Dissertations / Theses on the topic 'Cooperative control'

In this thesis project, we consider the cooperative control of multi-agent systems under limited communication between the individual agents. In particular, quantized values of the relatives states between neighboring agents are used as the control parameters for each agent. As an introductory part, the theoretical framework for the distributed consensus problem under perfect communication is reviewed with the focus on the system stability and convergence. We start from the common problem setup that single integrator agents with a static tree communication topology, where the stability constraints and convergence guaranty are derived for dierent quantization models: uniform, logarithmic and dynamic. Then the conclusions are extended to switching tree topologies, tree topologies with disconnected time intervals and nally general undirected graphs. The control performance like  onvergence rate and the area of convergence set are compared between systems with and without quantization eects, and also among the systems with dierent quantizers. Furthermore, similar techniques are applied to other system dynamics with quantized control inputs. We investigate additional constraints on the stability of the corresponding discrete time system due to the presence of quantization eects. Explicit upper bounds on the sampling time that guarantee convergence are derived. As expected, the sampling frequency has to be increased accordingly under dierent quantization models. The multi-agent system composed of second-order agents under general undirected communication graphs is also taken into account with quite dierent analytical tools. Finally we switch to an alternative model that takes the relative quantized states as control parameters instead. Distinctive convergence properties are found between these two models and detailed comparisons are made. Throughout this report, all results obtained are supported by numerical simulations.

In this work, the problem of deploying a team of mobile sensing agents to provide coverage of an environment is addressed. We propose a novel distributed algorithm to generate a sequence of waypoints for each agent, based on intermittent communication between the agents. The algorithm is shown to converge to an equilibrium configuration, while a measure of the environment coverage is shown to be monotonically nondecreasing. To fulfill the task of moving the agents to the designated waypoints, we develop a non-linear control algorithm based on backstepping, as well as a path planning strategy that uses potential field navigation and collision avoidance. All the proposed algorithms are tested in a simulated environment and on real-world aerial robots.

Thesis (Ph. D.) -- University of Maryland, College Park, 2004.
Thesis research directed by: Electrical Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

In this thesis, a distributed extremum seeking and cooperative control algorithm is designed for mobile agents to disperse themselves optimally in maintaining communication quality and maximizing their coverage. The networked mobile agents locally form a virtual multiple-input multiple-output (MIMO) communication system, and they cooperatively communicate among them by using the decode and forward cooperative communication technique. The outage probability is used as the measure of communication quality, and it can be estimated real-time. A general performance index balancing outage probability and spatial dispersion is chosen for the overall system. The extremum seeking control approach is used to estimate and optimize the value of the performance index, and the cooperative formation control is applied to move the mobile agents to achieve the optimal solution by using only the locally-available information. Through the integration of cooperative communication and cooperative control, network connectivity and coverage of the mobile agents are much improved when compared to either non-cooperative communication approaches or other existing control results. Analytical analysis is carried out to demonstrate the performance and robustness of the proposal methodology, and simulation is done to illustrate its effectiveness.
M.S.E.E.
Masters
Electrical Engineering and Computing
Engineering and Computer Science
Electrical Engineering

Thesis (MS)--Texas A&M University, 2003.
"Major Subject: Aerospace Engineering" Title from author supplied metadata (automated record created on Jul. 18, 2005.) Vita. Abstract. Includes bibliographical references.

Dissertation (Ph.D. in Aeronautical and Astronautical Engineering) Naval Postgraduate School, September 1993.
Dissertation supervisor(s): Agrawal, Brij N. "September 1993." Includes bibliographical references. Also available online.

The cooperative control of large-scale multi-agent systems has gained a significant interest in recent years from the robotics and control communities for multi-vehicle control. One motivator for the growing interest is the application of spatially and temporally distributed multiple unmanned aerial vehicle (UAV) systems for distributed sensing and collaborative operations. In this research, the multi-vehicle control problem is addressed using a decentralised control system. The work aims to provide a decentralised control framework that synthesises the self-organised and coordinated behaviour of natural swarming systems into cooperative UAV systems. The control system design framework is generalised for application into various other multi-agent systems including cellular robotics, ad-hoc communication networks, and modular smart-structures. The approach involves identifying suitable relationships that describe the behaviour of the UAVs within the swarm and the interactions of these behaviours to produce purposeful high-level actions for system operators. A major focus concerning the research involves the development of suitable analytical tools that decomposes the general swarm behaviours to the local vehicle level. The control problem is approached using two-levels of abstraction; the supervisory level, and the local vehicle level. Geometric control techniques based on differential geometry are used at the supervisory level to reduce the control problem to a small set of permutation and size invariant abstract descriptors. The abstract descriptors provide an open-loop optimal state and control trajectory for the collective swarm and are used to describe the intentions of the vehicles. Decentralised optimal control is implemented at the local vehicle level to synthesise self-organised and cooperative behaviour. A deliberative control scheme is implemented at the local vehicle level that demonstrates autonomous, cooperative and optimal behaviour whilst the preserv ing precision and reliability at the local vehicle level.

In this dissertation, we present novel solutions to cooperative control of autonomous multi-agent network topologies pertaining to the area of hostile target tracking by multiple unmanned aerial vehicles (UAVs). The present work assumes an undirected graph comprising point-mass UAVs with time-varying communication topology among agents. The level of information sharing ability among agents in a multi-agent network, i.e. the network connectivity, plays pivotal role in group dynamics. A neighborhood information based decentralized controller is proposed in order to drive UAVs into a symmetric formation of polygon shape surrounding a mobile target, simultaneously with maintaining and controlling connectivity during the formation process. Appropriate controller parameter selection schemes, both for controller weights and gains, are adapted for dynamic topologies to maintain the connectivity measure above zero at all times. A challenging task of tracking a desired connectivity profile along with the formation control, is accomplished by using time-varying controller gains throughout agents dynamics. We next present a generalized formation controller, which in fact generates a family of UAV trajectories satisfying the control criteria. The proposed decentralized controller contains additional tuning parameters as fractional powers on proportional and derivative terms, rendering flexibility in achieving the control objective. The proposed controller with proper fractional powers, results in gradual state changes in UAV dynamics by using limited control inputs. Moreover, we extend our work by addressing a ground target tracking and reacquiring problem using the visual information gathered by flying UAV. The proposed guidance law uses line-of-sight guidance to track the target pushing it towards the image center captured by UAV, and exploits UAV-target mutual information to reacquire the target in case it steers away from the field-of-view for a short time. The convergence of the closed loop systems under the proposed controllers are shown using Lyapunov theory. Simulation results validate the effectiveness and novelty of the proposed control laws.

In addition to the above, this work focuses on categorizing multi-agent topologies in concern with the network dynamics and connectivity to analyze, realize, and visualize multi-agent interactions. In order to explore various useful agents reconfiguration possibilities without compromising the network connectivity, the present work aims at determining distinct topologies with the same connectivity or isoconnected topologies. Different topologies with identical connectivity are found out with the help of analytic techniques utilizing matrix algebra and calculus of variation. Elegant strategies for preserving connectivity in a network with a single mobile agent and rest of the stationary members, are proposed in this work as well. The proposed solutions are validated with the help of sufficient examples. For visual understanding of how agents locations and topology configurations influence the network connectivity, a MATLAB based graphical user interface is designed to interact with multi-agent graphs in a user-friendly manner.

