A low-complexity iterative channel estimation and detection technique for doubly selective channels

In this paper, we propose a low-complexity iterative joint channel estimation, detection and decoding technique for doubly selective channels. The key is a segment-by-segment frequency domain equalization (FDE) strategy under the assumption that channel is approximately static within a short segment. Guard gaps (for cyclic prefixing or zero padding) are not required between adjacent segments, which avoids the power and spectral overheads due to the use of cyclic prefix (CP) in the conventional FDE technique. A low-complexity bi-directional channel estimation algorithm is also developed to exploit correlation information of time-varying channels. Simulation results are provided to demonstrate the efficiency of the proposed algorithms. © 2008 IEEE

Intersymbol and Intercarrier Interference in OFDM Systems: Unified Formulation and Analysis

A unified matrix formulation is presented for the analysis of intersymbol and intercarrier interference in orthogonal frequency-division multiplexing (OFDM) systems. The proposed formulation relies on six parameters and allows studying various schemes, including those with windowing in the transmitter and/or in the receiver (called windowed OFDM systems), which may add cyclic suffix and/or cyclic prefix (CP), besides the conventional CP-OFDM. The proposed framework encompasses seven different OFDM systems. It considers the overlap-and-add procedure performed in the transmitter of windowed OFDM systems, being jointly formulated with the channel convolution. The intersymbol and intercarrier interference, caused when the order of the channel impulse response is higher than the number of CP samples, is characterized. A new equivalent channel matrix that is useful for calculating both the received signal and the interference power is defined and characterized. Unlike previous works, this new channel matrix has no restrictions on the length of the channel impulse response, which means that the study is not constrained to the particular case of two or three data blocks interfering in the received signal. Theoretical expressions for the powers of three different kinds of interference are derived. These expressions allow calculating the signal-to-interference-plus-noise ratio, useful for computing the data rate of each OFDM system. The proposed formulation is applied to realistic examples, showing its effectiveness through comparisons based on numerical performance assessments of the considered OFDM systems

Transmisión multiportadora sin prefijo cíclico para comunicaciones radio de alta capacidad

Este trabajo fin de grado está orientado en una de las líneas de investigación seguidas en la actualidad para conseguir aumentar la capacidad de los sistemas de comunicaciones. Se ha centrado el trabajo en el marco de la modulación multiportadora Orthogonal Frequency Division Multiplexing (OFDM) por ser una de las técnicas más utilizadas en la actualidad en sistemas de banda ancha. El objetivo será aumentar la tasa de transmisión de un sistema de comunicaciones suprimiendo o reduciendo el prefijo cíclico, Cyclic Prefix (CP), perteneciente a OFDM. Se analizan y muestran las consecuencias en forma de interferencias que se tienen como resultado de no utilizar un CP mayor que la respuesta al impulso del canal. Las interferencias que se han encontrado son las denominadas como ISI (Inter-Symbol Interference) por no introducir un intervalo de guarda suficiente entre los símbolos OFDM e ICI (Inter-Carrier Interference) por la pérdida de ortogonalidad entre las subportadoras, provocando estas unos pésimos resultados en el sistema. Para poder combatir estos efectos se explicará y se implementará un algoritmo de cancelación de interferencias llamado “Cancelación residual de ISI” (RISIC). Este algoritmo consiste en realizar una combinación de cancelación de cola y una reconstrucción cíclica. Se ha analizado si los resultados obtenidos compensan el aumento de complejidad introducida en el sistema. Además, en este trabajo se investiga cómo esta técnica puede ser implementada en estándares actuales como Wimax (Worldwide Interoperability for Microwave Access) o LTE (Long Term Evolution), y así poder conseguir mayores velocidades de transmisión de datos.This final degree project deals with the research lines followed at present for increasing the capacity of communications systems. We have focused our work in multicarrier modulation Orthogonal Frequency Division Multiplexing (OFDM) for being one of the techniques used today in broadband systems. The goal that we want to achieve will be to increase the transmission rate of a communication system by eliminating or reducing the Cyclic Prefix (CP) pertaining to OFDM. Consequences are analyzed and displayed in the form of interference effects as result as not using a CP higher than the channel impulse response. The interferences that have been found are named ISI (Inter-Symbol Interference). This is because ISI doesn't introduce enough guard interval between OFDM symbols and ICI (Inter-Carrier Interference) for the loss of orthogonality between subcarriers and it provokes dismal system results. To combat these effects will be discussed and implemented an algorithm called interference cancellation "Residual ISI Cancellation" (RISIC). This algorithm is to perform a combination of tail cancellation cyclic reconstruction. We have analyzed if the results outweigh the increased complexity introduced into the system. In addition, this final degree project investigates how this technique can be implemented in existing standards such as WiMAX (Worldwide Interoperability for Microwave Access) or Long Term Evolution (LTE), so you can achieve higher data transmission speeds.Ingeniería de Sistemas de Comunicacione

