Systems consisting of multiple robots are traditionallydifficult to optimize. This project considers such a systemin a simulated warehouse setting, where the robots are todeliver boxes while avoiding collisions. Adding such collisionconstraints complicates the problem. For dynamical multi-agentsystems as these, reinforcement learning algorithms are oftenappropriate. We explore and implement a reinforcement learningalgorithm, called multi-agent rollout, that allows for re-planningduring operation. The algorithm is paired with a base policyof following the shortest path. Simulation results with up to10 robots indicates that the algorithm is promising for largescalemulti-robot systems. We have also discussed the possibilityof using neural networks and partitioning to further increaseperformance. / System med flera robotar har traditionellt sett ansetts mycket svåra att optimera. I detta projekt undersöks ett sådant system i en simulerad lagerlokal, där robotarna skall förflytta lådor samtidigt som de undviker kollisioner. För dessa dynamiska system med flera robotar är förstärkande inlärning ofta lämpligt. Vi undersöker och implementerar en förstärkandeinlärningsalgoritm kallad ”multi-agent rollout” vilken möjliggör omdirigering under drift. Algoritmen används tillsammans med en så kallad ”base policy” som alltid väljer kortaste vägen. Baserat på simulationsresultaten med upp till tio robotar verkar algoritmen lovande för storskaliga flerrobotsystem. Det diskuteras även om möjligheten av att använda neurala nätverk och partitionering för att vidare öka prestandan. / Kandidatexjobb i elektroteknik 2021, KTH, Stockholm

For the past couple of years researchers around theworld have tried to develop fully autonomous vehicles. One of theproblems that they have to solve is how to navigate in a dynamicworld with ever-changing variables. This project was initiated tolook into one scenario of the path planning problem; overtakinga human driven vehicle. Model Predictive Control (MPC) hashistorically been used in systems with slower dynamics but withadvancements in computation it can now be used in systems withfaster dynamics. In this project autonomous vehicles controlledby MPC were simulated in Python based on the kinematic bicyclemodel. Constraints were posed on the overtaking vehicle suchthat the two vehicles would not collide. Results show that anovertake, that keeps a proper distance to the other vehicle andfollows common traffic laws, is possible in certain scenarios. / Under de senaste åren har forskare världen över försökt utveckla fullt autonoma fordon. Ett av problemen som behöver lösas är hur man navigerar i en dynamisk värld med ständigt förändrande variabler. Detta projekt startades för att titta närmare på en aspekt av att planera en rutt; att köra om ett mänskligt styrt fordon. Model Predictive Control (MPC) har historiskt sett blivit använt i system med långsammare dynamik, men med framsteg inom datorers beräkningskraft kan det nu användas i system med snabbare dynamik. I detta projekt simulerades självkörande fordon, styrda av MPC, i Python. Fordonsmodellen som används var kinematic bicycle model. Begränsningar sattes på det omkörande fordonet så att de två fordonen inte kolliderar. Resultaten visar att en omkörning, som håller avstånd till det andra fordonet samt följer trafikregler, är möjligt i vissa scenarion. / Kandidatexjobb i elektroteknik 2020, KTH, Stockholm

The usage of autonomous ground vehicles is growingextensively. Therefore, it is important to gain a better understand-ing for the fundamentals of their communication network. Inthis paper, three important areas will be considered: Trajectorytracking, formation control and collision avoidance. Firstly,trajectory tracking is implemented for unicycles to direct them toa reference path. Secondly, formation control is examined for twoapproaches: A method based on virtual structure with a presettrajectory for unicycles and a method based on displacement forpoint agents. Finally, collision avoidance is incorporated withthe displacement-based controller. For this case, agents keepformation without colliding within formation and with staticobstacles in the workspace. The proposed controllers are verifiedthrough simulations in MATLAB. / Användningen av autonoma markfordon har ökat kraftigt de senaste åren. Således är det viktigt att få en bättre förståelse för grunderna i deras kommunikationsnätverk. I detta projekt studeras tre essentiella områden: projektilspårning, formationshållning och undvikning av kollisioner. Först och främst implementeras projektilspårning för en enhjuling där den styrs mot en önskad projektilbana. Därefter betraktas formationshållning genom två metoder: Den första metoden handlar om virtuella strukturer med en förutbestämd bana för enhjulingar. Den andra metoden baseras på en förskjutningsbaserad regulator. Slutligen införs undvikning av kollisioner tillsammans med den förskjutningsbaserade regulatorn för att uppnå ett kollisionsundvikande robotsystem. Samtliga objektiv inom de tre områdena nås med varierande precision. / Kandidatexjobb i elektroteknik 2020, KTH, Stockholm

