
Quantum Algorithms for Learning and Testing Juntas
|
quant-ph cs.LG
|
In this article we develop quantum algorithms for learning and testing
juntas, i.e. Boolean functions which depend only on an unknown set of k out of
n input variables. Our aim is to develop efficient algorithms:
- whose sample complexity has no dependence on n, the dimension of the domain
the Boolean functions are defined over;
- with no access to any classical or quantum membership ("black-box")
queries. Instead, our algorithms use only classical examples generated
uniformly at random and fixed quantum superpositions of such classical
examples;
- which require only a few quantum examples but possibly many classical
random examples (which are considered quite "cheap" relative to quantum
examples).
Our quantum algorithms are based on a subroutine FS which enables sampling
according to the Fourier spectrum of f; the FS subroutine was used in earlier
work of Bshouty and Jackson on quantum learning. Our results are as follows:
- We give an algorithm for testing k-juntas to accuracy $\epsilon$ that uses
$O(k/\epsilon)$ quantum examples. This improves on the number of examples used
by the best known classical algorithm.
- We establish the following lower bound: any FS-based k-junta testing
algorithm requires $\Omega(\sqrt{k})$ queries.
- We give an algorithm for learning $k$-juntas to accuracy $\epsilon$ that
uses $O(\epsilon^{-1} k\log k)$ quantum examples and $O(2^k \log(1/\epsilon))$
random examples. We show that this learning algorithms is close to optimal by
giving a related lower bound.
|
0707.3482
|
A Bayesian Framework for Combining Valuation Estimates
|
q-fin.ST cs.CE nlin.AO nlin.CD nlin.SI physics.pop-ph physics.soc-ph stat.AP
|
Obtaining more accurate equity value estimates is the starting point for
stock selection, value-based indexing in a noisy market, and beating benchmark
indices through tactical style rotation. Unfortunately, discounted cash flow,
method of comparables, and fundamental analysis typically yield discrepant
valuation estimates. Moreover, the valuation estimates typically disagree with
market price. Can one form a superior valuation estimate by averaging over the
individual estimates, including market price? This article suggests a Bayesian
framework for combining two or more estimates into a superior valuation
estimate. The framework justifies the common practice of averaging over several
estimates to arrive at a final point estimate.
|
0707.3507
|
Workspace and Kinematic Analysis of the VERNE machine
|
cs.RO
|
This paper describes the workspace and the inverse and direct kinematic
analysis of the VERNE machine, a serial/parallel 5-axis machine tool designed
by Fatronik for IRCCyN. This machine is composed of a three-degree-of-freedom
(DOF) parallel module and a two-DOF serial tilting table. The parallel module
consists of a moving platform that is connected to a fixed base by three
non-identical legs. This feature involves (i) a simultaneous combination of
rotation and translation for the moving platform, which is balanced by the
tilting table and (ii) workspace whose shape and volume vary as a function of
the tool length. This paper summarizes results obtained in the context of the
European projects NEXT ("Next Generation of Productions Systems").
|
0707.3509
|
Upper bound of loss probability in an OFDMA system with randomly located
users
|
math.PR cs.IT cs.NI math.IT
|
For OFDMA systems, we find a rough but easily computed upper bound for the
probability of loosing communications by insufficient number of sub-channels on
downlink. We consider as random the positions of receiving users in the system
as well as the number of sub-channels dedicated to each one. We use recent
results of the theory of point processes which reduce our calculations to the
first and second moments of the total required number of sub-carriers.
|
0707.3531
|
e-Science initiatives in Venezuela
|
cs.CE cs.DC
|
Within the context of the nascent e-Science infrastructure in Venezuela, we
describe several web-based scientific applications developed at the Centro
Nacional de Calculo Cientifico Universidad de Los Andes (CeCalCULA), Merida,
and at the Instituto Venezolano de Investigaciones Cientificas (IVIC), Caracas.
