Spectrally efficient FDM communication signals and transceivers: design, mathematical modelling and system optimization

This thesis addresses theoretical, mathematical modelling and design issues of Spectrally Efficient FDM (SEFDM) systems. SEFDM systems propose bandwidth savings when compared to Orthogonal FDM (OFDM) systems by multiplexing multiple non-orthogonal overlapping carriers. Nevertheless, the deliberate collapse of orthogonality poses significant challenges on the SEFDM system in terms of performance and complexity, both issues are addressed in this work. This thesis first investigates the mathematical properties of the SEFDM system and reveals the links between the system conditioning and its main parameters through closed form formulas derived for the Intercarrier Interference (ICI) and the system generating matrices. A rigorous and efficient mathematical framework, to represent non-orthogonal signals using Inverse Discrete Fourier Transform (IDFT) blocks, is proposed. This is subsequently used to design simple SEFDM transmitters and to realize a new Matched Filter (MF) based demodulator using the Discrete Fourier Transforms (DFT), thereby substantially simplifying the transmitter and demodulator design and localizing complexity at detection stage with no premium at performance. Operation is confirmed through the derivation and numerical verification of optimal detectors in the form of Maximum Likelihood (ML) and Sphere Decoder (SD). Moreover, two new linear detectors that address the ill conditioning of the system are proposed: the first based on the Truncated Singular Value Decomposition (TSVD) and the second accounts for selected ICI terms and termed Selective Equalization (SelE). Numerical investigations show that both detectors substantially outperform existing linear detection techniques. Furthermore, the use of the Fixed Complexity Sphere Decoder (FSD) is proposed to further improve performance and avoid the variable complexity of the SD. Ultimately, a newly designed combined FSD-TSVD detector is proposed and shown to provide near optimal error performance for bandwidth savings of 20% with reduced and fixed complexity. The thesis also addresses some practical considerations of the SEFDM systems. In particular, mathematical and numerical investigations have shown that the SEFDM signal is prone to high Peak to Average Power Ratio (PAPR) that can lead to significant performance degradations. Investigations of PAPR control lead to the proposal of a new technique, termed SLiding Window (SLW), utilizing the SEFDM signal structure which shows superior efficacy in PAPR control over conventional techniques with lower complexity. The thesis also addresses the performance of the SEFDM system in multipath fading channels confirming favourable performance and practicability of implementation. In particular, a new Partial Channel Estimator (PCE) that provides better estimation accuracy is proposed. Furthermore, several low complexity linear and iterative joint channel equalizers and symbol detectors are investigated in fading channels conditions with the FSD-TSVD joint equalization and detection with PCE obtained channel estimate facilitating near optimum error performance, close to that of OFDM for bandwidth savings of 25%. Finally, investigations of the precoding of the SEFDM signal demonstrate a potential for complexity reduction and performance improvement. Overall, this thesis provides the theoretical basis from which practical designs are derived to pave the way to the first practical realization of SEFDM systems

OFDM based air interfaces for future mobile satellite systems

This thesis considers the performance of OFDM in a non-linear satellite channel and mechanisms for overcoming the degradations resulting from the high PAPR in the OFDM signal in the specific satellite architecture. It was motivated by new S-DMB applications but its results are applicable to any OFDM system via satellites. Despite many advantages of OFDM, higher PAPR is a major drawback. OFDM signals are therefore very sensitive to non-linear distortion introduced by the power amplifiers and thus, significantly reduce the power efficiency of the system, which is already crucial to satellite system economics. Simple power amplifier back-off to cope with high OFDM PAPR is not possible. Two transmitter based techniques have been considered: PAPR reduction and amplifier linearization.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Next generation passive optical networks based on orthogonal frequency division multiplexing techniques