Collision avoidance systems (CASs) are being developed and fielded to reduce the number and severity of rear-end crashes. Kinematic algorithms within CASs evaluate sensor input and apply assumptions describing human-response timing and deceleration to determine when an alert should be presented. This dissertation presents an analytic assessment of dynamic function and performance CASs and associated driver performance for preventing automotive rear-end crashes. A method for using naturalistic data in the evaluation of CAS algorithms is described and applied to three algorithms. Time-series parametric data collected during 13 rear-end crashes and 70 near-crashes are input into models of collision avoidance algorithms to determine when the alerts would have occurred. Algorithm performance is measured by estimating how much of the driving population would be able to respond in the time available between when an alert would occur and when braking was needed. A sensitivity analysis was performed to consider the effect of alternative inputs into the assessment method. The algorithms were found to warn in sufficient time to permit 50–70% of the population to avoid collision in similar scenarios. However, the accuracy of this estimate was limited because the tested algorithms were found to alert too frequently to be feasible. The response of the assessment method was most sensitive to differences in assumed response-time distributions and assumed driver braking levels. Low-speed crashes were not addressed by two of the algorithms. Analysis of the events revealed that the necessary avoidance deceleration based on kinematics was generally less than 2 s in duration. At the time of driver response, the time remaining to avoid collision using a 0.5g average deceleration ranged from â 1.1 s to 2.1 s. In 10 of 13 crashes, no driver response deceleration was present. Mean deceleration for the 70 near-crashes was 0.37g and maximum was 0.72g. A set of the events was developed to measure driver response time. The mean driver response time was 0.7 s to begin braking and 1.1 s to reach maximum deceleration. Implications for collision countermeasures are considered, response-time results are compared to previous distributions and future work is discussed. / Ph. D.

Air corridors are an integral part of the advanced air mobility infrastructure. They are the virtual highways in the sky for transportation of people and cargo in the controlled airspace at an altitude of around 1000 ft. to 2000 ft. above the ground level. This paper presents fundamental insights into the design of air corridors with high operational efficiency as well as zero collisions. It begins with the definitions of air cube, skylane or track, intersection, vertiport, gate, and air corridor. Then, a multi-layered air corridor model is proposed. Traffic at intersections is analyzed in detail with examples of vehicles turning in different directions. The concept of capacity of an air corridor is introduced along with the nature of distribution of locations of vehicles in the air corridor and collision probability inside the corridor are discussed. Finally, the results of simulations of traffic flows are presented.

This thesis presents an evaluation of the performance, in terms of throughput and latency, of two Media Access Control (MAC) mechanisms in Underwater Acoustic Networks (UANs), using a model designed in the COTS simulation tool OPNET 10.5. The carrier sense multiple access with collision avoidance is the predominant approach for implementing the MAC mechanism in UANs. However, the underwater acoustic environment is characterized by extreme propagation delays and limited bandwidth, which suggests that an Aloha-like scheme may merit consideration. The performance of these two schemes was compared with respect to two topologies: tree and grid. The results showed that an Aloha-like scheme that does not segment messages outperforms the contention-based scheme under all load conditions, in terms of both throughput and latency, for the two topologies. This thesis is the first to establish that Aloha-like MAC mechanisms can be more than a limited alternative for lightly loaded networks; more specifically, they can be the preferred choice for an environment with large propagation delays. / Lieutenant Commander, Portuguese Navy

The problem of aircraft conflict detection and resolution (CDR) in uncertainty is addressed in this thesis. The main goal in CDR is to provide safety for the aircraft while minimizing their fuel consumption and flight delays. In reality, a high degree of uncertainty can exist in certain aircraft-aircraft encounters especially in cases where aircraft do not have the capabilities to communicate with each other. Through the use of a probabilistic approach and a multiple model (MM) trajectory information processing framework, this uncertainty can be effectively handled. For conflict detection, a randomized Monte Carlo (MC) algorithm is used to accurately detect conflicts, and, if a conflict is detected, a conflict resolution algorithm is run that utilizes a sequential list Viterbi algorithm. This thesis presents the MM CDR method and a comprehensive MC simulation and performance evaluation study that demonstrates its capabilities and efficiency.

Over the past few decades, wireless local area networks (WLANs) have been widely deployed for data communication in indoor environments such as offices, houses, and airports. In order to fairly and efficiently use the unlicensed frequency band that Wi-Fi devices share, the devices follow a set of channel access rules, which is called a wireless medium access control (MAC) protocol. It is known that wireless devices following the 802.11 standard MAC protocol, i.e. the distributed coordination function (DCF), suffer from packet collisions when multiple nodes simultaneously transmit. This significantly degrades the throughput performance. Recently, several studies have reported access techniques to reduce the number of packet collisions and to achieve a collision free WLAN. Although these studies have shown that the number of collisions can be reduced to zero in a simple way, there have been a couple of remaining issues to solve, such as dynamic parameter adjustment and fairness to legacy DCF nodes in terms of channel access opportunity. Recently, In-Band Full Duplex (IBFD) communication has received much attention, because it has significant potential to improve the communication capacity of a radio band. IBFD means that a node can simultaneously transmit one signal and receive another signal in the same band at the same time. In order to maximize the performance of IBFD communication capability and to fairly share access to the wireless medium among distributed devices in WLANs, a number of IBFD MAC protocols have been proposed. However, little attention has been paid to fairness issues between half duplex nodes (i.e. nodes that can either transmit or receive but not both simultaneously in one time-frequency resource block) and IBFD capable nodes in the presence of the hidden node problem.

