Publication: Estimación de canal y desplazamiento de frecuencia en sistemas MIMO-OFDM con prefijo cíclico insuficiente
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2015-12
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2015-12-16
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Abstract
La presente Tesis aborda el análisis de la estimación de canal y desplazamiento de frecuencia en sistemas MIMO-OFDM (Multiple-Input Multiple-Output Orthogonal Frequency Division Multiplexing) en condiciones de prefijo cíclico insuficiente. La motivación fundamental es estudiar uno de los sistemas más utilizados actualmente, como es MIMO-OFDM, pero con la intención de mejorar una de las características intrínsecas que le proporciona mayores ventajas, como es el uso del prefijo cíclico. Dicho prefijo cíclico permite mantener la ortogonalidad entre subportadoras y evita la aparición de interferencias entre símbolos y entre portadoras, facilitando también la ecualización en entornos multitrayecto, siempre que su longitud sea mayor que la del canal. Sin embargo, produce un decremento de la capacidad del sistema, al no llevar información por ser una réplica de las últimas muestras de cada símbolo OFDM, y disminuye la eficiencia de la estimación de parámetros.
La Tesis se centra, por tanto, en analizar un sistema MIMO-OFDM con prefijo cíclico insuficiente, menor a la longitud del canal, que puede ser debido a una supresión intencionada para incrementar la capacidad, a canales largos altamente dispersivos o para eliminar patrones repetitivos de la señal que facilitan la interceptabilidad de la señal OFDM.
Por consiguiente, para abordar el objetivo principal de esta Tesis es necesario analizar aquellos inconvenientes que surgen de la existencia de un prefijo cíclico insuficiente.
Por un lado, se analiza el impacto sobre las comunicaciones MIMO-OFDM en términos de interferencias en la parte de datos de la estructura de trama así como sobre la estimación de canal y desplazamiento de frecuencia. También se evalúan las secuencias utilizadas como preámbulo para dichas estimaciones tanto desde el punto de vista de sus características y utilización como desde el punto de vista de rango de adquisición del desplazamiento de frecuencia. Se demuestra claramente que los estimadores propuestos convergen a sus límites teóricos y que las secuencias seleccionadas tienen el mayor rango de adquisición posible.
Por otro lado, una vez analizado el efecto sobre la parte de datos de la trama, también se analiza el impacto sobre el propio preámbulo que se utiliza para labores de estimación y se presentan los resultados correspondientes a la tasa de error, demostrando que el rendimiento obtenido se equipara al de aquellos sistemas con prefijo cíclico suficiente. Debido a que las estrategias propuestas incrementan la complejidad del sistema, se analiza con detalle el coste computacional de la implementación del esquema de receptor propuesto concluyéndose que el coste que supone es muy inferior comparado con el ahorro que propicia en términos de capacidad y energía.
Finalmente, se propone la implementación de algunas de las estrategias definidas anteriormente en un sistema real como es el enlace descendente del estándar LTE (Long Term Evolution). En primer lugar, se propone una estructura de trama y receptor aplicados a una aproximación simplificada del estándar. Posteriormente, se adaptan las estrategias propuestas a una versión más fidedigna de dicho estándar. Los resultados obtenidos en ambos casos ponen de manifiesto las notables mejoras que implica la utilización de los receptores propuestos dando lugar a rendimientos del sistema similares a los obtenidos en el caso de haber utilizado un prefijo cíclico suficiente.
