10 research outputs found
Multiuser Optimal Transmit Beamforming: Performance Studies, Antennas Selection, A Genetic Algorithm Approach
RÉSUMÉ
La formation de faisceaux est une technique très prometteuse utilisant un grand nombre d'antennes pour transmettre un signal vers un ou plusieurs utilisateurs. L'objectif est d'augmenter la puissance du signal chez l'utilisateur souhaité et de réduire la puissance d'interférence chez les utilisateurs non visés. Étant donné que la transmission de la formation de faisceaux augmente la puissance dans une direction spécifique, cela permet à un accès multiple par division spatiale de servir plusieurs utilisateurs simultanément. Cependant, le problème est de garder un équilibre entre maximiser la puissance du signal et minimiser la puissance d'interférence dans les systèmes multi-utilisateurs. Cette thèse décrit une structure simple qui fournit une base théorique pour un système de formation de faisceau optimal. Dans cette thèse, nous étudions les propriétés des systèmes linéaires et optimaux dans différents scénarios, tels que les rapports des signaux faibles et élevés au bruit, des nombres multiple d'antennes, le canal à évanouissement de Rayleigh et les retards multiples. Nous analysons les scénarios lorsque la formation de faisceaux linéaires fonctionnent comme une formation de faisceau optimale. Ensuite, nous proposons une méthode simple pour sélectionner le nombre minimum d'antennes suffisantes pour satisfaire aux exigences de qualité de service des utilisateurs. Lorsque le nombre d’antennes à la station de base est très grand, il ne sera peut-être pas nécessaire d’utiliser toutes les antennes pour desservir seulement quelques utilisateurs. Cette situation incite à choisir un nombre d’antennes limité. Cependant, le nombre choisi peut ne pas suffire à satisfaire les exigences de qualité de service des utilisateurs en raison de fortes interférences, de conditions de canal et du nombre d'utilisateurs. Pour résoudre ce problème NP-difficile, il faut faire une recherche exhaustive ou une recherche heuristique des méthodes itératives avec un coût de complexité informatique acceptable. Ainsi, nous proposons un cadre simple pour sélectionner un ensemble d'antennes suffisantes pour satisfaire les besoins de l'utilisateur. Enfin, nous proposons un algorithme génétique pour une formation de faisceaux optimale avec une complexité d'implémentation faible. Considérant l'algorithme de réduction de branche comme une référence, nous comparons la performance de l'algorithme proposé dans différents scénarios.----------
ABSTRACT
Transmit beamforming is a very promising technique to transmit the signal from a large array of antennas to one or multiple users. The goal is to increase the signal power at the desired user and reduce the interference power at the non-intended users. Since transmit beamforming increases the power to a specific direction, it allows for space division multiple access to serve multiple users simultaneously. However, the problem is to keep the balance between maximizing the signal power and minimizing the interference power in multi-user systems. This thesis describes a simple structure that provides a theoretical foundation for optimal beamforming scheme. In this thesis, we study the properties of linear and optimal beamforming schemes in different scenarios such as low to high signal to noise ratio ranges, multiple number of antennas, simple Rayleigh fading channel, Rayleigh fading channel with Doppler effects. We analyze the scenarios when linear beamforming performs as an optimal beamforming. Next, we propose a simple method to select the minimum number of antennas that is enough to satisfy the quality of service requirements of the users. In case of massive number of antennas at base station, it may not be necessary to use all antennas to serve only few users. That situation motivates the selection of a set of limited number of antennas. However, the number of chosen antennas may not be enough to satisfy the quality of service requirements of the users due to strong interference, channel conditions and number of users. To solve this NP-hard problem, it requires an exhaustive search or heuristic search, iterative methods with a cost of computational complexity. Thus, we propose a simple framework to select a set of antennas that is enough to satisfy the user’s requirements. Finally, we propose a genetic algorithm for optimal beamforming with low implementation complexity. Considering the branch reduce and bound algorithm as a benchmark, we compare the performance of the proposed algorithm in different scenarios
Alocação de recursos para sistemas móveis multi-utilizador e multi-antena
Doutoramento em Engenharia ElectrotécnicaThe thesis addresses the sum rate or spectral e ciency maximization problem
in cellular systems with two main components, multiple antennas and
multiple users. In order to solve such a problem, several resource allocation
techniques are studied and developed for di erent cellular scenarios. The
antennas at the transmitters are arranged in several con gurations, i.e.,
co-located or distributed and for such arrangements di erent levels of coordination
and cooperation between transmitters are investigated. Accounting
for more receiver antennas than transmitter antennas implies that system
optimization must select the best transmitter-receiver match (combinatorial
problem) which can be solved with di erent degrees of cooperation between
transmitters. The system models studied can be classi ed either as interference
limited or as power limited systems.
