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Random vibration is present in many fields of engineering, although one where it has particular relevance is the industry of ground transportation. The interaction between the wheels and the road induces vibration that transmits to all the components in the vehicle, as well as to the passengers, producing mechanical damage, noise and discomfort. It is hence important to address the effect of vibration during the design and validation phases of development of the vehicle. For that purpose, a method for the synthesis of signals that can be used for numerical simulation or physical testing of the system being designed is needed. In this thesis, an algorithm for the synthesis of random vibration signals is developed, that takes into account the multiaxial and non-Gaussian nature of the process allowing individual adjustment of the kurtosis of the different degrees-of-freedom involved. The proposed algorithm avoids distortion of the spectrum when the kurtosis is being adjusted hence allowing for independent control of the spectral and kurtosis properties of the process. Theoretical expressions relating the control paramet-ers to the kurtosis and the spectrum of the process are provided. An additional benefit of the proposed algorithm is the possibility to modify the higher-order moments beyond the kurtosis.

The incessant demand for enhanced communication services has rendered current RF spectrum allocation policies obsolete. The astronomic price of licensed channels stands in sharp contradiction with their heavy underutilization and propels the adoption of novel spectrum management paradigms where spectral resources are assigned in a dynamic fashion, possibly respecting hierarchical relations among multiple classes of users. In those scenarios, it is expected that emerging communication systems will leverage spectrum awareness information to drastically improve spectrum usage eficiency. Spectrum sensing comprises a collection of signal processing procedures intended to characterize spectrum occupation along time, frequency and space based on the observations of the RF environment reported by one or more sensors. It is the purpose of the present thesis to contribute to this field by putting forward a number of statistical methods that capitalize on the special features of different communication scenarios to reliably obtain detailed occupancy information at low implementation costs. We first propose a family of techniques aimed to detect, relying on the noisy observations of a sensor with one or multiple antennas, the presence of constant magnitude and/or bandlimited transmissions in a frequency band of interest, as motivated by the new regulations of the Federal Communications Commission. Next, we address the problem of detecting constant-magnitude and Gaussian-distributed waveforms in time-varying channels, relevant in this context since spectrum sensing algorithms typically require long observation windows to meet the stringent performance requirements enforced by spectrum regulations in low-SNR conditions. We then apply sub-Nyquist acquisition techniques to characterize the occupancy state of a wide frequency band via inexpensive sensing architectures with minimal computational resources, where spectral prior information typically available in practice -- e.g. spectral masks, roll-off factors, etc. --, is exploited to enable compression. To minimize the sampling rate, we extensively analyze the general problem of recovering second-order statistical information of wide-sense stationary processes from compressed measurements. Our results in this direction are of application well beyond spectrum sensing contexts. Finally, we look at the problem of spectrum cartography, where the goal is to construct power spectrum maps characterizing the spectrum utilization not only along frequency and time, but also across space. We propose several methods capable of learning those maps based on the highly compressed observations reported by a collection of inexpensive sensors. A presente tese de doutoramento enmárcase no contexto de sistemas de sensado espectral con especial atención a esquemas de acceso dinámico ao espectro. Baixo este último termo englóbanse aqueles sistemas que pretenden incrementar a e ciencia de uso dos recursos espectrais existentes mediante unha flexibilización das políticas empregadas para a súa asignación. Nos esquemas empregados na actualidade, a autoridade reguladora do espectro electromagnético asigna bandas de frecuencia a grandes operadores, tipicamente mediante un sistema de poxas multimillonarias. Paradoxalmente, recentes campañas de medidas puxeron de manifesto o elevado grado de infrautilización que impera nestas bandas con licenza. Na vista destes resultados, varias comunidades de expertos suxiren considerar a opción de que usuarios sen licenza exploten estes baleiros espectrais para as súas actividades de comunicación a condición de que non degraden a calidade da comunicación do usuario con licenza cando este último accede ao canal. Por esta e outras razóns, tórnase necesario investigar mecanismos que permitan aos chamados usuarios secundarios coñecer, en cada momento, o estado do canal. O problema principal radica en empregar un conxunto de observacións do canal, proporcionadas por un sensor espectral, para dar resposta a unha ou máis preguntas do tipo: hai algún usuario primario a operar na banda b? se é así, con que potencia se recibe? Para proporcionar tal resposta, un riguroso conxunto de ferramentas estatísticas ponse en funcionamento para crear funcións dos sinais observados que devolven decisións que satisfacen certos requirimentos, tamén formulados en termos estatísticos. O noso abano inclúe metodoloxías importadas da teoría da detección, como o cociente xeneralizado de verosimilitudes ou a procura de tests invariantes, da teoría da estimación, como os estimadores de máxima verosimilitude, ou da teoría de aprendizaxe estatístico, como a máquina de vectores soporte.

Radio coginitiva es una nueva tecnología que posibilitará esquemas de acceso dinámico a bandas espectrales actualmente en explotación bajo licencia. Estos esquemas de acceso dinámico aprovechan los huecos espectrales existentes para su uso por parte de sistemas secundarios, y, al mismo tiempo, limitan la interferencia generada a los usuarios primarios. El desarrollo de un esquema de Radio cognitiva requiere nuevos algoritmos para la gestión de interferencia. Esta tesis aborda dos aspectos fundamentales de este problema: - En primer lugar, la monitorización espectral, que permite detectar y rastrear a los usuarios primarios. Se estudia el incremento de prestaciones derivado del uso de múltiples antenas, procesado simultáneo de mútlitples canales, así como el uso de la información disponible sobre la canalización y modulación de las transmisiones primarias. - En segundo lugar se propone y analiza un esquema particular para la gestión de interferencia en redes de radio cognitiva en las que se permite cierta interacción entre usuarios primarios y secundarios.

We develop a method for self-interference mitigation in wideband full-duplex multiple-input multiple-output regenerative relays, taking into account transmitter impairments and limited receiver dynamic range. The method combines a cancellation and suppression architecture by incorporating feedforward filters at both sides of the relay. The design criterion is to maximize the signal-to-interference-plus-noise ratio (SINR) at the relay input. On the transmit side, linear constraints are imposed in order to control the effect on the information signal at destination. Simulation results show the effectiveness of the proposed method both in terms of self-interference reduction and SINR improvement. Refereed/Peer-reviewed