Héctor Poveda

Frequency synchronization and channel equalization for an OFDM-IDMA uplink system

This paper deals with a system combining orthogonal frequency division multiplexing (OFDM) and interleave-division multiple access (IDMA). OFDM makes the system robust against intersymbol interference, whereas IDMA combats the multiple access interference. Nevertheless, two problems have to be solved: 1/ carrier frequency offset (CFO) must be estimated/corrected to guarantee the orthogonality between subcarriers. 2/ the conventional IDMA receiver requires a priori knowledge of the channel. Therefore, the CFO and the channel must be estimated. Our contribution is twofold. Firstly, we suggest a sigma point Kalman filter to solve the estimation issue. In addition, we propose a new scheme where a CFO correction is no longer necessary, unlike common OFDM approaches. We show that without a CFO correction, the transmitted bits can be recovered by modifying the conventional IDMA receiver. When considering an OFDM-IDMA network over Rayleigh fading channels, simulation results show the efficiency of the proposed algorithm.

A multi-(users, carriers, antennas) IDMA receiver: A way to address the bandwidth scarcity issue

This paper deals with a system combining orthogonal frequency division multiplexing (OFDM), interleave-division multiple access (IDMA) and space time block coding (STBC). Less investigated in the literature than multicarrier code division multiple access or OFDMA, OFDM-IDMA can be viewed as a promising technique to solve the problem of bandwidth scarcity. With a specific frame organization to estimate the carrier frequency offsets (CFO) and the channels, we suggest a new receiver architecture. Indeed, based on a modification of the standard IDMA receiver where a CFO correction is no longer necessary unlike common OFDM approaches, we show that the transmitted bits can be still recovered. When considering a STBC-OFDM-IDMA uplink system over different Rayleigh channels, simulation results show the efficiency of the proposed algorithm. These results confirm the utility of this technique as a mid-term solution to solve the constant increase of data rate and user density per cell in cellular systems.

A Non-Pilot Aided Iterative Carrier Frequency Offset Estimator Using Optimal Filtering for an Interleaved OFDMA Uplink System

In an uplink transmission of a coded orthogonal frequency division multiple access (C-OFDMA) system, channel estimation, time and frequency synchronization has to be addressed. For this purpose a control data, i.e. a known training sequence called “preamble” and pilot sub-carriers are used. As an alternative to the classic scheme and in order to maximize the data rate, we propose a non-pilot aided estimator based on an iterative architecture that does not require pilot sub-carriers. Our approach combines 1/ the so-called minimum mean square error successive detector to estimate the signal sent by each user 2/ a recursive method estimating the CFOs. Various algorithms such as the extended Kalman filter, the sigma-point Kalman filters and the extended H∞ filter are tested and their performances are compared in terms of convergence speed and estimation accuracy. When considering an interleaved OFDMA uplink system over a Rayleigh fading channel, simulation results clearly show the efficiency of the proposed algorithm in terms of CFO estimation and bit error rate performances.

Robust frequency synchronization for an OFDMA uplink system disturbed by a Cognitive Radio system interference

In Cognitive Radio (CR) systems, spectrum sensing plays a key role to determine the free frequency bands. However, when the primary-user (PU) signal spectrum exhibits localized fading, PU detection cannot be guaranteed. In addition, as the CR may use the PU faded frequencies, the PU spectrum can be disturbed by a narrow-band interference (NBI) and synchronization algorithms used for the PU carrier frequency offset (CFO) estimation suffer degradations. In this paper, we propose a new scheme that jointly allows the CR-NBI to be detected and the PU-CFOs and the channels to be estimated in an orthogonal frequency division multiple access (OFDMA) system. It combines a sigma point Kalman filter and a test aiming at detecting a variation of the measurement-noise covariance matrix. Simulation results confirm that the proposed algorithm can accurately detect the CR-NBI and estimate the PU-CFOs.

Programa de doble titulación en ingeniería entre la UTP de Panamá y la ENSEIRB-MATMECA de Francia: Primer programa de doble titulación entre Centroamérica y Francia

Resumen—En este trabajo presentamos diversos aspectos de la implementación del primer acuerdo de doble titulación en ingenierı́a entre una institución de educación superior de Francia y una en Centroamérica. Entre los temas abordados se destacan la definición de doble titulación, las diferencias de los sistemas educativos de ambos paı́ses y en general de Europa y América Latina. Además de mostrar los beneficios y el impacto esperado de esta iniciativa, se presentan los retos administrativos y legales que se han presentado durante la implementación de este programa.

