Signal Detection and Estimation for MIMO radar and Network Time Synchronization

The theory of signal detection and estimation concerns the recovery of useful information from signals corrupted by random perturbations. This dissertation discusses the application of signal detection and estimation principles to two problems of significant practical interest: MIMO (multiple-input multiple output) radar, and time synchronization over packet switched networks. Under the first topic, we study the extension of several conventional radar analysis techniques to recently developed MIMO radars. Under the second topic, we develop new estimation techniques to improve the performance of widely used packet-based time synchronization algorithms. The ambiguity function is a popular mathematical tool for designing and optimizing the performance of radar detectors. Motivated by Neyman-Pearson testing principles, an alternative definition of the ambiguity function is proposed under the first topic. This definition directly associates with each pair of true and assumed target parameters the probability that the radar will declare a target present. We demonstrate that the new definition is better suited for the analysis of MIMO radars that perform non-coherent processing, while being equivalent to the original ambiguity function when applied to conventional radars. Based on the nature of antenna placements, transmit waveforms and the observed clutter and noise, several types of MIMO radar detectors have been individually studied in literature. A second investigation into MIMO radar presents a general method to model and analyze the detection performance of such systems. We develop closed-form expressions for a Neyman-Pearson optimum detector that is valid for a wide class of radars. Further, general closed-form expressions for the detector SNR, another tool used to quantify radar performance, are derived. Theoretical and numerical results demonstrating the value of the proposed techniques to optimize and predict the performance of arbitrary radar configurations are presented.There has been renewed recent interest in the application of packet-based time synchronization algorithms such as the IEEE 1588 Precision Time Protocol (PTP), to meet challenges posed by next-generation mobile telecommunication networks. In packet based time synchronization protocols, clock phase offsets are determined via two-way message exchanges between a master and a slave. Since the end-to-end delays in packet networks are inherently stochastic in nature, the recovery of phase offsets from message exchanges must be treated as a statistical estimation problem. While many simple intuitively motivated estimators for this problem exist in the literature, in the second part of this dissertation we use estimation theoretic principles to develop new estimators that offer significant performance benefits. To this end, we first describe new lower bounds on the error variance of phase offset estimation schemes. These bounds are obtained by re-deriving two Bayesian estimation bounds, namely the Ziv-Zakai and Weiss-Weinstien bounds, for use under a non-Bayesian formulation. Next, we describe new minimax estimators for the problem of phase offset estimation, that are optimum in terms of minimizing the maximum mean squared error over all possible values of the unknown parameters.Minimax estimators that utilize information from past timestamps to improve accuracy are also introduced. These minimax estimators provide fundamental limits on the performance of phase offset estimation schemes.Finally, a restricted class of estimators referred to as L-estimators are considered, that are linear functions of order statistics. The problem of designing optimum L-estimators is studied under several hitherto unconsidered criteria of optimality. We address the case where the queuing delay distributions are fully known, as well as the case where network model uncertainty exists.Optimum L-estimators that utilize information from past observation windows to improve performance are also described.Simulation results indicate that significant performance gains over conventional estimators can be obtained via the proposed optimum processing techniques

Towards secure & robust PNT for automated systems

This dissertation makes four contributions in support of secure and robust position, navigation, and timing (PNT) for automated systems. The first two relate to PNT security while the latter two address robust positioning for automated ground vehicles. The first contribution is a fundamental theory for provably-secure clock synchronization between two agents in a distributed automated system. All one-way synchronization protocols, such as those based on the Global Positioning System (GPS) and other Global Navigation Satellite Systems (GNSS), are shown to be vulnerable to man-in-the-middle delay attacks. This contribution is the first to identify the necessary and sufficient conditions for provably secure clock synchronization. The second contribution, also related to PNT security, is a three-year study of the world-wide GPS interference landscape based on data from a dual-frequency GNSS receiver operating continuously on the International Space Station (ISS). This work is the first publicly-reported space-based survey of GNSS interference, and unveils previously-unreported GNSS interference activity. The third contribution is a novel ground vehicle positioning technique that is robust to GNSS signal blockage, poor lighting conditions, and adverse weather events such as heavy rain and dense fog. The technique relies on sensors that are commonly available on automated vehicles and are insensitive to lighting and inclement weather: automotive radar, low-cost inertial measurement units (IMUs), and GNSS. Remarkably, it is shown that, given a prior radar map, the proposed technique operating on data from off-the-shelf all-weather automotive sensors can maintain sub-50-cm horizontal position accuracy during 60 min of GNSS-denied driving in downtown Austin, TX. This dissertation’s final contribution is an analysis and demonstration of the feasibility of crowd-sourced digital mapping for automated vehicles. Localization techniques, such as the one described in the previous contribution, rely on such digital maps for accuracy and robustness. A key enabler for large-scale up-to-date maps is enlisting the help of the very consumer vehicles that need the map to build and update it. A method for fusing multi-session vision data into a unified digital map is developed. The asymptotic limit of such a map’s globally-referenced position accuracy is explored for the case in which the mapping agents rely on low-cost GNSS receivers performing standard code-phase-based navigation. Experimental validation along a semi-urban route shows that low-cost consumer vehicles incrementally tighten the accuracy of the jointly-optimized digital map over time enough to support sub-lane-level positioning in a global frame of reference.Electrical and Computer Engineerin