Abstract: This thesis addresses the problem of graceful degradation for air traffic management systems (ATMS). The graceful degradation is the process by which the safety of the airspace is ensured in the event of failures or operational degradation in the system. After listing the main areas where failures and degradation can affect the ATMS, an ontology of the ATMS is proposed. The ontology allows to introduce failures at different levels, track their propagation throughout the system, and measure their operational impact. Then, two operational degradations are studied: The first degradation studied is a reduction in the landing capacity at San Francisco International Airport. The aircraft queueing process for terminal area is modeled and optimized to ensure a graceful degradation. The second degradation encompasses Communication, Navigation and Surveillance systems failures. The graceful degradation is ensured by increasing the spacing distance between aircraft, using novel algorithms of avoidance under uncertainties. Those algorithm also serve as probes to compare the degradation capabilities of different traffic configurations such as Miles-In-Trail and Free-Flight arrivals. Finally, this thesis focuses on monitoring the airspace for potential degradation. The ability and the difficulty of en-route traffic configuration are evaluated using degradation maps. Those maps can be used controller to rapidly and efficiently steer traffic from nominal mode of operations to mode of operations under abnormal conditions. Finally, a monitoring tool for terminal area is presented: the conformance of current flight to pre-identified typical operations is determined in real time. As the number of non-conforming aircraft increases, the complexity seen by air traffic controllers increases, and can become a threat for the airspace safety.

Τα τελευταία χρόνια εξελίξεις στην τεχνολογία της μικροηλεκτρονικής και των ψηφιακών επικοινωνιών έκαναν δυνατή τη δημιουργία αυτόνομων μικροσκοπικών συσκευών, εξοπλισμένων με αισθητήρες, επεξεργαστή και δυνατότητα ασύρματης επικοινωνίας, που ονομάζονται μικροαισθητήρες. Τα ασύρματα δίκτυα μικροαισθητήρων αποτελούνται από ένα μεγάλο πλήθος μικροαισθητήρων που τοποθετούνται (συνήθως με τυχαίο τρόπο) σε μία περιοχή ενδιαφέροντος προκειμένου να μετρήσουν την τιμή ενός μεγέθους ενδιαφέροντος. Λόγω των περιορισμένων δυνατοτήτων των συσκευών, τόσο σε υπολογιστικούς πόρους, μνήμη και εμβέλεια επικοινωνίας όσο και κυρίως σε αποθέματα ενέργειας, είναι αναγκαία η ανάπτυξη πρωτοκόλλων επικοινωνίας που λαμβάνουν υπόψη τους περιορισμούς αυτούς. Στο παρόν κείμενο παρουσιάζουμε νέα πρωτόκολλα για ασύρματα δίκτυα μικροαισθητήρων. Τα πρωτόκολλα καλύπτουν πολλές σημαντικές πτυχές του προβλήματος της επικοινωνίας σε αυτή την κατηγορία δικτύων, όπως είναι η αποτελεσματική διάδοση δεδομένων, η αποφυγή συγκρούσεων και η εξοικονόμηση ενέργειας. Ο σχεδιασμός τέτοιων πρωτοκόλλων απαιτεί ανάπτυξη και χρήση μη τετριμμένων αλγοριθμικών τεχνικών, όπως είναι η προσαρμοστικότητα, η ικανοποίηση \\\\EN{trade-offs}, η ετερογένεια του δικτύου ως σχεδιαστικό εργαλείο κλπ. Η ανάλυση των πρωτοκόλλων γίνεται με την διεξαγωγή εξαντλητικών πειραμάτων προσομοίωσης μεγάλης κλίμακας και την μέτρηση της απόδοσής τους σε συγκεκριμένες μετρικές. Οι επιδόσεις των πρωτοκόλλων αξιολογούνται σε σύγκριση με ευρέως αποδεκτά αντίστοιχα πρωτόκολλα που έχουν προταθεί στη διεθνή βιβλιογραφία. / In recent years, advances in technology in the areas of microelectronics and digital communications made possible the creation of tiny autonomous devices, that are equipped with sensors, a processor and wireless communication capabilities, which are called sensors. Wireless sensor networks are comprised of a large number of sensor devices which are deployed (usually in a random manner) in an area of interest, in order to measure the value of a condition. Due to the limited capabilities of the devices, in processing, memory, communication range and mostly energy supplies, it is necessary to develop communication protocols that take into account these limitations. In this work we present new protocols for wireless sensor networks. Our protocols cover many important aspects of the problem of efficient communication in this type of networks, such as efficient data propagation, collision avoidance and energy conservation. The design of such protocols requires the development and use of non-trivial algorithmic techniques, such as adaptation, satisfaction of trade-offs, using the heterogeneity of the network as a design tool etc. The analysis of the protocols is performed by conducting thorough, large scale simulation experiments and measuring their performance in various metrics. The performance of the protocols is evaluated in comparison to well known widely accepted protocols proposed in the international literature.