To this end, the present work succeeds to determine solutions to challenging multi-UAV cooperative control problems, such as: (1) Symmetric formation control surrounding a mobile target; (2) Maintaining, improving and controlling the network connectivity during a mission; and (3) Categorizing different multi-agent topologies to unravel useful reconfiguration options for a group. The proposed theories with appropriate analysis, and the simulation results suffice to show the contribution and novelty of this work.

Cooperative control for miniature air vehicles (MAVs) is currently a highly researched topic. There are many application for which MAVs are well suited, including fire monitoring, surveillance and reconaissance, and search and rescue missions. All of these applications can be carried out more effictively by a team of MAVs than by a single vehicle. As technologies for microcontrollers and small sensors have improved so have the capabilities of MAVs. This improvement in MAV performance abilities increases the possibility for cooperative missions. The focus of this research was on cooperative timing missions. The issues faced when dealing with multi-MAV flight include information transfer, real time path planning, and maintenance of a fleet of flight-worthy MAVs. Additional challenges associated with timing missions include path following and velocity control. Two timing scenarios were studied and both of these scenarios were flight tested. The first scenario was a sequenced arrival of the MAVs over a target at a predetermined fly-through heading. The second scenario was a simultaneous arrival of the team ofMAVs over a known target location. The ideas of coordination functions and coordination variables have been employed to achieve coordination. Experimental results verify the feasibility of real time coperative control for a team of MAVs. Initial cooperative timing tests revealed the need for more accurate path following. Accordingly, a new method for path following using vector fields was developed. A vector field of desired ground track headings is calculated and commanded ground track headings are calculated such that ground track heading error and lateral following error decay asymptotically even in the presence of constant wind disturbances. The utilization of ground track heading and ground speed in the path following control, in combination with the vector field methods is what makes this zero-error following possible. Methods for following straight lines and orbits as well as combinations of the lines and circular arcs are presented. The assertions that minimal following errors result when using these methods have been verified experimentally.

Au cours des dernières années, le réseau électrique connait une transformation rapide avec la pénétration massive des unités de production renouvelables et distribuées. Le concept de microgrids (micro-réseau électrique) est un élément clés de cette transition énergétique. Ces micro-réseaux sont constitués par un ensemble de plusieurs unités de production distribuées (DGUs), d'unités de stockage (SUs) et de charges interconnectées par des lignes électriques. Un microgrid peut être installé dans plusieurs endroits, par exemple dans des maisons, des hôpitaux, des quartiers, etc. et fonctionne soit en mode connecté au réseau principale, soit en mode isolé (autonome). Les microgrids sont confrontés à plusieurs défis liés à la garantie de la stabilité, la cybersécurité, l'optimisation des coûts énergétiques, la gestion de l'énergie, la qualité de l'énergie, etc. Dans ce travail, nous concentrons notre attention sur le contrôle des microgrids à courant continu en mode de fonctionnement autonome. La principale contribution de cette thèse est l’établissement de lois de commande par retour d’état distribuées assurant un partage de courant proportionnel entre les unités de production, une régulation de la tension moyenne des lignes et un équilibrage simultané des états de charge des éléments de stockage. En partant de l'hypothèse que les agents (DGU ou SU) ont les mêmes paramètres physiques, la preuve de la convergence exponentielle et globale est donnée en l’absence d’une connaissance de la charge présente sur le réseau. La thèse est divisée en trois parties. La première partie présente le concept des microgrids, un état de l’art sur leurs stratégies de contrôle et les préliminaires mathématiques nécessaires tout au long du manuscrit. La deuxième partie constitue la contribution théorique de cette thèse et aborde la synthèse de lois de contrôle distribuées, garantissant les objectifs envisagés en l’absence d’une connaissance de la charge variable sur le réseau et même en cas de perturbation constantes au niveau de l’entrée de commande. Cette garantie est apportée en considérant trois actions intégrales distribuées de type consensus. Dans la troisième partie, les contrôleurs proposés sont évalués dans différents scénarios par le biais de simulation Matlab/Simulink et de tests Hardware-in-the-Loop (HIL) en temps réel. Les résultats montrent que les objectifs de contrôle sont atteints avec succès, ce qui illustre l'efficacité de la méthodologie de contrôle proposée
In recent years, the power grid has undergone a rapid transformation with the massive penetration of renewable and distributed generation units. The concept of microgrids is a key element of this energy transition. Microgrids are made up of a set of several distributed generation units (DGUs), storage units (SUs) and loads interconnected by power lines. A microgrid can be installed in several locations, for example in houses, hospitals, a neighborhood or village, etc., and operates either in connected mode to the main grid or in isolated (autonomous) mode. Microgrids are facing several challenges related to stability assurance, cyber-security, energy cost optimization, energy management, power quality, etc. In this work, we focus our attention on the control of islanded direct current microgrids. The main contribution is the design of a new distributed control approach to provably achieve current sharing, average voltage regulation and state-of-charge balancing simultaneously with global exponential convergence. The main tools are consensus in multi-agent systems, passivity, Lyapunov stability, linear matrix inequalities, etc. The thesis is divided into three parts. The First part presents the concept of microgrids, a literature review of their control strategies and the mathematical preliminaries required throughout the manuscript. The second part deals with the design of the proposed distributed control approach to achieve the considered objectives. The system is augmented with three distributed consensus-like integral actions, and a distributed-based static state feedback control architecture is proposed. Starting from the assumption that the agents (DGUs or SUs) have the same physical parameters, we provide proof of global exponential convergence. Moreover, the proposed control approach is distributed, i.e., each agent exchange relative information with only its neighbors through sparse communication networks. The proposed controllers do not need any information about the parameters of the power lines neither the topology of the microgrid. The control objectives are reached despite the unknown load variation and constant disturbances. In the third part, the proposed distributed controllers are assessed in different scenarios through Matlab/Simulink simulation and real-time Hardware-in-the-Loop experiment. The results show that the control objectives are successfully achieved, illustrating the effectiveness of the proposed control methodology

Cette thèse considère principalement trois problèmes dans le domaine du contrôle distribué coopératif des systèmes multi-agents(SMA): le consensus, la navigation en formation et le maintien en formation d'un groupe d'agents lorsqu'un agent disparait. Nous proposons 3 algorithmes pour résoudre le problème du calcul distribué d'un consensus à partir de l'approche leadeur-suiveur dans le contexte SMA à dynamique non-linéaire. La référence est définie comme un leader virtuel dont on n'obtient, localement, que les données de position et de vitesse. Pour résoudre le problème du suivi par consensus pour les SMA à dynamique non-linéaire, nous considérons le suivi par consensus pour SMA de premier ordre. On propose des résultats permettant aux suiveurs de suivre le leadeur virtuel en temps fini en ne considérant que les positions des agents. Ensuite, nous considérons le suivi par consensus de SMA de second. Dans le cas de la planification de trajectoire et la commande du mouvement de la formation multi-agents. L'idée est d'amener la formation, dont la dynamique est supposée être en 3D, d'une configuration initiale vers une configuration finale (trouver un chemin faisable en position et orientation) en maintenant sa forme tout le long du chemin en évitant les obstacles. La stratégie proposée se décompose en 3 étapes. Le problème du Closing-Rank se traduit par la réparation d'une formation rigide multi-agents "endommagée" par la perte de l'un de ses agents. Nous proposons 2 algorithmes d'autoréparation systématique pour récupérer la rigidité en cas de perte d'un agent. Ces réparations s'effectuent de manière décentralisée et distribuée n'utilisant que des informations de voisinage.