The different strategies that have been followed for implementing quantum
chemistry and atomic physics applications are presented. We also briefly
discuss a damage portal based on dynamic, nonlinear, finite elements of lumped
damage mechanics and a biomedical portal developed within the framework of the
\textit{E-Infrastructure shared between Europe and Latin America} (EELA)
initiative for searching common sequences and inferring their functions in
parasitic diseases such as leishmaniasis, chagas and malaria.
|
0707.3534
|
The Kinetostatic Optimization of a Novel Prismatic Drive
|
cs.RO
|
The design of a mechanical transmission taking into account the transmitted
forces is reported in this paper. This transmission is based on Slide-o-Cam, a
cam mechanism with multiple rollers mounted on a common translating follower.
The design of Slide-o-Cam, a transmission intended to produce a sliding motion
from a turning drive, or vice versa, was reported elsewhere. This transmission
provides pure-rolling motion, thereby reducing the friction of rack-and-pinions
and linear drives. The pressure angle is a suitable performance index for this
transmission because it determines the amount of force transmitted to the load
vs. that transmitted to the machine frame. To assess the transmission
capability of the mechanism, the Hertz formula is introduced to calculate the
stresses on the rollers and on the cams. The final transmission is intended to
replace the current ball-screws in the Orthoglide, a three-DOF parallel robot
for the production of translational motions, currently under development for
machining applications at Ecole Centrale de Nantes.
|
0707.3550
|
A Six Degree-Of-Freedom Haptic Device Based On The Orthoglide And A
Hybrid Agile Eye
|
cs.RO
|
This paper is devoted to the kinematic design of a new six degree-of-freedom
haptic device using two parallel mechanisms. The first one, called orthoglide,
provides the translation motions and the second one, called agile eye, produces
the rotational motions. These two motions are decoupled to simplify the direct
and inverse kinematics, as it is needed for real-time control. To reduce the
inertial load, the motors are fixed on the base and a transmission with two
universal joints is used to transmit the rotational motions from the base to
the end-effector. Two alternative wrists are proposed (i), the agile eye with
three degrees of freedom or (ii) a hybrid wrist made by the assembly of a
two-dof agile eye with a rotary motor. The last one is optimized to increase
its stiffness and to decrease the number of moving parts.
|
0707.3552
|
Analyse Comparative des Manipulateurs 3R \`a Axes Orthogonaux
|
cs.RO
|
A family of 3R orthogonal manipulators without offset on the third body can
be divided into exactly nine workspace topologies. The workspace is
characterized in a half-cross section by the singular curves. The workspace
topology is defined by the number of cusps and nodes that appear on these
singular curves. Based on this classification, we evaluate theses manipulators
by the condition number related to the joint space and the proportion of the
region with four inverse kinematic solutions compared to a sphere containing
all the workspace. This second performance number is in relation with the
workspace. We determine finally le topology of workspace to which belong
manipulators having the best performance number values.
|
0707.3553
|
An Exhaustive Study of the Workspace Topologies of all 3R Orthogonal
Manipulators with Geometric Simplifications
|
cs.RO
|
This paper analyses the workspace of the three-revolute orthogonal
manipulators that have at least one of their DH parameters equal to zero. These
manipulators are classified into different groups with similar kinematic
properties. The classification criteria are based on the topology of the
workspace. Each group is evaluated according to interesting kinematic
properties such as the size of the workspace subregion reachable with four
inverse kinematic solutions, the existence and the size of voids, and the size
of the regions of feasible paths in the workspace.
|
0707.3559
|
Practical Approach to Knowledge-based Question Answering with Natural
Language Understanding and Advanced Reasoning
|
cs.CL cs.AI cs.HC cs.IR
|
This research hypothesized that a practical approach in the form of a
solution framework known as Natural Language Understanding and Reasoning for
Intelligence (NaLURI), which combines full-discourse natural language
understanding, powerful representation formalism capable of exploiting
ontological information and reasoning approach with advanced features, will
solve the following problems without compromising practicality factors: 1)
restriction on the nature of question and response, and 2) limitation to scale
across domains and to real-life natural language text.