In interference limited systems the resource allocation is carried out independently
by each transmitter which yield power leakage to unintended
receivers. For this kind of systems, the access network using distributed
antenna architectures is examined. The properties of distributed antenna
in cellular systems as well as the gains they provide in terms of frequency
reuse and throughput are assessed. Accounting for multiple user scenarios,
several techniques and algorithms for transmitter-receiver assignment,
power allocation, and rate allocation are developed in order to maximize
the spectral e ciency.
In power limited systems the transmitters jointly allocate resources among
transmit and receive antennas. The transmitters are equipped with multiple
antennas and signal processing is implemented in order to suppress inter-user
interference. Single-cell and multi-cell systems are studied and the problem
of sum rate maximization is tackled by decoupling the user selection and
the resource allocation (power and precoding) processes. The user selection
is a function of the type of precoding technique that is implemented
and the level of information that can be processed at the transmitter. The
developed user selection algorithms exploit information provided by novel
channel metrics which establish the spatial compatibility between users.
Each metric provides a di erent trade-o between the accuracy to identify
compatible users, and the complexity required to compute it. Numerical
simulations are used to assess the performance of the proposed user selection
techniques (metrics and algorithms) whose performance are compared
to state-of-the-art techniques.Esta tese descreve o problema da maximização da taxa de transmissão ou
e ciência espectral em sistemas moveis tomando em atenção duas características fundamentais destes, o número de antenas e utilizadores.
A fim de resolver este tipo de problema, várias técnicas de alocação de recursos
foram estudadas e propostas para diferentes cenários. As antenas nos transmissores
estão organizadas em diferentes configurações, podendo ser localizadas
ou distribuídas e para estes esquemas, diferentes níveis de cooperação
e coordenação entre transmissores foram investigados. Assumindo mais antenas
receptoras do que antenas transmissoras, implica que a otimização do
sistema seleccione as melhores combinações de transmissor-receptor (problema
combinatório), o que pode ser concretizado usando diferentes graus
de cooperação entre transmissores. Os modelos de sistemas estudados, podem
ser classificados como sistemas limitados por interferência ou sistemas
limitados por potência.
Em sistemas limitados por interferência a alocação de recursos e feita independentemente
para cada transmissor o que resulta em perda de energia
para os receptores não tomados em consideração. Para este tipo de sistemas,
e considerado o caso em que a rede de acesso e constituída por antenas
distribuídas. Os ganhos obtidos devido ao uso de antenas distribuídas,
quer em termos do planeamento de frequências quer da maximização da taxa
de transmissão são considerados. Assumindo esquemas multi-utilizador,
várias técnicas e algoritmos de transmissão-recepção, alocação de potência
e de taxa de transmissão foram desenvolvidos para maximizar a e ciência
espectral.
Para sistemas limitados em potência os transmissores alocam os recursos
quer de antenas de transmissão quer de recepção conjuntamente. Os transmissores
estão equipados com várias antenas e o processamento de sinal e
implementado de modo a eliminar a interferência entre utilizadores. Sistemas
de célula única e de múltiplas células foram estudados. Para estes foi
considerado o problema da maximização de taxa de transmissão o qual foi
resolvido heuristicamente, através do desacoplamento do problema em duas
partes, uma onde se efectua a seleção de utilizadores e outra onde se considera
a alocação de recursos. A seleção de utilizadores e feita em função do
tipo de técnicas de pré-codificação implementadas e do nível de informação
que o transmissor possui. Os algoritmos de seleção de utilizadores desenvolvidos
verificam a compatibilidade espacial entre utilizadores, usando para
tal métricas propostas. Cada uma das métricas oferece um trade-off diferente
entre a precisão para identificar um utilizador compatível e a complexidade
necessária para a implementar. Foram usadas simulações numéricas
para avaliar a performance das técnicas de seleção de utilizadores propostas
(métricas e algoritmos), performance que foi comparada com as técnicas
mais inovadoras
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Identification and Mitigation of Information Leakage Caused by Side Channel Vulnerabilities in Network Stack
Keeping users sensitive information secure and private in todays network is challenging. Networks are large, complicated distributed systems and are subject to a wide variety of attacks, such as eavesdropping, identity spoofing, hijacking, etc. What is worse, encrypting data is often not enough in light of advanced threats such as side channel attacks, which enable malicious attackers to infer sensitive data from insignificant network information unexpectedly. For this purpose, we pro- pose series of techniques to prevent such information leakage at different layers in network stacks, and raise awareness of its severity. More specifically, 1) we propose a practical physical (PHY) layer security framework FOG, for effective packet header obfuscation using MIMO, to keep eavesdroppers from receiving any meaningful packet information; 2) we identify and fix a subtle yet serious pure off-path side channel vulnerability (CVE-2016-5696) introduced in both