Cooperative planning control is an active topic of research, with many practical applications including multi-robot systems, transportation, multi-point surveillance and biological systems. The contributions of this thesis lie in the scope of three topics: formation control, time-constrained cooperative planning control and probabilistic control synthesis, all of the them in the framework of multi-agent systems. In the first part of the thesis, given a team of rigid-bodies, we propose decentral-ized control protocols such that desired position and orientation-based formationbetween neighboring agents is achieved. Inter-agent collisions and collisions betweenagents and static obstacles of the workspace are guaranteed to be avoided by theproposed control scheme. Furthermore, the connectivity between the agents thatare initially connected is preserved. In the second part of the thesis, we considera team of agents, modeled by coupled single-integrator dynamics. Each agent isassigned with individual high-level tasks, given in Metric Interval Temporal Logic(MITL). By abstracting the motion of each agent into Transition Systems (TS), wepropose decentralized control methodologies that guarantee the satisfaction of thedesired tasks of each agent. In the final part, a coupled multi-agent system underthe presence of uncertainties and model errors is considered. Each agent is modeledby a Markov Decision Process (MDP) and is assigned with a high-level task givenin Probabilistic Computational Tree Logic (PCTL). The goal is to design controlpolicies such that each agent is performing a desired task. By clustering the agentsinto dependency clusters, we propose control algorithms that guarantee that thedesired specifications are fulfilled. Numerical simulations conducted in MATLABverify the claimed results.

QC 20170512

This thesis deals with distributed control strategies for cooperative control of multi-robot systems. Specifically, distributed coordination strategies are presented for groups of mobile robots. The formation control problem is initially solved exploiting artificial potential fields. The purpose of the presented formation control algorithm is to drive a group of mobile robots to create a completely arbitrarily shaped formation. Robots are initially controlled to create a regular polygon formation. A bijective coordinate transformation is then exploited to extend the scope of this strategy, to obtain arbitrarily shaped formations. For this purpose, artificial potential fields are specifically designed, and robots are driven to follow their negative gradient. Artificial potential fields are then subsequently exploited to solve the coordinated path tracking problem, thus making the robots autonomously spread along predefined paths, and move along them in a coordinated way. Formation control problem is then solved exploiting a consensus based approach. Specifically, weighted graphs are used both to define the desired formation, and to implement collision avoidance. As expected for consensus based algorithms, this control strategy is experimentally shown to be robust to the presence of communication delays. The global connectivity maintenance issue is then considered. Specifically, an estimation procedure is introduced to allow each agent to compute its own estimate of the algebraic connectivity of the communication graph, in a distributed manner. This estimate is then exploited to develop a gradient based control strategy that ensures that the communication graph remains connected, as the system evolves. The proposed control strategy is developed initially for single-integrator kinematic agents, and is then extended to Lagrangian dynamical systems.

This thesis deals with distributed control strategies for cooperative control of multi-robot systems. Specifically, distributed coordination strategies are presented for groups of mobile robots. The formation control problem is initially solved exploiting artificial potential fields. The purpose of the presented formation control algorithm is to drive a group of mobile robots to create a completely arbitrarily shaped formation. Robots are initially controlled to create a regular polygon formation. A bijective coordinate transformation is then exploited to extend the scope of this strategy, to obtain arbitrarily shaped formations. For this purpose, artificial potential fields are specifically designed, and robots are driven to follow their negative gradient. Artificial potential fields are then subsequently exploited to solve the coordinated path tracking problem, thus making the robots autonomously spread along predefined paths, and move along them in a coordinated way. Formation control problem is then solved exploiting a consensus based approach. Specifically, weighted graphs are used both to define the desired formation, and to implement collision avoidance. As expected for consensus based algorithms, this control strategy is experimentally shown to be robust to the presence of communication delays. The global connectivity maintenance issue is then considered. Specifically, an estimation procedure is introduced to allow each agent to compute its own estimate of the algebraic connectivity of the communication graph, in a distributed manner. This estimate is then exploited to develop a gradient based control strategy that ensures that the communication graph remains connected, as the system evolves. The proposed control strategy is developed initially for single-integrator kinematic agents, and is then extended to Lagrangian dynamical systems.

Recent developments in the theoretical field of multi-agent systems and cooperative control have driven a growing interest towards the area of autonomous transportation, surveillance and other applications. This thesis is an attempt to close the gap between the fields of theoretical and experimental control systems. To close this gap it is essential to create a reliable software and hardware infrastructure that can be used to test the applicability and performance of the controllers developed in theory under artificial constrains. In this thesis we will present a feasible implementation of an experimental testbed, covering both the hardware and software components of it. To build this testbed and show its operability, scenarios of multi-agent systems such as platooning and surveillance were implemented using scale models of Scania trucks and quadrotors. In this thesis we study the problem starting with the theoretical analysis of the vehicle dynamics, performing simulations and finally carrying out the experiments in the testbed. The result of this thesis is a reliable and versatile testbed that can be used to perform demonstrations of multi-agent robotic systems. The core of the program is developed in LabVIEW, the wireless communication is done using NesC and TinyOS, the simulations are developed in Simulink and the Visualization Engine is written in C++. The path taken to build this testbed has proven to be successful, allowing us to control multiple vehicles simultaneously under the intuitive LabVIEW programming environment. This document serves as a guide for those who wish to carry out experiments using the infrastructure developed here or those who wish to improve upon the existing work.

An innovative auxiliary control system is developed to cooperate with an unskilled welder in a manual GTAW in order to obtain a consistent welding performance. In the proposed system, a novel mobile sensing system is developed to non-intrusively monitor a manual GTAW by measuring three-dimensional (3D) weld pool surface. Specifically, a miniature structured-light laser amounted on torch projects a dot matrix pattern on weld pool surface during the process; Reflected by the weld pool surface, the laser pattern is intercepted by and imaged on the helmet glass, and recorded by a compact camera on it. Deformed reflection pattern contains the geometry information of weld pool, thus is utilized to reconstruct its $3$D surface. An innovative image processing algorithm and a reconstruction scheme have been developed for (3D) reconstruction. The real-time spatial relations of the torch and the helmet is formulated during welding. Two miniature wireless inertial measurement units (WIMU) are mounted on the torch and the helmet, respectively, to detect their rotation rates and accelerations. A quaternion based unscented Kalman filter (UKF) has been designed to estimate the helmet/torch orientations based on the data from the WIMUs. The distance between the torch and the helmet is measured using an extra structure-light low power laser pattern. Furthermore, human welder's behavior in welding performance has been studied, e.g., a welder`s adjustments on welding current were modeled as response to characteristic parameters of the three-dimensional weld pool surface. This response model as a controller is implemented both automatic and manual gas tungsten arc welding process to maintain a consistent full penetration.