This Thesis tackles the analysis of channel and frequency offset estimation in MIMOOFDM (Multiple-Input Multiple-Output Orthogonal Frequency Division Multiplexing) systems in conditions of insufficient cyclic prefix. The main motivation is to study one of the most currently used systems, as is MIMO-OFDM, but with the intention of improving one of its intrinsic characteristics that provides more advantages, as the use of the cyclic prefix is. This cyclic prefix allows to keep the orthogonality between subcarriers and avoid the appearance of inter symbol and inter carrier interferences, making easier the equalization in multi-path environments provided that its length is longer than the channel length. However, it produces a decrease in the system capacity, since it does not bear any information because it is a replica of some last samples in every OFDM symbol, and decrease the efficiency of parameters estimation. Therefore, this Thesis is focused on analyzing a MIMO-OFDM system with insufficient cyclic prefix, shorter than the channel length, which can arise due to an intentional suppression to increase the system capacity, to highly dispersive long channels or to eliminate repetitive patterns in the OFDM signal that ease the signal interception. Consequently, to tackle the main objective of this Thesis it is necessary to analyze those inconveniences arisen from the existence of an insufficient cyclic prefix. On the one hand, the impact in MIMO-OFDM communications is analyzed in terms of interferences in the data part of the frame structure and in terms of channel and frequency offset estimation. In addition, those sequences used as preamble are assessed in relation with these estimations both concerning their characteristics and handling and considering which is the acquisition range of the frequency offset. It is clearly demonstrated that the proposed estimators converge to their theoretical limits and that the selected sequences have the highest possible acquisition range. On the other hand, once the effect over the data part of the frame has been analyzed, it is also evaluated the impact over the own preamble used for estimation tasks and the results corresponding to the error rate are presented, showing that the performance is similar to that one obtained in systems with insufficient cyclic prefix. Due to the fact that the proposed strategies increase the system complexity, the computational cost of the proposed receiver has been analyzed with detail to conclude that the implicit cost is much lower compared with the saving in terms of capacity or energy. Finally, the implementation of some of the previously defined schemes has been proposed for a real system, such as the down link in the LTE (Long Term Evolution) standard. In first place, it is proposed a frame structure and receiver for a simplified version of the standard. Later, these proposed strategies are adapted to a more similar version of the standard. The results in both cases prove the dramatic improvements of the utilization of these two proposed receivers giving rise to system performances comparable to those obtained in the case of having used a insufficient cyclic prefix.
This Thesis tackles the analysis of channel and frequency offset estimation in MIMOOFDM (Multiple-Input Multiple-Output Orthogonal Frequency Division Multiplexing) systems in conditions of insufficient cyclic prefix. The main motivation is to study one of the most currently used systems, as is MIMO-OFDM, but with the intention of improving one of its intrinsic characteristics that provides more advantages, as the use of the cyclic prefix is. This cyclic prefix allows to keep the orthogonality between subcarriers and avoid the appearance of inter symbol and inter carrier interferences, making easier the equalization in multi-path environments provided that its length is longer than the channel length. However, it produces a decrease in the system capacity, since it does not bear any information because it is a replica of some last samples in every OFDM symbol, and decrease the efficiency of parameters estimation. Therefore, this Thesis is focused on analyzing a MIMO-OFDM system with insufficient cyclic prefix, shorter than the channel length, which can arise due to an intentional suppression to increase the system capacity, to highly dispersive long channels or to eliminate repetitive patterns in the OFDM signal that ease the signal interception. Consequently, to tackle the main objective of this Thesis it is necessary to analyze those inconveniences arisen from the existence of an insufficient cyclic prefix. On the one hand, the impact in MIMO-OFDM communications is analyzed in terms of interferences in the data part of the frame structure and in terms of channel and frequency offset estimation. In addition, those sequences used as preamble are assessed in relation with these estimations both concerning their characteristics and handling and considering which is the acquisition range of the frequency offset. It is clearly demonstrated that the proposed estimators converge to their theoretical limits and that the selected sequences have the highest possible acquisition range. On the other hand, once the effect over the data part of the frame has been analyzed, it is also evaluated the impact over the own preamble used for estimation tasks and the results corresponding to the error rate are presented, showing that the performance is similar to that one obtained in systems with insufficient cyclic prefix. Due to the fact that the proposed strategies increase the system complexity, the computational cost of the proposed receiver has been analyzed with detail to conclude that the implicit cost is much lower compared with the saving in terms of capacity or energy. Finally, the implementation of some of the previously defined schemes has been proposed for a real system, such as the down link in the LTE (Long Term Evolution) standard. In first place, it is proposed a frame structure and receiver for a simplified version of the standard. Later, these proposed strategies are adapted to a more similar version of the standard. The results in both cases prove the dramatic improvements of the utilization of these two proposed receivers giving rise to system performances comparable to those obtained in the case of having used a insufficient cyclic prefix.
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Keywords
MIMO-OFDM, Multiple-Input Multiple-Output Orthogonal Frequency Division Multiplexing, Prefijo cíclico insuficiente, Estimación de canal