|
0707.3560
|
Integration of a Balanced Virtual Manikin in a Virtual Reality Platform
aimed at Virtual Prototyping
|
cs.RO
|
The work presented here is aimed at introducing a virtual human controller in
a virtual prototyping framework. After a brief introduction describing the
problem solved in the paper, we describe the interest as for digital humans in
the context of concurrent engineering. This leads us to draw a control
architecture enabling to drive virtual humans in a real-time immersed way, and
to interact with the product, through motion capture. Unfortunately, we show
this control scheme can lead to unfeasible movements because of the lack of
balance control. Introducing such a controller is a problem that was never
addressed in the context of real-time. We propose an implementation of a
balance controller, that we insert into the previously described control
scheme. Next section is dedicated to show the results we obtained. Finally, we
propose a virtual reality platform into which the digital character controller
is integrated.
|
0707.3562
|
Balanced Virtual Humans Interacting with their Environment
|
cs.RO
|
The animation of human avatars seems very successful; the computer graphics
industry shows outstanding results in films everyday, the game industry
achieves exploits... Nevertheless, the animation and control processes of such
manikins are very painful. It takes days to a specialist to build such animated
sequences, and it is not adaptive to any type of modifications. Our main
purpose is the virtual human for engineering, especially virtual prototyping.
As for this domain of activity, such amounts of time are prohibitive.
|
0707.3563
|
Virtual reality: A human centered tool for improving Manufacturing
|
cs.RO
|
Manufacturing is using Virtual Reality tools to enhance the product life
cycle. Their definitions are still in flux and it is necessary to define their
connections. Thus, firstly, we will introduce more closely some definitions
where we will find that, if the Virtual manufacturing concepts originate from
machining operations and evolve in this manufacturing area, there exist a lot
of applications in different fields such as casting, forging, sheet
metalworking and robotics (mechanisms). From the recent projects in Europe or
in USA, we notice that the human perception or the simulation of mannequin is
more and more needed in both fields. In this context, we have isolated some
applications as ergonomic studies, assembly and maintenance simulation, design
or training where the virtual reality tools can be applied. Thus, we find out a
family of applications where the virtual reality tools give the engineers the
main role in the optimization process. We will illustrate our paper by several
examples where virtual reality interfaces are used and combined with
optimization tools as multi-agent systems.
|
0707.3564
|
A New Six Degree-of-Freedom Haptic Device based on the Orthoglide and
the Agile Eye
|
cs.RO
|
The aim of this paper is to present a new six degree-of-freedom (dof) haptic
device using two parallel mechanisms. The first one, called orthoglide,
provides the translation motions and the second one produces the rotational
motions. These two motions are decoupled to simplify the direct and inverse
kinematics, as it is needed for real-times control. To reduce the inertial
load, the motors are fixed on the base and a transmission with two universal
joints is used to transmit the rotational motions from the base to the
end-effector. The main feature of the orthoglide and of the agile eye mechanism
is the existence of an isotropic configuration. The length of the legs and the
range limits of the orthoglide are optimized to have homogeneous performance
throughout the Cartesian workspace, which has a nearly cubic workspace. These
properties permit to have a high stiffness throughout the workspace and
workspace limits that are easily understandable by the user.
|
0707.3574
|
L'orthoglide : une machine-outil rapide d'architecture parall\`ele
isotrope
|
cs.RO
|
This article presents the Orthoglide project. The purpose of this project is
the realization of a prototype of machine tool to three degrees of translation.
The characteristic of this machine is a parallel kinematic architecture
optimized to obtain a compact workspace with homogeneous performance. For that,
the principal criterion of design which was used is the isotropy.
|
0707.3575
|
An exploratory study of Google Scholar
|
cs.DL cs.IR
|
The paper discusses and analyzes the scientific search service Google Scholar
(GS). The focus is on an exploratory study which investigates the coverage of
scientific serials in GS. The study shows deficiencies in the coverage and
up-to-dateness of the GS index. Furthermore, the study points up which Web
servers are the most important data providers for this search service and which
information sources are highly represented. We can show that there is a
relatively large gap in Google Scholars coverage of German literature as well
as weaknesses in the accessibility of Open Access content.