Secure GPS clock synchronization in smart grids

Tese de mestrado, Segurança Informática, Universidade de Lisboa, Faculdade de Ciências, 2015As smart grids resultaram da integração da rede elétrica atual no mundo digital. Isso traz várias vantagens às redes elétricas, como uma instalação, configuração e manutenção mais simples e eficiente, mas também a fácil integração na rede de novas tecnologias. Enquanto as redes elétricas continuam a crescer em dimensão e complexidade, elas tornam-se mais importantes para a sociedade e subsequentemente mais sujeitas a ataques distintos. Alguns dos objetivos mais importantes da smart grid são: acomodar uma grande variedade de tecnologias de produção de eletricidade como a eólica, solar e geotérmica; ser resiliente a ataques físicos e ciber-ataques; ter mecanismos de deteção, análise e resposta automática a incidentes; dar mais poder ao consumidor final sobre como e quando a energia pode ser comprada ou consumida. Para implementar actividades relacionadas com a monitorização do estado da smart grid, vários componentes especializados são geograficamente distribuídos pela rede. Um dos dispositivos críticos é o Phase Measurement Unit (Unidade de Medição de Fase) (PMU). Este dispositivo é usado para estimar o estado da smart grid num determinado momento, recolhendo várias métricas sobre a qualidade do sinal elétrico. Para se conseguir criar uma imagem geral da rede inteira, todos estes dispositivos necessitam de ser sincronizados no tempo, assegurando assim que as medições são efetuadas aproximadamente no mesmo instante. A sincronização do tempo desempenha um papel crucial na estabilidade e no funcionamento correto de todos os componentes da smart grid. Dada a importância da sincronização de tempo, e a falta de qualquer tipo de proteção nas soluções atuais, este sistema torna-se num alvo potencial para atacantes. Em conformidade com os standards, a precisão dos relógios dos PMU’s devem ter um erro máximo na ordem dos 30 µs. Isso garante que a informação recolhida sobre o estado da smart grid é válida. Hoje em dia este requisito é satisfeito usando equipamentos GPS em cada sítio onde se encontra um PMU. Quando o GPS foi concebido, não se pensou que podia vir a ter o sucesso e o impacto atual e, portanto, assegurar a sua segurança não foi um ponto importante. Ao longo do tempo passou a ser usado em infraestruturas críticas, o que introduz eventuais problemas graves de segurança. As smart grids são uma destas estruturas críticas onde o GPS está a ser usado sem qualquer tipo de proteção. Atualmente existe também uma versão segura do GPS que é empregue pelas forças militares. Os dispositivos que conseguem decifrar este sinal só estão disponíveis ao exército. Por além disso, todos os detalhes sobre o funcionamento do algoritmo de cifra são mantidos em segredo. Ao longo dos anos foram desenvolvidos vários tipos de ataques ao GPS. O mais básico é o Blocking que consiste simplesmente em impedir a comunicação entre a antena do recetor e o sinal GPS. Isso pode ser conseguido de uma maneira tão simples como tapar a antena com um bocado de metal. Um ataque que tenta também quebrar a ligação com o satélite é o Jamming. A ideia deste ataque é introduzir ruído suficiente para que o recetor não consiga distinguir o sinal original. Estes dois tipos de ataques só conseguem perturbar o funcionamento do recetor GPS. Um tipo de ataque mais potente é o Spoofing. Este ataque consegue modificar o sinal original vindo do satélite de forma a enganar o recetor. Assim é possível fazer com que o recetor GPS mostre uma posição¸ ou tempo incorretos. Nesta dissertação também foi analisada uma evolução deste ataque que tem como alvo a alteração ilegítima dos dados contidos no sinal. Isso pode fazer como que o recetor falhe ou deixe de poder ser usado. Os algoritmos de sincronização de relógios existentes hoje em dia, nomeadamente o Network Time Protocol (NTP) e o Precision Time Protocol (PTP), não são suficientemente robustos, em termos de segurança ou precisão, para serem utilizados na smart grid. O NTP foi concebido para a sincronização de relógios em redes de grande escala mas não consegue fornecer a precisão necessária para os requisitos da smart grid. Por outro lado temos o PTP que consegue atingir uma precisão na ordem dos nanosegundos em certas condições, mas é muito sensível a atrasos e oscilações na rede. Isso faz com que o PTP só consiga garantir uma precisão de tempo na ordem dos nanosegundos em redes de pequena escala. A smart grid usa uma rede de alta velocidade com relativamente pouco tráfego, o que torna o PTP uma possível solução para algumas partes dessa rede. Em termos de segurançaa, o PTP não está preparado para ser utilizado num ambiente tão crítico como a smart grid, sendo suscetível a ataques. O foco desta investigação é encontrar um algoritmo resiliente a faltas, capaz de satisfazer os requisitos de sincronização de tempo necessários para o correto funcionamento da smart grid. Foi desenvolvida uma solução baseada no PTP, que consegue cumprir os requisitos de precisão temporal na smart grid e também consegue mitigar todos os tipos de ataques ao GPS que foram identificados. Para além disso, a solução também permite reduzir o número de recetores de GPS necessários para o funcionamento correto da smart grid.Smart grids resulted from the integration of computer technologies into the current power grid. This brings several advantages, allowing for a faster and more efficient deployment, configuration and maintenance, as well as easy integration of new energy sources (e.g., wind and solar). As smart grids continue to grow in size and complexity, they become subject to failures and attacks from different sources. Time synchronization plays a crucial role in the stability and correct functioning of many grid components. Considering how sensitive time synchronization is, the tight restrictions imposed for correct operation and the lack of any kind of protection, makes this service a potential prime target for attackers. Today most of the time synchronization requirements are met using relatively expensive GPS hardware placed in some locations of the smart grid. When GPS was first devised, nobody could have predicted the success and the impact that it would have and therefore, security was never an important concern. Through the years, it slowly gained entrance into more critical systems, where it was never intended to be used, which can lead to serious security problems. The smart grid is just one of these critical systems where GPS is being employed without any kind of protection. The focus of this research is trying to solve this problem, by proposing a more secure and robust clock synchronization algorithm. A solution based on the Precision Time Protocol (PTP) was developed that manages to fulfill the time synchronization requirements of the smart grid and is also capable of mitigating all types of identified GPS attacks. As an added benefit, the solution may also reduce the number of GPS receivers necessary for the correct operation of the smart grid, contributing to decrease costs