L'augmentation dans les dernières décennies du nombre de véhicules présents sur les routes ne s'est pas passée sans son lot d'impacts négatifs sur la société. Même s'ils ont joué un rôle important dans le développement économique des régions urbaines à travers le monde, les véhicules sont aussi responsables d'impacts négatifs sur les entreprises, car l'inefficacité du ot de traffic cause chaque jour d'importantes pertes en productivité. De plus, la sécurité des passagers est toujours problématique car les accidents de voiture sont encore aujourd'hui parmi les premières causes de blessures et de morts accidentelles dans les pays industrialisés. Ces dernières années, les aspects environnementaux ont aussi pris de plus en plus de place dans l'esprit des consommateurs, qui demandent désormais des véhicules efficaces au niveau énergétique et minimisant leurs impacts sur l'environnement. évidemment, les gouvernements de pays industrialisés ainsi que les manufacturiers de véhicules sont conscients de ces problèmes et tentent de développer des technologies capables de les résoudre. Parmi les travaux de recherche en ce sens, le domaine des Systèmes de Transport Intelligents (STI) a récemment reçu beaucoup d'attention. Ces systèmes proposent d'intégrer des systèmes électroniques avancés dans le développement de solutions intelligentes conçues pour résoudre les problèmes liés au transport automobile cités plus haut. Ce mémoire se penche donc sur un sous-domaine des STI qui étudie la résolution de ces problèmes gr^ace au développement de véhicules intelligents. Plus particulièrement, ce mémoire propose d'utiliser une approche relativement nouvelle de conception de tels systèmes, basée sur l'apprentissage machine. Ce mémoire va donc montrer comment les techniques d'apprentissage par renforcement peuvent être utilisées afin d'obtenir des contrôleurs capables d'effectuer le suivi automatisés de véhicules. Même si ces efforts de développement en sont encore à une étape préliminaire, ce mémoire illustre bien le potentiel de telles approches pour le développement futur de véhicules plus \intelligents".
The impressive growth, in the past decades, of the number of vehicles on the road has not come without its share of negative impacts on society. Even though vehicles play an active role in the economical development of urban regions around the world, they unfortunately also have negative effects on businesses as the poor efficiency of the traffic ow results in important losses in productivity each day. Moreover, numerous concerns have been raised in relation to the safety of passengers, as automotive transportation is still among the first causes of accidental casualties in developed countries. In recent years, environmental issues have also been taking more and more place in the mind of customers, that now demand energy-efficient vehicles that limit the impacts on the environment. Of course, both the governments of industrialized countries and the vehicle manufacturers have been aware of these problems, and have been trying to develop technologies in order to solve these issues. Among these research efforts, the field of Intelligent Transportation Systems (ITS) has been gathering much interest as of late, as it is considered an efficient approach to tackle these problems. ITS propose to integrate advanced electronic systems in the development of intelligent solutions designed to address the current issues of automotive transportation. This thesis focuses on a sub-field ITS since it studies the resolution of these problems through the development of Intelligent Vehicle (IV) systems. In particular, this thesis proposes a relatively novel approach for the design of such systems, based on modern machine learning. More specifically, it shows how reinforcement learning techniques can be used in order to obtain an autonomous vehicle controller for longitudinal vehiclefollowing behavior. Even if these efforts are still at a preliminary stage, this thesis illustrates the potential of using these approaches for future development of \intelligent" vehicles.
Inscrit au Tableau d'honneur de la Faculté des études supérieures

The increasing popularity of wireless communications and the higher data requirements of new types of service lead to higher demands on wireless networks. Relay based cellular networks have been seen as an effective way to meet users’ increased data rate requirements while still retaining the benefits of a cellular structure. However, maximizing the probability of providing service and spectrum efficiency are still major challenges for network operators and engineers because of the heterogeneous traffic demands, hard-to-predict user movements and complex traffic models. In a mobile network, load balancing is recognised as an efficient way to increase the utilization of limited frequency spectrum at reasonable costs. Cooperative control based on geographic load balancing is employed to provide flexibility for relay based cellular networks and to respond to changes in the environment. According to the potential capability of existing antenna systems, adaptive radio frequency domain control in the physical layer is explored to provide coverage at the right place at the right time. This thesis proposes several effective and efficient approaches to improve spectrum efficiency using network wide optimization to coordinate the coverage offered by different network components according to the antenna models and relay station capability. The approaches include tilting of antenna sectors, changing the power of omni-directional antennas, and changing the assignment of relay stations to different base stations. Experiments show that the proposed approaches offer significant improvements and robustness in heterogeneous traffic scenarios and when the propagation environment changes. The issue of predicting the consequence of cooperative decisions regarding antenna configurations when applied in a realistic environment is described, and a coverage prediction model is proposed. The consequences of applying changes to the antenna configuration on handovers are analysed in detail. The performance evaluations are based on a system level simulator in the context of Mobile WiMAX technology, but the concepts apply more generally.

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2010.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (p. 79-80).
Proto/Amorphous Cooperative Energy Management (PACEM) aims to build and deploy a highly scalable system for smart power grids that will enable efficient demand shaping for small-user networks. Two key problems are to provide distributed control algorithm for efficient demand shaping and to provide an incentive structure to encourage both users and the electric power sector to opt-in to PACEM. In this thesis, I address the first problem by designing ColoredPower, a probabilistic control algorithm. I implemented and tested ColoredPower in MIT Proto, building on previous work in spatial computing. Simulations in Proto show that ColoredPower operates within 3% error and provides a stable dynamic response time on the order of minutes. To address the second problem, I provide a model for user and power company incentives in PACEM, in the form of the Colored Procurement Mechanism, which enables further work in optimal algorithmic mechanism design.
by Vinayak V. Ranade.
M.Eng.

The possibility to manipulate the purely noise-induced behaviour in the large ensemble of globally coupled excitable systems is central to my research work. We employ globally coupled noise-driven FitzHugh-Nagumo units as a prototype of excitable system, which serve as a rough model of a neural network. Such a network is capable of demonstrating various kinds of behaviour with non-synchronized or synchronized units, with the mean field demonstrating periodic or chaotic small oscillations, or periodic or aperiodic spiking. Delayed feedback control applied through the mean field is shown capable of manipulating the basic features of the network behaviour, namely, to induce or suppress collective synchrony, to regularize the system behaviour in both synchronous and non-syncrhonous states, to shift the basic time scales of oscillations. These results are relevant to the control of unwanted behaviour in neural networks.