Estimacion de Frecuencia en Redes Eléctricas Inteligentes en Presencia de Desbalance de Voltaje basado en Modelos Multiples Interactivos

En este trabajo proponemos la estimacion de frecuencia en sistemas de redes inteligentes en presencia de desbalances de voltaje. Para esta estimacion proponemos la utilizacion de Modelos Multiples Interactivos (IMM) utilizando dos filtros de Kalman extendidos (EKF) operando en paralelo. Los filtros(EKF) son utilizados para modelar dos modos de operacion que hemos considerado para representar el sistema. Uno de los filtros de EKF permite caracterizar un modo de operacion con variaciones minimas o nulas de frecuencia. El segundo filtro EKF caracteriza un modo de operacion en que se dan una cambios de frecuencia mas significativas. Los resultados demuestran que el algoritmo permite detectar el cambio significativo de frecuencia y combinar las salidas de los filtros EKF de manera que tengamos un tiempo de respuesta adecuado a este cambio, es decir, dando mayor ponderacion el filtro correspondiente al segundo modo. En este trabajo comparamos los resultados del enfoque propuesto contrabajos previos utilizando un solo filtro EKF para demostrar sus ventajas en terminos de error de estimacion y tiempo de respuesta en las cambios de frecuencia

A Planning and Optimization Framework for Ultra Dense Cellular Deployments

To accommodate the ever-expanding wireless data traffic volumes, mobile network operators are complementing their macrocellular networks by deploying low-power base stations (or small cells) to offload traffic from congested macrocells and to reuse spectrum. To that end, Ultra Dense Network (UDN) deployments provide means to aggressively reuse spectrum, thus providing significant enhancements in terms of system capacity. However, these deployments entail several challenges, including the increased complexity in network planning and optimization. In this paper, we propose a versatile optimization framework for planning UDN deployments. The planning and optimization framework is underpinned by metrics that consider scalability in terms of number of users, cost of densification, and fairness. The proposed methodology is evaluated using a real-world UDN planning case. The numerical results expose a number of interesting insights, including the impact of different bandwidth allocation strategies and spatial service demand distribution on the performance of various network topologies. Specifically, we provide a performance comparison of the optimized UDN topologies versus random (unplanned), regular grid, and heuristically derived UDN topologies. This comparison further underlines the need for flexible network planning and optimization frameworks as different operator performance metrics of interest may require different radio access networks configurations.

Sistemas de Radio Inteligencia de Tipo Overlay: Análisis de Técnicas de Mitigación de Interferencia

Resumen– La radio inteligencia es un novedoso enfoque en los sistemas de comunicacion inalambrica que surge para solucionar la saturacion actual del espectro. Existen tres esquemas de acceso para la radio inteligencia: interweave, underlay y overlay. El esquema overlay puede permitir mayores velocidades de transmision y ancho de banda mediante la aplicacion de tecnicas de mitigacion o manejo de interferencias. A traves de estas tecnicas se garantiza la calidad y eficiencia en las transmisiones. Este trabajo presenta un estudio y un analisis de estas tecnicas que podran ser implementadas en los futuros sistemas de comunicaciones inalambricos. Se analizan las caracteristicas de algunas de las tecnicas mas comunes de mitigacion de interferencia en los esquemas overlay para sistemas de radio inteligencia. Los resultados de simulacion que se presentan en terminos de error de bit para sistemas overlay que utilizan tecnicas de precodificacion y filtrado espacial muestran el buen desempeno de estas tecnicas en los sistemas de comunicacion inalambricos.

Way to design an orthogonal frequency-division multiple access-base station receiver disturbed by a narrowband interfering cognitive radio signal

This study deals with an interweave cognitive-radio (CR) system, where an uplink orthogonal frequency-division multiple access system is considered for the primary users (PUs). Our purpose is to estimate the PU carrier frequency offsets (PU-CFOs) as well as the channels to estimate the transmitted symbols. However, in wideband wireless communications, the PU received-signal spectrum usually exhibits a localised fading because of the multipath propagation channel. When the PU faded frequencies are used by the CR system, the PU system is contaminated by a CR narrowband interference (CR-NBI). In that case, if a Kalman filter (KF)-based approach is used for the estimations of the CFOs and the channels, the state-space representation does not necessarily take into account the CR-NBI; this hence has a negative impact on the algorithm performance. Therefore the authors propose to make this solution more robust to the CR-NBI by detecting when it appears and disappears to avoid using the data disturbed by the CR-NBI. The authors’ contribution is to assume that the CR-NBI is because of the transmission of contiguous blocks and to propose various criteria based on the covariance matrix of the KF innovation.

Tensor based feature detection for recognition of poorly illuminated objects

In this work we present an extension of the SIFT algorithm to color images. In the extrema detection stage, an energy level descriptor based on the color tensor of the image is computed and used to locate keypoints candidates. Then, in the description stage, the color gradient magnitude and orientation of the samples around the keypoint are used to compute an orientation histogram to create the keypoint descriptor. A comparative study is carried out between the proposed algorithm and the classic SIFT and the C-SIFT algorithms in several illumination settings. The proposed algorithm presents a better performance in terms of accuracy when objects are poorly illuminated.