Distributed synchronization algorithms for wireless sensor networks

The ability to distribute time and frequency among a large population of interacting agents is of interest for diverse disciplines, inasmuch as it enables to carry out complex cooperative tasks. In a wireless sensor network (WSN), time/frequency synchronization allows the implementation of distributed signal processing and coding techniques, and the realization of coordinated access to the shared wireless medium. Large multi-hop WSN\u27s constitute a new regime for network synchronization, as they call for the development of scalable, fully distributed synchronization algorithms. While most of previous research focused on synchronization at the application layer, this thesis considers synchronization at the lowest layers of the communication protocol stack of a WSN, namely the physical and the medium access control (MAC) layer. At the physical layer, the focus is on the compensation of carrier frequency offsets (CFO), while time synchronization is studied for application at the MAC layer. In both cases, the problem of realizing network-wide synchronization is approached by employing distributed clock control algorithms based on the classical concept of coupled phase and frequency locked loops (PLL and FLL). The analysis takes into account communication, signaling and energy consumption constraints arising in the novel context of multi-hop WSN\u27s. In particular, the robustness of the algorithms is checked against packet collision events, infrequent sync updates, and errors introduced by different noise sources, such as transmission delays and clock frequency instabilities. By observing that WSN\u27s allow for greater flexibility in the design of the synchronization network architecture, this work examines also the relative merits of both peer-to-peer (mutually coupled - MC) and hierarchical (master-slave - MS) architectures. With both MC and MS architectures, synchronization accuracy degrades smoothly with the network size, provided that loop parameters are conveniently chosen. In particular, MS topologies guarantee faster synchronization, but they are hindered by higher noise accumulation, while MC topologies allow for an almost uniform error distribution at the price of much slower convergence. For all the considered cases, synchronization algorithms based on adaptive PLL and FLL designs are shown to provide robust and scalable network-wide time and frequency distribution in a WSN