With limited road infrastructure and increasing number of vehicles on the road, an intelligent transport system is needed to increase the throughput in traffic and minimize traffic jams in highly populated areas. The purpose of this project is to design and implement a control system that is capable of driving and following the preceding vehicle autonomously in the longitude direction only. The vehicle is also equipped with a vehicle to vehicle communication unit. With this information, all vehicles on the road can communicate with each other and are able to achieve shorter distances between vehicles and damp any disturbance caused by upstream traffic. A general structure on Cooperative Adaptive Cruise Control (CACC) is created by studying the research from The Netherlands Organization for Applied Scientific Research (TNO). A string stability criterion is used to determine if the system is suitable of driving in a platoon, where a string of vehicles are following a lead vehicle. This system is then implemented in a Volvo S60 and has participated in the 2011 Grand Cooperative Driving Challenge hosted in The Netherlands. The results show that the system has ability to increase throughput and damp disturbance on the upstream traffic by communicating with the other vehicles ahead. The system is also robust and simple enough to earn the 2nd place in the competition.
Grand Cooperative Driving Challenge

In this thesis, we present research on synthesizing autonomous driving with shared autonomy using Unity Engine. Adaptive Cruise Control (ACC) is considered as level 1 autonomous vehicle, which has been studied by academia and commercialized by industry. Cooperative Adaptive Cruise Control (CACC) system is an expansion of ACC, in which communication is set up between members to share driving information. Shared autonomy is a subject about human-computer interactivities. In our research, we developed a highly customizable 3D environment. We can simulate various driving scenarios and analyze the performance of different driving methods from human driving to CACC. The result of simulation proves the safety and efficiency of CACC, and the project also provides a potential of assisting the improvement of autonomous vehicles.

The objectives of this research are to extend cooperative control methods based on potential games to dynamic environments and to develop an experimental test bed to illustrate theoretical results. Cooperative control concerns coordinating a collective performance of multiple autonomous agents. Possible applications include mobile sensor networks, distributed computation, and unmanned vehicle teams. Prior work has explored game theory, specifically the framework of potential games, as an approach to cooperative control, but has been restricted to static environments. This research shows that potential game based cooperative control also can be applied to dynamic environment problems. The approach is illustrated on three example problems. The first one is a moving target tracking problem using a modified form of the learning algorithm, restrictive log-linear learning. The second example is mobile sensor coverage for an unknown dynamic environment. The last example is multi-agent path optimization using payoff based learning. The performances of the developed systems are studied by simulation. The last part of this thesis develops an experimental moving target tracking system using multiple mobile robots. Finally, the thesis concludes with suggestions for future research directions.

De nombreux systèmes électrochimiques sont composés d'un grand nombre d'éléments électroactifs en interaction. Si la réaction électrochimique possède une cinétique non linéaire, des comportements coopératifs complexes peuvent émerger suivant la nature et l’intensité des interactions entre les éléments du système. L'objectif de cette thèse est de comprendre l'influence de la taille finie de l’électrode et des interactions entre les microélectrodes sur le comportement coopératif d'un groupe de microélectrodes de platine soumis à un couplage global. Les réactions choisies pour cette étude sont l’électrooxydation du monoxyde de carbone (CO), une réaction avec une cinétique bistable et l’électrooxydation du formaldéhyde (HCHO), qui présente des oscillations de potentiel sous contrôle galvanostatique. Au cours de l’électrooxydation galvanodynamiques du CO sur une seule microélectrode de Pt, la branche S-NDR a pu être mise en évidence contrairement au comportement observé sur une macroélectrode de Pt. En outre, les nouveaux comportements coopératifs comme l'activation séquentielle des microélectrodes, des oscillations de courant et de potentiel spontanées et un régime de commutation dynamique entre les électrodes ont été découverts pour cette réaction lorsque quatre électrodes ont été couplées globalement. Pendant l’électrooxydation de HCHO, l'introduction du couplage global à deux électrodes conduit à des oscillations de courant en opposition de phase
Many electrochemical systems are composed of a large number of interacting electroactive elements. If the reaction taking place on them has nonlinear kinetics and their interactions allow them to exchange information, complex cooperative behaviors can emerge. The objective of this thesis is to understand the influence of finite-size effects and cooperative phenomena on the global behavior of a group of coupled Pt microelectrodes. The reactions chosen for this study were CO electrooxidation, a reaction with current bistability, and HCHO electrooxidation, which exhibits oscillations under galvanostatic control. During the galvanodynamic electrooxidation of CO on a single microelectrode the S-NDR branch could be evidenced, on macroelectrodes this is not possible due to the formations of stationary domains. Additionally, novel cooperative behaviors (i.e., sequential activation, oscillations and complex switching) were discovered for this reaction when four electrodes were globally coupled. During HCHO electrooxidation the introduction of global coupling to two electrodes led to anti-phase current oscillations

Automation has become increasingly prevalent in all forms of society. Activities that are too difficult for a human or to dangerous can be done by machines which do not share those downsides. In addition, tasks can be scheduled more precisely and accurately. Increases in the autonomy have allowed for a new level of tasks which are completed by teams of automated agents rather than a single one, called cooperative control. This has many benefits; but comes at the cost of increased complexity and coordination. The main thrust of research in this field is problem based, considering communication issues as a secondary feature. There is a gap considering problems in which many changes occur as rapidly as communication and the issues that arise as a result. This is the main motivation. This research presents an approach to cooperative control in highly variable systems and tackles some of the issues present in such a system. One of the most important issues is the communication network itself, which is used as an indicator for how healthy the system is an how well it may react to future changes. Therefore using the network as an input to control allows the system to navigate between conservative and aggressive techniques to improve performance while still maintaining robustness. Results are based on a test bed designed to simulate a wide variety of problem types based on: network type; numbers of actors; frequency of changes; impact of changes and method of change. The developed control method is compared to the baseline case ignoring cooperation as well as an idealized case assuming perfect system knowledge. The baseline represents sacrifices coordination to achieve a high level of robustness at reduced performance while the idealized case represents the best possible performance. The control techniques developed give a performance at least as good as the baseline case if not better for all simulations.