TCP specification and its implementation in Linux kernel, which prevents malicious attackers from exploiting it to indicate arbitrary connections state, reset the connection or even further hijack the connection; 3) we propose a principled TCP side channel vulnerability discovery solution based on model checking and program analysis, and automatically identify 12 new side channel vulnerabilities (and 3 old ones) from TCP implementation in Linux and FreeBSD kernel code. The ultimate goal is to help guide the future design and implementation of network stacks.Keeping users’ sensitive information secure and private in today’s network is challenging. Network nowadays are subject to a wide variety of attacks, such as eavesdropping, identity spoofing, denial of service, etc. What is worse, encrypting sensitive data is often not enough in light of advanced threats such as side channel attacks, which enable malicious attackers to infer sensitive data from “insignificant” network information unexpectedly. For this purpose, we propose series of techniques to prevent such information leakage at different layers in network stack, and raise awareness of its severity. In our first work, we propose a practical physical (PHY) layer security framework FOG, for effective packet header obfuscation using MIMO, to prevent eavesdroppers from receiving any packet headers to profile users. Secondly, we identify and fix a subtle yet serious pure off-path side channel vulnerability (CVE-2016-5696) introduced in both TCP specification and its implementation in Linux kernel. This vulnerability allows malicious attackers to indicate arbitrary TCP connection’s state, reset the connection or even further hijack the connection. Motivated by the fact that most previous TCP side channel vulnerabilities are manually identified, in our last work, we propose a principled TCP side channel vulnerability discovery solution based on model checking and program analysis. It automatically identifies 12 new side channel vulnerabilities (and 3 old ones) from TCP implementation in Linux and FreeBSD kernel code. The ultimate goal of my research is to help guide the future design and implementation of network stacks
D 3. 3 Final performance results and consolidated view on the most promising multi -node/multi -antenna transmission technologies
This document provides the most recent updates on the technical contributions and research
challenges focused in WP3. Each Technology Component (TeC) has been evaluated
under possible uniform assessment framework of WP3 which is based on the simulation guidelines
of WP6. The performance assessment is supported by the simulation results which are in their
mature and stable state. An update on the Most Promising Technology Approaches (MPTAs)
and their associated TeCs is the main focus of this document. Based on the input of all the TeCs in WP3, a consolidated view of WP3 on the role of multinode/multi-antenna transmission
technologies in 5G systems has also been provided. This consolidated view is further
supported in this document by the presentation of the impact of MPTAs on METIS scenarios
and the addressed METIS goals.Aziz, D.; Baracca, P.; De Carvalho, E.; Fantini, R.; Rajatheva, N.; Popovski, P.; Sørensen, JH.... (2015). D 3. 3 Final performance results and consolidated view on the most promising multi -node/multi -antenna transmission technologies. http://hdl.handle.net/10251/7675
Cell-Free Massive MIMO and Millimeter Wave Channel Modelling for 5G and Beyond
Huge demand for wireless throughput and number of users which are connected to the base station (BS) has been observed in the last decades. Massive multiple-input multiple-output (MIMO) is a promising technique for 5G for the following reasons; 1) high throughput; 2) serving large numbers of users at the same time; 3) energy efficiency. However, the low throughput of cell-edge users remains a limitation in realistic multi-cell massive MIMO systems. In cell-free massive MIMO, on the other hand, distributed access points (APs) are connected to a central processing unit (CPU) and jointly serve distributed users. This thesis investigates the performance of cell-free Massive MIMO with limited-capacity fronthaul links from the APs to the CPU which will be essential in practical 5G networks. To model the limited-capacity fronthaul links, we exploit the optimal uniform quantization. Next, closed-form expressions for spectral and energy efficiencies are presented. Numerical results investigate the performance gap between limited fronthaul and perfect fronthaul cases, and demonstrate that exploiting a relatively few quantization bits, the performance of limited-fronthaul cell-free Massive MIMO closely approaches the perfect fronthaul performance. Next, the energy efficiency maximization problem and max-min fairness problems are considered with per-user power and fronthaul capacity constraints. We propose an iterative procedure which exploits a generalized eigen vector problem and geometric programming (GP) to solve the max-min optimization problem. Numerical results indicate the superiority of the proposed algorithms over the case of equal power allocation. On the other hand, the performance of communication systems depends on the propagation channel. To investigate the performance of MIMO systems, an accurate small scale fading channel model is necessary. Geometry-based stochastic channel models (GSCMs) are mathematically tractable models to investigate the performance of MIMO systems
A vision-based optical character recognition system for real-time identification of tractors in a port container terminal
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