Context-awareness privacy in data communications

Internet users consume online targeted advertising based on information collected about them and voluntarily share personal information in social networks. Sensor information and data from smart-phones is collected and used by applications, sometimes in unclear ways. As it happens today with smartphones, in the near future sensors will be shipped in all types of connected devices, enabling ubiquitous information gathering from the physical environment, enabling the vision of Ambient Intelligence. The value of gathered data, if not obvious, can be harnessed through data mining techniques and put to use by enabling personalized and tailored services as well as business intelligence practices, fueling the digital economy. However, the ever-expanding information gathering and use undermines the privacy conceptions of the past. Natural social practices of managing privacy in daily relations are overridden by socially-awkward communication tools, service providers struggle with security issues resulting in harmful data leaks, governments use mass surveillance techniques, the incentives of the digital economy threaten consumer privacy, and the advancement of consumergrade data-gathering technology enables new inter-personal abuses. A wide range of fields attempts to address technology-related privacy problems, however they vary immensely in terms of assumptions, scope and approach. Privacy of future use cases is typically handled vertically, instead of building upon previous work that can be re-contextualized, while current privacy problems are typically addressed per type in a more focused way. Because significant effort was required to make sense of the relations and structure of privacy-related work, this thesis attempts to transmit a structured view of it. It is multi-disciplinary - from cryptography to economics, including distributed systems and information theory - and addresses privacy issues of different natures. As existing work is framed and discussed, the contributions to the state-of-theart done in the scope of this thesis are presented. The contributions add to five distinct areas: 1) identity in distributed systems; 2) future context-aware services; 3) event-based context management; 4) low-latency information flow control; 5) high-dimensional dataset anonymity. Finally, having laid out such landscape of the privacy-preserving work, the current and future privacy challenges are discussed, considering not only technical but also socio-economic perspectives.

Amplificador de RF Doherty assimétrico de 2-vias

Os atuais esquemas de modulação e acesso ao meio, tais como o Wide- Band Code-Division Multiple Access (WCDMA) ou Orthogonal Frequency- Division Multiple Access (OFDMA), que são otimizados para a gestão eficiente do espetro electromagnético e elevada taxa de transmissão, originam sinais de elevado Peak-to-Average Power Ratio (PAPR) e requisitos de linearidade rigorosos. As arquiteturas de amplificação tradicionais, i.e. baseadas no operação em modo de corrente do dispositivo ativo, são incapazes de satisfazer estes requisitos em simultâneo. Assim, o amplificador de potência (do inglês, Power Ampli_er (PA)) incorre numa degradação significativa de rendimento energético em favor de maior linearidade, aumentando simultaneamente os custos de operação das estacões base para os operadores de telecomunicações móveis e o impacte ambiental. Este trabalho foca-se no estudo da arquitetura Doherty, a principal solução encontrada para melhorar o compromisso linearidade/rendimento para aplicações em estações-base de comunicações móveis. Para tal, são expostos os princípios básicos de amplificadores de rádio frequência assim como a análise teórica do tradicional PA Doherty (do inglês, Doherty Power Amplifier (DhPA)) de duas vias e suas variantes. O estudo _e complementado com o projeto e implementação de um PA excitador, em classe-AB, e de um DhPA de elevada potência, colocando-se em prática a teoria e técnicas de projeto estudadas ao longo deste trabalho, aliadas aos desafios da implementação com dispositivos reais de elevada potência.