Cooperation is increasingly being applied in the control of interconnected multi-agent systems, and it introduces many benefits. In particular, cooperation can improve the efficiency of many types of missions, and adds flexibility and robustness against external disturbances or unknown obstacles. This thesis investigates cooperative maneuvers for aerial vehicles autonomously landing on moving platforms, and how to safely and robustly perform such landings on a real system subject to a variety of disturbances and physical and computational constraints. Two specific examples are considered: the landing of a fixed-wing drone on top of a moving ground carriage; and the landing of a quadcopter on a boat. The maneuvers are executed in a cooperative manner where both vehicles are allowed to take actions to reach their common objective while avoiding safety based spatial constraints. Applications of such systems can be found in, for example, autonomous deliveries, emergency landings, and search and rescue missions. Particular challenges of cooperative landing maneuvers include the heterogeneous and nonlinear dynamics, the coupled control, the sensitivity to disturbances, and the safety criticality of performing a high-velocity landing maneuver. The thesis suggests the design of a cooperative control algorithm for performing autonomous and cooperative landings. The algorithm is based on model predictive control, an optimization-based method where at every sampling instant a finite-horizon optimal control problem is solved. The advantages of applying this control method in this setting arise from its ability to include explicit dynamic equations, constraints, and disturbances directly in the computation of the control inputs. It is shown how the resulting optimization problem of the autonomous landing controller can be decoupled into a horizontal and a vertical sub-problem, a finding which significantly increases the efficiency of the algorithm. The algorithm is derived for two different autonomous landing systems, which are subsequently implemented in realistic simulations and on a drone for real-world flight tests. The results demonstrate both that the controller is practically implementable on real systems with computational limitations, and that the suggested controller can successfully be used to perform the cooperative landing under the influence of external disturbances and under the constraint of various safety requirements.
Samarbete tillämpas i allt högre utsträckning vid reglering av sammankopplade multiagentsystem, vilket medför både ökad robusthet och flexibilitet mot yttre störningar, samt att många typer av uppgifter kan utföras mer effektivt. Denna licentiatavhandling behandlar kooperativa och autonoma landningar av drönare på mobila landingsplatformar, och undersöker hur sådana landningar kan implementeras på ett verkligt system som påverkas av externa störningar och som samtidigt arbetar under fysiska och beräkningsmässiga begränsningar. Två exempel betraktas särskilt: först landingen av ett autonomt flygplan på en bil, därefter landning av en quadcopter på en båt. Landningarna utförs kooperativt, vilket innebär att båda fordonen har möjlighet att påverka systemet för att fullborda landningen. Denna typ av system har applikationer bland annat inom autonoma leveranser, nödlandningar, samt inom eftersöknings- och räddningsuppdrag. Forskningen motiveras av ett behov av effektiva och säkra autonoma landingsmanövrar, för fordon med heterogen och komplex dynamik som samtidigt måste uppfylla en mängd säkerhetsvillkor. I avhandlingen härleds  kooperativa regleralgoritmer för landningsmanövern. Reglermetoden som appliceras är modell-prediktiv reglerteknik, en optimeringsbaserad metod under vilken ett optimalt reglerproblem med ändlig horisont löses  varje samplingsperiod. Denna metod tillför här fördelar såsom explicit hantering av systemdynamik, och direkt inkludering av störningshantering och bivillkor vid beräkning av insignaler. På så sätt kan vi direkt i optimeringslösaren hantera säkerhetsvillkor och externa störningar. Det visas även hur lösningstiden för optimeringen kan effektiviseras genom att separera den horisontella och den vertikala dynamiken till två subproblem som löses sekvensiellt. Algoritmen implementeras därefter för två olika landingssystem, för att därefter tillämpas och utvärderas i realistiska simuleringsmiljöer med olika typer av störningar, samt med flygtester på en verklig plattform. Resultaten visar dels att reglermetoden ger önskade resultat med avseende både på störningshantering och uppfyllande av bivillkor från säkerhetskrav, och dels att algoritmen är praktiskt implementerbar även på system med begränsad beräkningskraft.

QC 20190315

The objective of this thesis is to present how implicit, nonverbal cues support coordinated action between two people. A set of five experiments shows how people utilise sensory information about a partner in order to control goal-directed action involving synchronous vertical hand movement. Chapter 1 reviews recent developments in studies of interpersonal interaction and highlights control issues that are critical in cooperative action. In Chapter 2, an experimental study addresses the importance of haptic feedback about object dynamics for learning behavioural characteristics of another person who manipulated the same object from the other side. In Chapter 3, the contributions of feedback and feedforward control are assessed as the interaction of the task partners are experimentally controlled using a humanoid robot which serves the role of task partner. Chapter 4 proposes error-based learning as a model of cooperative action wherein subsequent motor response is regressed on current error in order to improve coordination between partners. Chapter 5 shows that adaptation rate of a participant is modulated with respect to the rate of the task partner so that the net adaptation of the two partners becomes optimal using a computer simulated task partner. In Chapter 6, this joint adaptation model is applied to continuous movement to demonstrate the generalisability of the model. The last chapter discusses the contribution of the empirical chapters and reviews the theoretical and methodological contribution of the thesis as a whole to the field of cognitive neuroscience. In conclusion, the thesis provides strong evidence that movement characteristics of a partner expressed both within and across trials determine the way a person engages in a joint task.

A variety of cooperative medium access control (MAC) schemes are designed for the sake of improving the achievable transmit rate and for reducing the transmit energy dissipation of cooperative communication systems relying on realistic greedy - rather than altruistic - relay nodes (RNs). Based on the system’s objective functions (OF), novel distributed relay selection schemes are developed for selecting the best relay node (RN) set. In order to investigate the effect of the proposed MAC schemes on the performance of the cooperative communication systems considered, the system’s stability is analysed with the aid of queueing theory. Specifically, we first consider a cooperative spectrum leasing system (CSLS) supporting a licensed source node (SN) and a licensed destination node (DN) as well as multiple unlicensed greedy RNs, which require rewards for providing cooperative transmission assistance. A ’win-win’ (WW) cooperative framework (WWCF) is formulated for sake of improving the achievable transmit rate and for simulanteously minimizing the energy dissipation of the co- operative spectrum leasing system considered. Based on the proposed WWCF, the licensed SN intends to lease part of its spectrum to the unlicensed RNs in exchange for cooperative support, leading to an improved transmit rate, while simultaneously reducing the transmit power. The unlicensed RNs also have an incentive to provide cooperative transmission assistance for the SN, since in exchange for relaying assistance they are allowed to access the licensed spectrum for transmitting their own data, and even to maintain their own target Quality of Service (QoS). Furthermore, a distributed WW cooperative MAC protocol is developed for implementing the proposed WWCF by designing a specific signalling procedure and the format of both the data frame control messages as well as a distributed relay selection scheme. More explicitly, a novel backoff algorithm is designed for distributively selecting the best RN in order to optimize the system’s OF formulated by our WWCF. Our simulation results demonstrated that both substantial rate improvements and considerable energy savings are achieved by implementing the proposed distributed WW cooperative MAC protocol. However, encountering a low service rate at the MAC layer may excessively increase the length of queue in the buffer storing the incoming packet. Hence, the queueing system may become unstable due to the low service rate limited by an inferior MAC protocol design. Hence we conceived a queueing model for our cooperative spectrum leasing system relying on the proposed distributed WW cooperative MAC protocol. In order to simplify the stability analysis, some idealized simplifying assumptions are invoked and a non-Markovian analysis method is used for investigating the transmission probability of each node and for deriving the average departure rates at both the SN and the RNs operating under the control of the proposed distributed WW cooperative MAC protocol. Our simulation results confirmed that an increased stable throughput is provided by the proposed distributed WW cooperative MAC protocol for both the SN and RNs compared to the benchmark schemes. As an improved extension of the proposed WWCF, a WW reciprocal-selection-based framework (WWRSF) is formulated for a cooperative spectrum leasing system hosting multiple licensed transmission pairs and multiple unlicensed transmission pairs. The SN of a licensed pair of nodes is referred as the primary transmitter (PT), while the SN of an unlicensed transmission pair is termed as the secondary transmitter (ST). Based on the proposed WWRSF, the PT intends to lease its spectral resources to an appropriate secondary transmitter (ST) in exchange for cooperative transmission assistance for the sake of minimizing its transmit power and simultaneously satisfying its transmit rate requirement. The ST has an incentive to collaborate with the best PT for the sake of minimizing the ST’s transmit power under the constraint of its QoS requirement, whilst simultaneously winning a transmission opportunity for its own traffic. Based on the OFs of the proposed WWRSF, a distributed WW reciprocal-selection-based medium access scheme (DWWRS-MAS) is designed, which is capable of producing the best cooperative pairs set for the sake of reducing the transmit power of both the PT and of the ST in each cooperative pair, whilst simultaneously satisfying their transmit rate requirements. This is achieved with the aid of the proposed distributed reciprocal selection between the active PTs and STs, which have the capability of providing successful cooperative transmission assistance. Moreover, we analyse both the queueing stability and the algorithmic stability of our cooperative spectrum leasing system exploiting our DWWRS-MAS. In comparison to the benchmark schemes considered in the literature, the proposed DWWRS-MAS is capable of achieving a performance, which is comparable to that of the optimal schemes in terms of the system’s transmit power and system’s achievable transmit rate.