Keywords: Search engines, Digital libraries, Worldwide Web, Serials,
Electronic journals
|
0707.3584
|
The effect of fading, channel inversion, and threshold scheduling on ad
hoc networks
|
cs.IT math.IT
|
This paper addresses three issues in the field of ad hoc network capacity:
the impact of i)channel fading, ii) channel inversion power control, and iii)
threshold-based scheduling on capacity. Channel inversion and threshold
scheduling may be viewed as simple ways to exploit channel state information
(CSI) without requiring cooperation across transmitters. We use the
transmission capacity (TC) as our metric, defined as the maximum spatial
intensity of successful simultaneous transmissions subject to a constraint on
the outage probability (OP). By assuming the nodes are located on the infinite
plane according to a Poisson process, we are able to employ tools from
stochastic geometry to obtain asymptotically tight bounds on the distribution
of the signal-to-interference (SIR) level, yielding in turn tight bounds on the
OP (relative to a given SIR threshold) and the TC. We demonstrate that in the
absence of CSI, fading can significantly reduce the TC and somewhat
surprisingly, channel inversion only makes matters worse. We develop a
threshold-based transmission rule where transmitters are active only if the
channel to their receiver is acceptably strong, obtain expressions for the
optimal threshold, and show that this simple, fully distributed scheme can
significantly reduce the effect of fading.
|
0707.3665
|
A Comparative Study of Parallel Kinematic Architectures for Machining
Applications
|
cs.RO
|
Parallel kinematic mechanisms are interesting alternative designs for
machining applications. Three 2-DOF parallel mechanism architectures dedicated
to machining applications are studied in this paper. The three mechanisms have
two constant length struts gliding along fixed linear actuated joints with
different relative orientation. The comparative study is conducted on the basis
of a same prescribed Cartesian workspace for the three mechanisms. The common
desired workspace properties are a rectangular shape and given kinetostatic
performances. The machine size of each resulting design is used as a
comparative criterion. The 2-DOF machine mechanisms analyzed in this paper can
be extended to 3-axis machines by adding a third joint.
|
0707.3666
|
Kinematic Analysis of a New Parallel Machine Tool: the Orthoglide
|
cs.RO
|
This paper describes a new parallel kinematic architecture for machining
applications: the orthoglide. This machine features three fixed parallel linear
joints which are mounted orthogonally and a mobile platform which moves in the
Cartesian x-y-z space with fixed orientation. The main interest of the
orthoglide is that it takes benefit from the advantages of the popular PPP
serial machines (regular Cartesian workspace shape and uniform performances) as
well as from the parallel kinematic arrangement of the links (less inertia and
better dynamic performances), which makes the orthoglide well suited to
high-speed machining applications. Possible extension of the orthoglide to
5-axis machining is also investigated.
|
0707.3673
|
The Computation of All 4R Serial Spherical Wrists With an Isotropic
Architecture
|
cs.RO
|
A spherical wrist of the serial type is said to be isotropic if it can attain
a posture whereby the singular values of its Jacobian matrix are all identical
and nonzero. What isotropy brings about is robustness to manufacturing,
assembly, and measurement errors, thereby guaranteeing a maximum orientation
accuracy. In this paper we investigate the existence of redundant isotropic
architectures, which should add to the dexterity of the wrist under design by
virtue of its extra degree of freedom. The problem formulation leads to a
system of eight quadratic equations with eight unknowns. The Bezout number of
this system is thus 2^8 = 256, its BKK bound being 192. However, the actual
number of solutions is shown to be 32. We list all solutions of the foregoing
algebraic problem. All these solutions are real, but distinct solutions do not
necessarily lead to distinct manipulators. Upon discarding those algebraic
solutions that yield no new wrists, we end up with exactly eight distinct
architectures, the eight corresponding manipulators being displayed at their
isotropic posture.
