Analysis of a contention resolution multiple-access algorithm for handling mixed traffic on wireless networks

Existing wireless standards, like DECT and GSM, are based on TDMA. Packet multiple-access algorithms perform statistical multiplexing, leading to greater efficiency than basic TDMA which assigns a fixed amount of channel resources to each user. Moreover, these methods permit one to handle in a more flexible way different kinds of information. Most of the proposed packet access methods, applied to radio networks, are based on slotted ALOHA algorithms. We propose a methodology for applying packet contention resolution multiple-access techniques to wireless networks. The proposed techniques are able to handle the access of mixed traffic types. We define and analyze a multiple-access algorithm for handling the access of a mixed set of users, including a group of speech users and a population of users that generate a random traffic approximated by a stream of Poisson.     

A Search Algorithm for Calculating Validated Reliability Bounds

The search algorithm presented allows the CDF of a dependent variable to be bounded with 100% confidence, and allows for a guaranteed evaluation of the error involved. These reliability bounds are often enough to make decisions, and often require a minimal number of function evaluations. The procedure is not intrusive, i.e. it can be equally applied when the function is a complex computer model (black box). The proposed procedure can handle input information consisting of probabilistic, interval-valued, set-valued, or random-set-valued information, as well as any combination thereof. The function as well as the joint pdf of the input variables can be of any type.     

How the Location of the Range Sensor Affects EKF-based Localization

The article proposes a solution to map-based self-localization for an autonomous robot operating in cluttered and crowded environments. To detect features for localization, 2D laser range-finders traditionally scan a plane parallel to the floor. This work hypothesizes the existence of a ??low frequency cross-section?? of the 3D Workspace where cluttered and dynamic environments become ??more regular?? and ??less dynamic??. The contribution of the article is twofold. First, an ??unevenness index?? U is introduced to quantitatively measure the complexity of the environment as it would be perceived if the laser range-finder were located at different heights from the floor. The article shows that, by choosing the laser scanning plane to statistically minimize U (in most cases, above the heads of people), it is possible to deal more efficiently with non-linearities in the measurement model, moving objects and occluded features. Second, it is demonstrated that, when adopting an extended Kalman filter for position tracking (a very common and widely used technique in real-world scenarios), the a posteriori covariance of the estimated robot pose converges faster, on average, when U is lower, which leads to better localization performance. Experimental results show hours of continuous robot operation in real-world, cluttered and crowded environments.     

The General Product Machine: a New Model for Symbolic FSM Traversal

Proving the equivalence of two Finite State Machines (FSMs) has many applications to synthesis, verification, testing, and diagnosis. Building their product machine is a theoretical framework for equivalence proof. There are some cases where product machine traversal, a necessary and sufficient check, is mandatory. This is much more complex than traversing just one of the component machines. This paper proposes an equivalence-preserving function that transforms the product machine in theGeneral Product Machine (GPM). Using the GPM in symbolic state space traversal reduces the size of the BDDs and makes image computation easier. As a result, GPM traversal is much less expensive than product machine traversal, its cost being close to dealing with a single machine.     

On the Use of Random Set Theory to Bracket the Results of Monte Carlo Simulations

Based on Random Set Theory, procedures are presented for bracketing the results of Monte Carlo simulations in two notable cases: (i) the calculation of the entire distribution of the dependent variable; (ii) the calculation of the CDF of a particular value of the dependent variable (e.g. reliability analyses). The presented procedures are not intrusive in that they can be equally applied when the functional relationship between the dependent variable and independent variables is known analytically and when it is a complex computer model (black box). Also, the proposed procedures can handle probabilistic (with any type of input joint PDF), interval-valued, set-valued, and random set-valued input information, as well as any combination thereof.When exact or outer bounds on the function image can be calculated, the bounds on the CDF of the dependent variable guarantee 100% confidence, and allow for an explicit and exact evaluation of the error involved in the calculation of the CDF. These bounds are often enough to make decisions, and require a minimal amount of functional evaluations. A procedure for effectively approximating the CDF of the dependent variable is also proposed.An example shows that, compared to Monte Carlo simulations, the number of functional evaluations is reduced by orders of magnitude and that the convergence rate increases tenfold.     

Numerical Study of the Effect of Verification Core Hole on the Point Bearing Capacity of Drilled Shafts

This paper presents a numerical investigation of the effect of a verification core hole on the point bearing capacity of drilled shafts installed in clay shales. The verification core extracted at the shaft tip may reduce the point bearing capacity of drilled shafts as a result of degradation of clay shales and imperfect core hole infill. Finite-element analyses were conducted using the Mohr-Coulomb model with total stress material parameters estimated from laboratory tests. A series of load-displacement curves was calculated for 1 cycle of air drying and wetting; different drying durations and different core hole conditions were considered; and the point bearing capacity was determined at 3 and 5% shaft diameter displacements. The numerical analyses indicate that the point bearing capacity of drilled shafts with a verification core hole does not decrease for most cases, and the maximum reduction merely reaches 5%. Recommendations are made to reduce the effect of the verification core extracted at the shaft bottom during construction.     

Temporal integration in structure from motion.

A temporal integration model is proposed that predicts the results reported in 4 psychophysical experiments. The main findings were (a) the initial part of a structure-from-motion (SFM) sequence influences the orientation evoked by the final part of that sequence (an effect lasting for more than 1 s), and (b) for oscillating SFM sequences, perceived slant is affected by the oscillation frequency and by the sign of the final gradient. For contracting optic flows (i.e., rotations away from the image plane), the sequence with the lowest oscillation frequency appeared more slanted; for expanding optic flows (i.e., rotations toward the image plane), the sequence with the highest oscillation frequency appeared more slanted. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Perceiving surface slant from deformation of optic flow.

Perceived surface orientation and angular velocity were investigated for orthographic projections of 3-D rotating random-dot planes. It was found that (a) tilt was accurately perceived and (b) slant and angular velocity were systematically misperceived. It was hypothesized that these misperceptions are the product of a heuristic analysis based on the deformation, one of the differential invariants of the first-order optic flow. According to this heuristic, surface attitude and angular velocity are recovered by determining the magnitudes of these parameters that most likely produce the deformation of the velocity field, under the assumption that all slant and angular velocity magnitudes have the same a priori probability. The results of the present investigation support this hypothesis. Residual orientation anisotropies not accounted for by the proposed heuristic were also found. (PsycINFO Database Record (c) 2010 APA, all rights reserved)