Broadly speaking, wireless ad hoc networks are permeated by cooperative behaviors. In such systems, indeed, nodes have to continuously pool their resources to achieve goals that are of general interest, such as routing packets towards a destination that would otherwise be out of reach for an information source, or coordinating medium access so as to successfully share a common spectrum. Recently, however, the idea of collaboration has gathered a renewed and increasing deal of attention in the research community thanks to the development of innovative concepts, most notably the idea of cooperative relaying, that have been shown to unleash significant improvements, going beyond some intrinsic limitations that affect wireless communications systems. While these emerging solutions have been thoroughly studied from a theoretical perspective, the advantages they offer can be reaped in real world implementations only if additional coordination among nodes is provided, so that cooperating terminals are allowed to offer their help obeying the rules that control medium access, and without disrupting the normal activity of the network. Along this line of reasoning, this thesis focuses on the design and study of link layers that implement cooperation in ad hoc networks. The contribution of our work is twofold. On the one hand, we introduce novel and beneficial collaborative strategies, while on the other hand we investigate how the intrinsic nature of different medium access control policies can be more or less beneficial to cooperative behaviors. In the first part, we present an innovative approach to cooperation in networks with multi-antenna equipped nodes that rely on directional transmissions and receptions. Our solution proposes terminals to share information on ongoing communications to better coordinate medium access at the link layer, and manages to overcome issues such as node deafness that typically hamper the potential of large scale directional wireless systems. The central part of this work, conversely, concentrates on the simpler ad hoc scenario where omnidirectional communications are in place, and tackles some inefficiencies of the cooperative relaying paradigm. In particular, we introduce the novel concept of hybrid cooperative-network coded ARQ, which allows a relay to code data of its own together with a corrupted packet during a retransmission at no additional cost in terms of bandwidth. Such a solution encourages nodes to cooperate, since they are offered the possibility to pursue a goal of their interest while helping surrounding terminals. Moreover, the capability of exploiting retransmissions to serve additional traffic, achieved by smartly taking advantage of network coding techniques, triggers beneficial effects also at a network level in terms of sustainable throughput and reduced congestion. The potential of the proposed approach is first investigated by means of mathematical analysis, while subsequently extensive simulation campaigns test the effectiveness of link layers that implement it in a variety of networking environments. Taking the cue from a reasoned comparison of the results achieved by relaying schemes in different scenarios, we devote the final part of the thesis to the investigation of the impact that distinct spectrum control policies can have on collaborative behaviors. Combining once again mathematical analysis and simulations, we consider how the characteristics of completely distributed, e.g., carrier sense based, and centralized, e.g., time division based, systems influence the effectiveness of a given cooperative strategy. Not only does our study shed light on the relations that exist between cooperation and medium access, but also it provides important hints on how to efficiently design link layers capable of supporting such techniques. Finally, the appendix of this thesis reports the outcome of a research activity, carried out in collaboration with the IBM Zurich Research Laboratory (Switzerland), whose focus falls out of the topic of cooperative link layers and covers the design of energy-efficient routing protocols for wireless sensor networks.
Le reti wireless ad hoc presentano in generale moltissimi comportamenti di natura cooperativa, nei quali i nodi condividono le loro risorse per perseguire un interesse di utilità comune. Basti pensare, in tal senso, alle procedure di routing per la consegna di traffico multihop, o allo scambio di informazioni tra terminali necessario per riuscire a gestire in modo efficace uno spettro condiviso. Recentemente, inoltre, è progressivamente emerso un rinnovato e crescente interesse nella comunità di ricerca per il concetto di collaborazione tra nodi, grazie allo sviluppo di nuovi paradigmi, tra i quali in primis l'idea del relaying cooperativo, che si sono dimostrati in grado di mitigare brillantemente alcuni problemi tipici dei sistemi wireless, rendendo possibili significativi miglioramenti delle prestazioni. Sebbene tali soluzioni innovative siano state oggetto di notevole attenzione in letteratura, gli studi su di esse si sono concentrati principalmente su trattazioni di natura analitica, atte a dimostrarne le potenzialità e i vantaggi nell'ottica della teoria dell'informazione. Approcci di questo tipo tendono chiaramente a considerare, ai fini della trattabilità matematica, topologie semplificate quali reti a tre soli nodi, e spesso assumono un accesso al mezzo idealizzato. Nel momento in cui queste idee vogliano essere implementate in scenari reali, tuttavia, si rende necessario un profondo raffinamento della coordinazione a livello di rete, dal momento che i nodi cooperanti devono comunque sottostare alle regole che caratterizzano la gestione del canale (link layer), di modo da offrire il loro contributo senza ostacolare la normale attività della rete. Prendendo spunto da tale riflessione, questa tesi si concentra sulla definizione e l'analisi di link layer che implementino soluzioni cooperative in reti ad hoc. Due sono i principali contributi del lavoro. Se da un lato, infatti, sono introdotti paradigmi innovativi ed efficaci, dall'altro viene presentato uno studio articolato e completo su come diverse politiche di accesso al mezzo possano influenzare tali comportamenti cooperativi. La prima parte della tesi si focalizza sullo sviluppo di un nuovo approccio collaborativo per reti i cui terminali, dotati di sistemi multiantenna, siano in grado di effettuare trasmissioni e ricezioni direzionali. L'idea proposta prevede che i nodi condividano, tramite scambio di brevi pacchetti di controllo, informazioni sulle comunicazioni attive di cui sono a conoscenza, per poter favorire la maggior distribuzione possibilie di una percezione corretta dello stato del sistema al fine di garantire una migliore coordinazione nell'accesso al mezzo. Studi dedicati dimostrano come tale soluzione sia in grado di superare problemi, quali la sordità di nodo (deafness), che spesso limitano l'efficacia delle trasmissioni direzionali in reti con numero elevato di dispositivi, portando a importanti guadagni in termini di prestazione complessive. La parte centrale del lavoro, al contrario, prende in considerazione reti ad hoc con comunicazioni omnidirezionali, e affronta alcune inefficienze che caratterizzano il paradigma di relaying cooperativo. In particolare, viene introdotto per la prima volta il concetto innovativo di ARQ ibrido cooperativo-network coded, che permette a nodi che agiscano da relay di utilizzare le ritrasmissione di un pacchetto in vece di una sorgente, non in grado di consegnarlo, al fine di servire anche del proprio traffico. Tale approccio, a differenza del comportamento puramente altruistico richiesto dal relaying semplice, incoraggia i terminali a cooperare, offrendo loro la possibilità di perseguire un loro interesse contingente nell'atto stesso di aiutare altri nodi in difficoltà. Inoltre, la capacità di sfruttare il meccanismo di ritrasmissione per servire traffico addizionale, resa possibile dall'utilizzo di tecniche di combinazione lineare sui dati caratteristiche del network coding, getta le basi per benefici anche a livello di rete, quali un incremento del throughput sostenibile e una riduzione della congestione nell'utilizzo della banda. Le potenzialità della soluzione identificata sono dapprima studiate per mezzo di modelli matematici, seguendo le modalità tipicamente riscontrabili in letteratura. Successivamente sono proposti l'implementazione e lo studio simulativo di diversi link layer in grado di supportare tale forma di ARQ ibrido in contesti differenti, quali reti completamente distribuite e reti maggiormente strutturate. Traendo spunto da un confronto ragionato dei risultati ottenibili da schemi di relaying in scenari di rete diversi, la parte finale di questa tesi è dedicata alla discussione dell'impatto che politiche di accesso al mezzo distinte possono avere su comportamenti di natura cooperativa. Combinando ancora una volta analisi matematica e studi simulativi, viene affrontato il problema di come le caratteristiche intrinseche di sistemi basati su carrier sensing e su condivisione del mezzo a multiplazione di tempo influenzino l'efficacia di meccanismi di collaborazione tra nodi. Le osservazioni ottenute tramite questo approccio non solo mettono in luce la stretta relazione esistente tra politiche di gestione dello spettro e cooperazione, ma al tempo stesso forniscono importanti suggerimenti sulla progettazione di link layer in grado di supportare in modo efficace tali strategie. In appendice, infine, vengono riportati i risultati di attività di ricerca svolte in collaborazione con i laboratori di ricerca IBM di Zurigo (Svizzera) e incentrate su tematiche che si discostano leggermente dal fulcro della tesi, quali la progettazione di teniche di routing per reti wireless di sensori, con particolare attenzione all'efficienza energetica.

