Fundamental Limits of Cooperation

Cooperation is viewed as a key ingredient for interference management in wireless systems. This paper shows that cooperation has fundamental limitations. The main result is that even full cooperation between transmitters cannot in general change an interference-limited network to a noise-limited network. The key idea is that there exists a spectral efficiency upper bound that is independent of the transmit power. First, a spectral efficiency upper bound is established for systems that rely on pilot-assisted channel estimation; in this framework, cooperation is shown to be possible only within clusters of limited size, which are subject to out-of-cluster interference whose power scales with that of the in-cluster signals. Second, an upper bound is also shown to exist when cooperation is through noncoherent communication; thus, the spectral efficiency limitation is not a by-product of the reliance on pilot-assisted channel estimation. Consequently, existing literature that routinely assumes the high-power spectral efficiency scales with the log of the transmit power provides only a partial characterization. The complete characterization proposed in this paper subdivides the high-power regime into a degrees-of-freedom regime, where the scaling with the log of the transmit power holds approximately, and a saturation regime, where the spectral efficiency hits a ceiling that is independent of the power. Using a cellular system as an example, it is demonstrated that the spectral efficiency saturates at power levels of operational relevance.Comment: 27 page

Reverse pumping: theory and experimental validation on a multi-kites system

International audienceMost kite wind power systems have a great drawback that wind turbines do not have: they cannot stay in the air if the wind is not strong enough, [7-16]. As a consequence, most of the kite systems need to land when there is no wind, and to take-off once the wind is strong enough. These maneuvers are quite risky because generally the wind gets weak and turbulent close to the ground's surface. Moreover, as the wind can be strong enough at high altitude and weak close to the ground, it might lead to losses in energy production. From a material point of view, "classic" landings and takeoffs need a landing zone, ground handling or infrastructure (such as pylons) that reduces the advantages of kite systems. Some ideas, such as embedded motors or helium balloons, might solve this problem, but they have their own drawbacks such as the weight of the motor and its battery, the necessity of a conductive cable or the need to refill the balloons. The following paper studies a solution called "reverse pumping". It basically consists of providing kinetic energy to the kite by pulling the kite with a rope. This kinetic energy is then transformed into potential energy by gaining altitude. This technique allows to keep the kite airborne in total absence of wind. This paper will study the reverse pumping principle, the constrains on the aerodynamical model, flight simulations and will present the experimental setup used to validate the theoretical study

Crystal Structure of the ZrO Phase at Zirconium/Zirconium Oxide Interfaces

Zirconium-based alloys are used in water-cooled nuclear reactors for both nuclear fuel cladding and structural components. Under this harsh environment, the main factor limiting the service life of zirconium cladding, and hence fuel burn-up efficiency, is water corrosion. This oxidation process has recently been linked to the presence of a sub-oxide phase with well-defined composition but unknown structure at the metal–oxide interface. In this paper, the combination of first-principles materials modeling and high-resolution electron microscopy is used to identify the structure of this sub-oxide phase, bringing us a step closer to developing strategies to mitigate aqueous oxidation in Zr alloys and prolong the operational lifetime of commercial fuel cladding alloys

Energy production control of an experimental kite system in presence of wind gusts

International audienceThe growing need of energy, global warming and recent nuclear power plant accidents have shown that renewable energies need to be developed for tomorrow's world. Wind energy is generally harvested using wind turbines. Unfortunately, these systems have some drawbacks such as their cost, and the amount of steel and concrete used for construction. As their size grows, their complexity increases exponentially. This paper studies an alternative solution for the production of wind energy, using a kite's traction force. The aim of this paper is to control the amount of energy produced by the kite, and to be able to fly it safely in the presence of strong wind gusts. Our theoretical work has been implemented in a scale model flying autonomously in a wind tunnel. The proposed control strategy has led to control the system output power with an accuracy greater than 95%, with unknown wind speeds varying from 7.5 to 9 m/s

Design and control of a spheroidal underwater robot for the inspection of nuclear piping systems

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 59-60).While it is critical that nuclear plants frequently inspect their facilities for cracking, corrosion or other failure modes, humans cannot safely perform these tasks due to the hazardous conditions within the tanks and piping systems. In response, the d'Arbeloff Laboratory in the Mechanical Engineering department is designing a compact submersible robot that is capable of precise navigation and maneuvering in order to detect defects within water filled piping systems. The robot is spheroidal with a smooth surface and no external appendages. It propels itself with centrifugal pumps which suck in water from the environment, and pump it out in various directions. This thesis covers the design and implementation of the software, electrical, and a few mechanical systems of the robot. Specifically, it details the programming techniques for the microcontroller and graphical user interface code, circuit board design, wiring, and waterproofing. A robot prototype was built, and experiments have given useful data to construct a model to supplement the field of underwater robotic design.by Martin Lozano, Jr..S.B

A hypothesis-based algorithm for planning and control in non-Gaussian belief spaces

We consider the partially observable control problem where it is potentially necessary to perform complex information-gathering operations in order to localize state. One approach to solving these problems is to create plans in belief-space, the space of probability distributions over the underlying state of the system. The belief-space plan encodes a strategy for performing a task while gaining information as necessary. Most approaches to belief-space planning rely upon representing belief state in a particular way (typically as a Gaussian). Unfortunately, this can lead to large errors between the assumed density representation and the true belief state. We propose a new computationally efficient algorithm for planning in non-Gaussian belief spaces. We propose a receding horizon re-planning approach where planning occurs in a low-dimensional sampled representation of belief state while the true belief state of the system is monitored using an arbitrary accurate high-dimensional representation. Our key contribution is a planning problem that, when solved optimally on each re-planning step, is guaranteed, under certain conditions, to enable the system to gain information. We prove that when these conditions are met, the algorithm converges with probability one. We characterize algorithm performance for different parameter settings in simulation and report results from a robot experiment that illustrates the application of the algorithm to robot grasping

Thermodynamic Comparison of Magnetocaloric and Vapor Compression Domestic Wine Coolers

This paper presents a comparative study of magnetocaloric and vapor compression wine coolers. The vapor compression system is a commercially available 31bottle climate class N appliance, whereas the magnetic cooling device is a TRL5 prototype connected to the retrofitted insulated cabinet of the same VC cooler. Test procedures included temperature pulldown and standardized steadystate energy consumption tests. Detailed instrumentation of both systems allowed a lowlevel comparison in terms of firstand secondlaw based parameters that is unique in the literature. Although the vapor compression system is more efficient than the present magnetic cooling prototype, the behavior of its internally ideal COP led to significant, but realistic design improvements which were evaluated and quantified via a genetic algorithmbased optimization aimed at reducing the system mass and the power consumption. The numerical model, supported by the experimental data, indicate that magnetic cooling systems, although bulkier than their vapor compression counterpart, can be competitive in terms of thermodynamic performance