Sistemas OFDM de alta eficiencia espectral

En los últimos años, el desarrollo de los sistemas de comunicaciones ha tenido un gran auge debido a las mejoras en los sistemas de procesado digital de señales. Entre ellos, destacan los sistemas basados en las técnicas de modulación OFDM con un gran crecimiento y una expansión creciente. Las mejoras en esta técnica de transmisión de información son constantes gracias a una gran actividad investigadora. Por este motivo, surge recientemente el interés en llevar a cabo investigaciones con el objetivo de conseguir una mayor eficiencia espectral. Es cierto también, que en los últimos tiempos existe un crecimiento exponencial de la demanda de ancho de banda en cualquier tecnología. Por otro lado, la comunicación móvil se encuentra en un buen momento de crecimiento. El procesado digital de señal se ha visto en parte sobre-explotado en este punto, de forma que hay que abrirs nuevas líneas de búsqueda de otros métodos que nos lleven a conseguir esa mayor capacidad en sistemas tan relevantes como OFDM. Es esto último por lo que se presenta este trabajo, el cual se centra en la búsqueda de métodos alternativos al uso del prefijo cíclico (PC) de los sistemas OFDM, para conseguir de esta forma una mayor eficiencia espectral pero a la vez conservar la ortogonalidad de las subportadoras y de ese modo evitar la presencia de interferencias. Se han analizado diferentes alternativas propuestas en la literatura para eliminar las interferencias que provoca suprimir el prefijo cíclico, simulando las condiciones de canal, modulación y ruido consideradas por los autores. ________________________________________________________________________________________________________________________________In recent years, development of communications systems has boomed due to improvements in systems for digital signal processing. Among them are systems based on OFDM modulation techniques with a high growth and increasing expansion. Improvements in this technique of transmission are constant through a high research activity. For this reason, recently emerged interest in conducting research in order to achieve greater spectral efficiency. It is also true that in recent times there is an exponential growth in demand for bandwidth in any technology. On the other hand, mobile communication is in a great moment of growth. The digital signal processing has been partly over-exploited at this point, so it is needed to open new lines in the search for other methods that lead us in order to achieve this increased capacity as relevant as OFDM systems. This is what it is presented in this work, which focuses on finding alternative methods to the use of cyclic prefix (PC) in OFDM systems to achieve thus increased spectral efficiency but also preserve the orthogonality of the subcarriers and thereby avoid the presence of interference. Different alternatives proposed in the literature have been analyzed in order to eliminate interference caused by removing the cyclic prefix, simulating the channel conditions, modulation and noise considered by the authors.Ingeniería de Sistemas de Comunicacione

Robust frequency-domain turbo equalization for multiple-input multiple-output (MIMO) wireless communications

This dissertation investigates single carrier frequency-domain equalization (SC-FDE) with multiple-input multiple-output (MIMO) channels for radio frequency (RF) and underwater acoustic (UWA) wireless communications. It consists of five papers, selected from a total of 13 publications. Each paper focuses on a specific technical challenge of the SC-FDE MIMO system. The first paper proposes an improved frequency-domain channel estimation method based on interpolation to track fast time-varying fading channels using a small amount of training symbols in a large data block. The second paper addresses the carrier frequency offset (CFO) problem using a new group-wise phase estimation and compensation algorithm to combat phase distortion caused by CFOs, rather than to explicitly estimate the CFOs. The third paper incorporates layered frequency-domain equalization with the phase correction algorithm to combat the fast phase rotation in coherent communications. In the fourth paper, the frequency-domain equalization combined with the turbo principle and soft successive interference cancelation (SSIC) is proposed to further improve the bit error rate (BER) performance of UWA communications. In the fifth paper, a bandwidth-efficient SC-FDE scheme incorporating decision-directed channel estimation is proposed for UWA MIMO communication systems. The proposed algorithms are tested by extensive computer simulations and real ocean experiment data. The results demonstrate significant performance improvements in four aspects: improved channel tracking, reduced BER, reduced computational complexity, and enhanced data efficiency --Abstract, page iv