Dissertação apresentada na Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa para a obtenção do Grau de Mestre em Engenharia Informática
Since the introduction of Software Transactional Memory (STM), this topic has received a strong interest by the scientific community, as it has the potential of greatly facilitating concurrent programming by hiding many of the concurrency issues under the transactional layer, being in this way a potential alternative to the lock based constructs, such as mutexes and semaphores. The current practice of STM is based on keeping track of changes made to the memory and, if needed, restoring previous states in case of transaction rollbacks. The operations in a program that can be reversible,by restoring the memory state, are called transactional operations. The way that this reversibility necessary to transactional operations is achieved is implementation dependent on the STM libraries being used. Operations that cannot be reversed,such as I/O to external data repositories (e.g., disks) or to the console, are called nontransactional operations. Non-transactional operations are usually disallowed inside a memory transaction, because if the transaction aborts their effects cannot be undone. In transactional databases, operations like inserting, removing or transforming data in the database can be undone if executed in the context of a transaction. Since database I/O operations can be reversed, it should be possible to execute those operations in the context of a memory transaction. To achieve such purpose, a new transactional model unifying memory and database transactions into a single one was defined, implemented, and evaluated. This new transactional model satisfies the properties from both the memory and database transactional models. Programmers can now execute memory and database operations in the same transaction and in case of a transaction rollback, the transaction effects in both the memory and the database are reverted.

The purpose of this study was to assess the adoption response of cotton growers to Integrated Pest Management (IPM) programs, identify the extension strategies used to promote IPM, and provide descriptive comparison of IPM programs between the foremost eleven cotton-producing states in the United States. Integrated pest management is presented as an alternative agricultural production strategy referred to as sustainable agriculture. IPM programs for cotton were evaluated by use of a survey instrument and telephone interviews with the IPM coordinators for the eleven cotton-producing states. The most significant findings were that programs vary considerably between states; the most widely employed IPM techniques were selective use of chemicals, use of economic thresholds and sampling for pests; the Cooperative Extension Service worked closely with private consultants and grower associations; and adoption of a wide variety of IPM techniques has been extensive for cotton producers in the eleven foremost cotton-producing states.

CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO
Os avanços das tecnologias de captura, armazenamento e compressão de vídeo digital estão motivando o desenvolvimento e a disponibilização de novos serviços e sistemas para manipulação e gerenciamento de acervos de vídeo. Um exemplo disso são os sistemas de gerenciamento, edição e compartilhamento de versões utilizados pelos produtores de conteúdo audiovisual. Entretanto, tais funcionalidades são requisitos não encontrados em um único sistema. Este trabalho descreve um sistema que possibilita a edição cooperativa de dados audiovisuais no formato MPEG-2 permitindo o controle de versão, a visualização e manipulação do seu conteúdo por partes (segmentos). Esse sistema colaborativo tem ainda como vantagens a divisão de tarefas, a fusão das contribuições e a extração de informações da autoria de cada versão.
Technological advances in areas such as capture, storage and compression of digital video are stimulating the development of new services and systems for manipulation and management of huge amount of video data. An example of this, are the systems of management, editing and sharing of versions used by producers of audiovisual content. However, such functional requirements are not found in one system. This work describes a system that makes possible the cooperative edition of audiovisual data in MPEG-2 format, allowing the version control, visualization and manipulation of its content by segments. This collaborative system still has advantages as the division of tasks between editors, the fusion of different versions and the extraction of information of authorship from each version.

The performance of many protocols and applications of Vehicular Ad hoc Networks (VANETs), depends on vehicles obtaining enough fresh information on the status of their neighbouring vehicles. This should be fulfilled by exchanging Basic Safety Messages (BSMs) also called beacons using a shared channel. In dense vehicular conditions, many of the beacons are lost due to channel congestion. Therefore, in such conditions, it is necessary to control channel load at a level that maximizes BSM dissemination. To address the problem, in this thesis algorithms for adaptation of beaconing to control channel load are proposed. First, a position-based routing protocol for VANETs is proposed and the requirement of adaptive beaconing to increase the performance of the protocol is indicated. The routing protocol is traffic-aware and suitable for city environments and obtains real-time traffic information in a completely ad hoc manner without any central or dedicated control, such as traffic sensors, roadside units, or information obtained from outside the network. The protocol uses an ant-based algorithm to find a route that has optimum network connectivity. Using information included in small control packets called ants, vehicles calculate a weight for every street segment that is proportional to the network connectivity of that segment. Ant packets are launched by vehicles in junction areas. To find the optimal route between a source and destination, a source vehicle determines the path on a street map with the minimum total weight for the complete route. The correct functionality of the protocol design has been verified and its performance has been evaluated in a simulation environment. Moreover, the performance of the protocol in different vehicular densities has been studied and indicated that in dense vehicular conditions the performance of the protocol degrades due to channel load created by uncontrolled periodic beaconing. Then, the problem of beaconing congestion control has been formulated as non-cooperative games, and algorithms for finding the equilibrium point of the games have been presented. Vehicles as players of the games adjust their beacon rate or power or both, based on the proposed algorithms so that channel load is controlled at a desired level. The algorithms are overhead free and fairness in rate or power or both rate and power allocation are achieved without exchanging excess information in beacons. Every vehicle just needs local information on channel load while good fairness is achieved globally. In addition, the protocols have per-vehicle parameters, which makes them capable of meeting application requirements. Every vehicle can control its share of bandwidth individually based on its dynamics or requirements, while the whole usage of the bandwidth is controlled at an acceptable level. The algorithms are stable, computationally inexpensive and converge in a short time, which makes them suitable for the dynamic environment of VANETs. The correct functionality of the algorithms has been validated in several high density scenarios using simulations.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 1989.
GRSN 406710
Includes bibliographical references (leaves 146-147).
by Robert J. Polutchko.
M.S.