|
0707.3781
|
Bijective Faithful Translations among Default Logics
|
cs.AI cs.LO
|
In this article, we study translations between variants of defaults logics
such that the extensions of the theories that are the input and the output of
the translation are in a bijective correspondence. We assume that a translation
can introduce new variables and that the result of translating a theory can
either be produced in time polynomial in the size of the theory or its output
is polynomial in that size; we however restrict to the case in which the
original theory has extensions. This study fills a gap between two previous
pieces of work, one studying bijective translations among restrictions of
default logics, and the other one studying non-bijective translations between
default logics variants.
|
0707.3878
|
Plotkin construction: rank and kernel
|
cs.IT math.IT
|
Given two binary codes of length n, using Plotkin construction we obtain a
code of length 2n. The construction works for linear and nonlinear codes. For
the linear case, it is straightforward to see that the dimension of the final
code is the sum of the dimensions of the starting codes. For nonlinear codes,
the rank and the dimension of the kernel are standard mesures of linearity. In
this report, we prove that both parameters are also the sum of the
corresponding ones of the starting codes.
|
0707.3925
|
Use of a $d$-Constraint During LDPC Decoding in a Bliss Scheme
|
cs.IT math.IT
|
Bliss schemes of a run length limited (RLL) codec in combination with an LDPC
codec, generate LDPC parity bits over a systematic sequence of RLL channel bits
that are inherently redundant as they satisfy e.g. a $d=1$ minimum run length
constraint. That is the subsequences consisting of runs of length $d=1$, viz.
$...010...$ and $...101...$, cannot occur. We propose to use this redundancy
during LDPC decoding in a Bliss scheme by introducing additional $d$-constraint
nodes in the factor graph used by the LDPC decoder. The messages sent from
these new nodes to the variable or codeword bit nodes exert a ``force'' on the
resulting soft-bit vector coming out of the LDPC decoding that give it a
tendency to comply with the $d$-constraints. This way, we can significantly
reduce the probability of decoding error.
|
0707.3959
|
Four-Group Decodable Space-Time Block Codes
|
cs.IT math.IT
|
Two new rate-one full-diversity space-time block codes (STBC) are proposed.
They are characterized by the \emph{lowest decoding complexity} among the known
rate-one STBC, arising due to the complete separability of the transmitted
symbols into four groups for maximum likelihood detection. The first and the
second codes are delay-optimal if the number of transmit antennas is a power of
2 and even, respectively. The exact pair-wise error probability is derived to
allow for the performance optimization of the two codes. Compared with existing
low-decoding complexity STBC, the two new codes offer several advantages such
as higher code rate, lower encoding/decoding delay and complexity, lower
peak-to-average power ratio, and better performance.
|
0707.3972
|
Learning Probabilistic Models of Word Sense Disambiguation
|
cs.CL cs.AI
|
This dissertation presents several new methods of supervised and unsupervised
learning of word sense disambiguation models. The supervised methods focus on
performing model searches through a space of probabilistic models, and the
unsupervised methods rely on the use of Gibbs Sampling and the Expectation
Maximization (EM) algorithm. In both the supervised and unsupervised case, the
Naive Bayesian model is found to perform well. An explanation for this success
is presented in terms of learning rates and bias-variance decompositions.
|
0707.3979
|
Clifford Algebra of the Vector Space of Conics for decision boundary
Hyperplanes in m-Euclidean Space
|
cs.NE cs.CG
|
In this paper we embed $m$-dimensional Euclidean space in the geometric
algebra $Cl_m $ to extend the operators of incidence in ${R^m}$ to operators of
incidence in the geometric algebra to generalize the notion of separator to a
decision boundary hyperconic in the Clifford algebra of hyperconic sections
denoted as ${Cl}({Co}_{2})$. This allows us to extend the concept of a linear
perceptron or the spherical perceptron in conformal geometry and introduce the
more general conic perceptron, namely the {elliptical perceptron}. Using
Clifford duality a vector orthogonal to the decision boundary hyperplane is
determined. Experimental results are shown in 2-dimensional Euclidean space
where we separate data that are naturally separated by some typical plane conic
separators by this procedure. This procedure is more general in the sense that
it is independent of the dimension of the input data and hence we can speak of
the hyperconic elliptic perceptron.
|
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