In recent decades, the industry of communications has acquired huge significance, and nowadays constitutes an essential tool for the society information. Thus, the exponential growth in demand of broadband services and the increasing amount of information to be transmitted have spurred the evolution of the access network infrastructure to effectively meet the user needs in an effective way in terms of costs of both installation and maintenance. Passive optical networks (PON) are currently considered the most efficient and least costly alternative to deploy fiber to the home environment. In order to allow many users simultaneously coexist PONs based on time multiplexing (TDMA) have been developed. Looking ahead, however, it is expected that these techniques do not meet the requirements on access networks. In consequence, other multiple access techniques such as Wavelength Division Multiplexing Access (WDMA) or Orthogonal Frequency Division Multiplexing Access (OFDMA) are currently under study and development for use in the next generation of PONs. Particularly, in recent years OFDM has stood out among the scientific community to be considered a solution with great potential on future implementation of PONs. This is especially true due to the capacity of OFDM to work with multilevel modulations, its high tolerance to chromatic dispersion, and its high flexibility and granularity in terms of bandwidth management. Given the above, the aim of this Thesis is to study deeply the advantages and challenges of implementing the standard OFDM as an access network solution; likewise, it offers solutions to improve its performance. In order to evaluate the main structures and strategies for OFDM-based PON, a comparative analysis of all of them is performed firstly, highlighting their sensitivity levels, maximum range and number of users. A key aspect for network providers is the cost of operation, deployment and maintenance of networks. As a low-cost solution, this Thesis proposes a network model called Statistical-OFDMA-PON based on intensity modulation and direct detection. In addition, dynamic bandwidth management strategies are applied into this model getting an improvement in the power balance which in turn, allows to increase the maximum range and the scalability in number of users. One of the main OFDM problems is the Peak-to-Average Power Ratio (PAPR) which increases with the number of carriers. This thesis proposes a new algorithm based on folding the signal and transmitting auxiliary information in order to compensate the PAPR effect and thus increase the sensitivity of the optical system. On the other hand, OFDMA requires a large number of operations in the digital domain resulting in a high computational effort, which in turn results in an increased cost. For this reason, this Thesis presents a study on the optimization of the required resolution in the Digital-to-Analog / Analog-to-Digital Converters (DAC/ADCs) maintaining the transmission quality. The optimization of the computation time may make the OFDMA-based optical network more attractive for future PONs. Finally, another problem concerning the OFDM optical networks is their sensitivity to Phase Noise (PN). In this regard, this Thesis presents a study of the effect of the laser linewidth and its dependence on signal bandwidth. A mitigation technique based on pilot tones is implemented and the limiting values for the laser linewidth are found to be within the reach of present low-cost light sources.En estas últimas décadas, la industria de las comunicaciones ha adquirido gran importancia y hoy en día, constituye una herramienta imprescindible para el funcionamiento en la sociedad de la información. Así pues, el crecimiento exponencial en la demanda de servicios de banda ancha y la carga de información cada vez mayor que se necesita transmitir ha estimulado la evolución de las infraestructuras del tramo de acceso a la red para poder satisfacer las necesidades del usuario de forma efectiva en términos de costes de instalación y de mantenimiento. Las redes ópticas pasivas (Passive Optical Networks, PON) son actualmente consideradas la alternativa más eficiente y de menor coste para desplegar fibra hasta los hogares. Con el fin de permitir que muchos usuarios coexistan simultáneamente se han desarrollado PONs basadas en multiplexación en tiempo (Time Division Multiplexing Access, TDMA). De cara al futuro, sin embargo, se prevé que estas técnicas no permitan cubrir las exigencias sobre las redes de acceso. En consecuencia, otras técnicas de acceso múltiple al medio como el acceso múltiple por división de longitud de onda (Wavelength Division Multiplexing Access, WDMA) o el acceso múltiple por división de frecuencia ortogonal (Orthogonal Frequency Division Multiplexing Access, OFDMA) se encuentran actualmente en proceso de estudio y desarrollo para su uso en la futura generación de PONs. En concreto, en los últimos años OFDM se ha destacado entre la comunidad científica al considerarse una solución con gran potencial para su futura implantación en redes de acceso pasivas. Esto es especialmente cierto debido a la capacidad que el OFDM para trabajar con modulaciones multinivel, así como su alta tolerancia a la dispersión cromática y a la gran flexibilidad y granularidad que posibilita en términos de gestión del ancho de banda. Por todo lo anterior, el objetivo de esta Tesis es estudiar con profundidad las ventajas y los retos de aplicar el estándar OFDM como solución de red de acceso; del mismo modo, ofrece soluciones para mejorar su rendimiento. Con el objetivo de evaluar las principales estructuras basadas en OFDM-PON, en primer lugar se realiza un análisis comparativo de todas ellas destacando sus niveles de sensibilidad, máximo alcance y número de usuarios. Un aspecto fundamental para los proveedores de red es el coste de operación, despliegue y mantenimiento de las redes. Como solución de bajo coste, esta Tesis propone un modelo de red llamado Statistical-OFDMA-PON que se basa en modulación de intensidad y detección directa. Además, este modelo se completa con estrategias de gestión dinámica del ancho de banda de los usuarios que conforman la estructura de red propuesta consiguiendo una mejora en el balance de potencias que permite aumentar distancia y número de usuarios. Uno de los principales problemas del OFDM es el alto nivel de la relación de potencia de pico a potencia media (Peak-to-Average Power Ratio, PAPR) creciente con el número de portadoras. Esta Tesis propone un nuevo algoritmo basado en el pliegue de la señal y la transmisión de información auxiliar para compensar el efecto del PAPR aumentando así la sensibilidad del sistema óptico. Por otro lado, OFDMA requiere un número elevado de operaciones en el dominio digital resultando en un alto esfuerzo computacional que a su vez se traduce en un aumento del coste. Por esta razón, esta Tesis presenta un estudio sobre la optimización de la resolución requerida en los conversores analógicodigital (Digital-to-Analog/Analog-to-Digital Converters, DAC/ADCs) manteniendo la calidad de transmisión. La optimización del tiempo de cómputo requerido puede dotar de un mayor atractivo la solución de red óptica basada en OFDMA. Finalmente, otro de los problemas que presentan las redes ópticas OFDM es su sensibilidad frente al ruido de fase (Phase Noise, PN). En este aspecto, esta Tesis presenta un estudio del efecto del ancho de línea del láser y su dependencia con el ancho de banda de la señal. Técnicas de mitigación basadas en tonos piloto han sido implementadas y se han encontrado los valores limitantes del ancho de línea dentro del alcance de los láseres de bajo coste

D13.1 Fundamental issues on energy- and bandwidth-efficient communications and networking

Deliverable D13.1 del projecte europeu NEWCOM#The report presents the current status in the research area of energy- and bandwidth-efficient communications and networking and highlights the fundamental issues still open for further investigation. Furthermore, the report presents the Joint Research Activities (JRAs) which will be performed within WP1.3. For each activity there is the description, the identification of the adherence with the identified fundamental open issues, a presentation of the initial results, and a roadmap for the planned joint research work in each topic.Preprin